THE 
Epidermal Organs 
of Plants 
THEIR MORPHOLOGY AND PHYSIOLOGY 
BY 
CHARLES F. COX, f. r. m. s. 
Vice-president of the new vork microscopical society, corresponding member of 
THE SOClfil'4 BEI.GF. DE M1CROSCOPIE, ETC., ETC. 
Reprinted from the 
AMERICAN MONTHLY MICROSCOPICAL JOURNAL 
Jui.y, 1881 THE 
Epidermal Organs 
of Plants 
THEIR MORPHOLOGY AND PHYSIOLOGY 
BY 
CHARLES F. COX, f. r. m. s. 
VICE-PRESIDENT OF THE NEW YORK MICROSCOPICAL SOCIETY, CORRESPONDING MEMBER OF 
TH2 SOCl£r£ BELGE DE MICROSCOPIE, ETC., ETC. 
Reprinted from the 
AMERICAN MONTHLY MICROSCOPICAL JOURNAL 
July, 1881 The Epidermal Organs of Plants. 
It is, of course, well known that 
those parts of plants which are ex- 
posed to external influences—to light, 
heat, evaporation, etc.—become com- 
pacted and hardened into a rather 
dense and rigid tissue called the epi- 
dermis. This differentiation is not 
well marked in roots and submerged 
parts of aquatic plants, but even in 
such parts we recognize at least a 
theoretical difference between the 
most exterior layers of cells and those 
of the so-called parenchyma lying 
beneath. 
Upon the aerial portions of plants 
there is generally a tissue external 
to the epidermis—of the nature of 
a very thin, usually homogeneous 
and transparent pellicle or skin,— 
termed the cuticle. 
The epidermis, however, may be 
regarded, physiologically, as the most 
external organized and organ-produc- 
ing substance of the plant. 
From this epidermis are developed 
all external appendages, known as 
hairs, villi, papillae, tentacles, bris- 
tles, thorns, scales and glands, with 
analogous and homologous organs, 
including probably the sporangia of 
ferns, and even the ovules of phane- 
rogams. 
It has been proposed to class all 
such lateral growths from the epi- 
dermis under a single name : Trich- 
omes. This grouping, however, not 
being entirely satisfactory in all 
respects, is merely tentative; and 
there is good reason to think 
that, when microscopical botany has 
been thoroughly worked up, the 
whole theory of the epidermis and its 
appendages will have to be reconsider- 
ed and much modified. 
Hairs are the most attractive of 
the epidermal appendages, and are 
exceedingly varied in form, as well 
as often charmingly beautiful in 
appearance. They have long been 
favorite objects of observation with 
microscopists, though I cannot say 
that they have long been objects of 
scientific study and investigation; for, 
as far as I am aware, they have sel- 
dom attracted attention except for 
their external symmetry and beauty. 
This mere prettiness of plant-hairs 
cannot be adequately expressed in 
description ; and, so far from won- 
dering that it has attracted attention, 
I am surprised that it has not attract- 
ed more ; for I am sure that no class 
of microscopic objects offers more 
various beauties of outline, ornamen- 
tation and color, than do these ex- 
quisite gems of the vegetable world. 
The outlines of different kinds 
of plant-hairs have often been figured, 
and I will not consume time and 
space in any attempts at an extended 
description here. It may be well, 
however, to mention the more char- 
acteristic forms. 
To begin with, we have what may 
be called the right hair, which is a 
simple, perpendicular, or more or less 
appressed, unicellular growth, such as 
may be found on Lithospermum hir- 
tum or Lithospermum canescens. 
Jointed hairs are similar in outline 
to the right hairs ; but instead of be- 
ing unicellular, they are composed of 
a series of cells or nodes. Such 
hairs are found upon many of the 4 
Composite,—for instance, Pluchea 
bifrons. 
In segmented hairs the component 
cells are separated by deeper con- 
strictions than in jointed hairs ; and 
they resemble, when filled with their 
living fluids, strings of translucent 
beads. Such hairs grow upon the 
stamens of Tradescantia Virginica 
and upon the leaves of Polemonium 
cceruleum. 
Cleft hairs resemble right hairs 
which have been split part way down, 
and the separated portions of which 
have become more or less depressed. 
In some cases this depression pro- 
ceeds so far that the hair appears to 
be a long, doubly terminated, hori- 
zontal cell attached to the leaf by a 
pedicel at its centre. Such hairs 
may be seen on the leaves of Clematis 
erecta, Astragalus vesicarens, and of 
Bentharnia fragifera. 
Branched hairs are multicellular 
hairs in which growth takes place in 
many directions, or not entirely in one 
general direction. Examples of this 
mode of growth are found upon Alter- 
nanthera lanugmosa, and upon Ver- 
bascum thapsus and many others of the 
Scrophulariacse. 
A variety of this species is the 
radiate, in which the branching 
is confined to nearly horizontal 
planes. This form of hair occurs 
upon the upper side of the leaf of 
Shephcrdia Canadensis, and upon many 
of the Polypodiaceae. 
These branching modes of growth 
(especially the radiate) give rise to 
many of the forms commonly includ- 
ed under the term stellate ; but I pre- 
fer to reserve that name for unicellu- 
lar, star-shaped hairs, such as are 
found upon Dentzia, Vesicaria and 
some species of Alyssum. 
Peltate hairs are those partly mem- 
branous, partly radiate, scale-like 
hairs, which occur upon all the Elcea- 
gnacece, upon some of the Crotons 
and upon Si da lepidota. 
I apply the term rosette to those 
less scale-like, though nearly peltate, 
hairs such as are found upon Croton 
caramba. These differ from the pel- 
tate hairs in having fewer lines of 
radiating growth, and in being less 
membranous or scale-like. They 
approach closely to the forms of the 
glands of Callicarpa, of Carya, or of 
the Myricaceae. 
When several simple hairs are de- 
veloped from adjacent epidermal 
cells, I term them grouped. Some- 
times these grouped hairs consist of 
but two simple hairs in contact at 
their bases, but often five or six or 
more form a group which, when seen 
from above, give the appearance of a 
stellate form. Such stellately grouped 
hairs are common among the Mal- 
vaceae and the Cistaceae. In Helian- 
themum canum, var. genuina, we have 
a good example of stellately grouped, 
short hairs, associated with simple 
groups of two, three or four long 
right hairs. 
Closely allied to this form is the 
one which I call mammillate. In 
this species what would be the cen- 
tral hair of a group is fully devel- 
oped, but the surrounding epidermal 
cells are only partly developed into 
hairs, most frequently just enough to 
slightly raise them above the general 
surface of the epidermis, thus produ- 
cing a protuberance or mamma upon 
the surface of the leaf, from the centre 
of which the fully developed, simple 
hair seems to spring. This is a char- 
acteristic form amongst the Borra- 
ginacece, and is most strikingly exhib- 
ited on the leaf of Onosmodium Vir- 
ginianum. In some species, however, 
all the hairs are aborted, and what 
might otherwise be a group of 
hairs, is only a group of partly 
developed epidermal cells. This is 
exemplified in Echinacea angustifolia. 
On the other hand, it happens in 
some species that all the grouped 
epidermal-cells, or a large number of 
them, become fully developed hairs, 
but only the central one grows per- 
perpendicular to the surface of the 
leaf, the others radiating about its 
base in a nearly horizontal position, 
or at an acute angle to the surface. 5 
A fine example of this is seen in 
Onosma montana. 
Among the Compositae a very strik- 
ing, but not a very common, form of 
hair is what may be called serrate. 
In its simpler variety its outline some- 
what resembles that of wool; but in 
its more pronounced phase it suggests 
the hair of some animal among 
the rodents, particularly of the Indian 
bat. This is the case with the leaf- 
hairs of Hieracium tomentosum and 
of Adenostylcs lanatus. 
Glandular hairs (hairs bearing 
glands at their extremities) are found 
upon a large number of plants, par- 
ticularly upon the so-called “ insect- 
ivorous ” plants, and also upon many 
others which possess glands rather 
for the purpose of secreting or ex- 
creting than for absorbing, such as 
the tomato and Rhodora. 
Tentacular hairs are the most dif- 
ferentiated and specialized of all,— 
they being endowed with the power 
of movement in response to stimula- 
tion, besides the power of secreting 
and absorbing. These are best ex- 
emplified in the Droseraceae. 
At the other extreme of the scale 
of development are what I term pa- 
pillae, which, together with what may 
be called villi, are perhaps not so 
much of the nature of immature or 
undeveloped hairs, as of organs inter- 
mediate between hairs and stomata. 
Instances of the occurrence of these 
forms—which are hard to separate, 
the latter being apparently only less 
differentiated than the former—may 
be found on many of the Ericacece, 
particularly on Rhodora Canadensis 
and Rhododendron Nuttal/ii, and also 
on Clematis recta and on Porsooni 
salicina. 
There are, of course, many other 
forms of hairs, some of them inter- 
mediate varieties to those I have 
named, and some of them connecting 
links between hairs and glands or 
other epidermal organs ; and several 
of these often occur upon the same 
plant or part of the plant. 
It would be an easy matter to 
arrange a series of specimens show- 
ing an almost imperceptible running 
of one of these forms into another, 
in perfect gradation all the way from 
the true sunken gland up to the most 
developed and specialized forms of 
radiate, stellate, peltate or tentacular 
hairs. 
The difference between a secreting, 
glandular hair and a true gland seems 
to consist mainly in the fact that one 
is entirely sessile, or sunken, while 
the other is more or less pedicellate. 
Sachs states that all reservoirs of oil, 
resin, gum, etc., whether internal or 
external, are only slightly different 
developments of the same organ ; but 
if we include among such reser- 
voirs the turpentine-cells of the Coni- 
ferae, as has been suggested by some 
writers, we shall have to rearrange 
somewhat our present ideas of what 
are epidermal organs. 
As may have been inferred from 
what I have already said, the forms 
of hairs, glands, etc., are more or less 
characteristic of families, orders, gen- 
era, and in some cases of species 
even. 
This is the most interesting, and 
scientifically the most important point 
in this whole subject; but it has not yet 
been even approximately elaborated, 
and there is vast room for research 
here and great promise of valuable 
results. 
Take the forms of hairs peculiar to 
the Elaeagnaceae, for example, they are 
as characteristic of the order as any 
other part of the plant ; indeed, in 
this particular case, I think they are 
the most characteristic and distin- 
guishing part. And yet little or noth- 
ing is made of these microscopical 
characteristics in classification, al- 
though enough has already been 
learned concerning them to show that 
they are more intimately connected 
with the physiological nature and natu- 
ral affinities of the plant than are many 
less intrinsic qualities or charac- 
ters, which are given a prominent 
place in the usual system of classifi- 
cation. To be sure, reference is some- 6 
times made to the “ silvery ” appear- 
ance of some species, and to the “ scur- 
fy ” appearance of others, to the naked 
eye, but the characteristic feature of 
Shepherdia or Elceagnus or Hippophcz 
is seldom even referred to in such 
botanical works as I have happened 
to consult. 
The hairs of the Malvacece also are 
very characteristic. In general, to 
the casual observer, they seem to 
come under the very inclusive desig- 
nation of stellate. But they are not 
stellate, in the proper limitation of 
that term, or as the hairs of Deutzia, 
for example, are stellate. In the 
genus Sida, and in one or two other 
genera, they are what I have called 
peltate, and in still other genera they 
tend toward what I have called the 
radiate form. But throughout the larger 
part of the order they are what I have 
termed grouped hairs. Upon the 
upper side of the leaf they may be 
grouped only in twos or threes, or the 
hairs may be even solitary ; but upon 
the under side of the leaf, or upon the 
calyx, the groups will probably con- 
sist of four, five, six or more hairs, 
disposed in such a way as to give 
rise to the so-called stellate appear- 
ance, although this stellate grouping 
will often be associated with the sim- 
pler aggregations, or with the single 
right hairs. But a tendency to stellate 
grouping, in some part of the plant 
or at some stage of its existence, is 
decidedly characteristic of the Mal- 
low family as a whole. 
In the genus Althcea there is a dis- 
position to complexity of form and 
luxuriance of growth on every part of 
the plant—even on the petals. 
In Malva there is less complexity 
of form and less luxuriousness of 
growth, the stellate groups being most 
pronounced upon the calyx. 
In Abutilon there is still less hairi- 
ness and still less tendency to stellate 
grouping. 
In the genus Hibiscus the hairs 
almost disappear from the leaves (es- 
pecially from the adult ones), though 
the stellate groups still form rather 
abundantly upon the calyx and the 
seed-pod. 
These facts are sometimes vaguely 
referred to in the text-books under 
the heading of pubescence; but usu- 
ally Hibiscus is spoken of as smooth 
or glabrous, because the leaf in its 
adult stage is hairless, or nearly so, 
to the unaided senses, while the 
truth is that hairiness in some part of 
the plant, in some stage of its exist- 
ence, and, as a general rule, hairiness 
of a particular kind, is almost if not 
quite as much a characteristic of this 
genus, as well as of its order, as are 
its “ alternate, stipulate leaves and 
regular flowers, with monadelphous 
stamens.” 
Nothing can be more characteristic 
of the Borraginaceae, as an order, 
than their hairinesss as seen under 
the microscope. This is generally 
of the kind I have described as 
mammilate ; and so striking is this 
in almost every genus and species I 
have examined, that the minutest 
fragment of a leaf large enough to 
hold a hair, would in most cases be 
enough to determine at least the 
order to which it belonged. 
Amongst the Cruciferae, too, there 
are some very distinct groups of 
genera marked off by the peculiari- 
ties of their leaf-hairs, so that one 
who has once become familiar with 
Alyssum, Ve sic aria, Draba and Arabis, 
for instance, would have little difficul- 
ty in determining the place of a speci- 
men of one of them, by microscopical 
examination alone. As far as my ex- 
perience has gone, the grouping of 
plants by their microscopical charac- 
teristics has been found to accord in 
the main with the classifications al- 
ready made by botanical science on 
other grounds ; but in running through 
the order Cruciferae, for example, one 
is often struck by the wide dissimilarity 
discoverable by the microscope, be- 
tween certain groups of genera; and 
one is led to wonder whether plants 
so different in their minute struc- 
tures can really be near relatives, 
even though their mode of infiores- 7 
cence and some other attributes may 
be similar. 
Nevertheless, the microscopical 
characteristics of plants may be 
determined and tabulated so as to be 
of much value for purposes of identi- 
fication ; and I am inclined to 
think that such labor will some day 
result in a system by which either 
extant or extinct orders, at least, can 
be identified to a certainty ; and such 
a system would be of no little impor- 
tance in many lines of scientific in- 
vestigation, as will at once be seen. 
Thus, in geological science, buried 
parts of plants would throw light 
upon the age and character of the 
deposits in which they were found, 
and in zoological science even the half- 
digested food of beasts, birds or in- 
sects might be made to disclose the 
habits of the eaters. 
But, as has already been fore- 
shadowed, it is not only the mere 
form or figure of the hairs that is 
characteristic. Their distribution upon 
different parts of the plants is also 
a matter of regular and peculiar 
habit, as well as their existence or 
non-existence at different stages of 
the plant’s history. 
In many plants the young leaves 
or stems are hairy or scaly, while the 
adult parts are quite smooth. 
We recall the fact that the ramenta 
of ferns commonly fall off when the 
frond is fully developed ; and this 
is also true of the hairs of many 
phanerogams. But even when the 
hairs are not shed, the adult leaf will be 
relatively much less hairy than the 
young leaf, from the fact that the 
hairs are developed coincidently with 
the leaf itself, and that their number 
does not increase as the leaf increases 
in size. The same number of hairs 
is, therefore, spread over a much 
greater surface in the adult leaf than 
in the young, growing leaf ; and the 
young leaf is therefore the more pub- 
escent, relatively to its surface. This 
has an important bearing, as will 
be observed when I come to speak of 
the physiological side of my subject. 
It follows from the fact I have just 
mentioned, that to obtain the most 
striking specimens of vegetable hairs, 
the microscopist generally selects the 
young, terminal or rapidly growing 
leaves, stems or petioles. 
Another point as to distribution, I 
have barely referred to already, and 
that is that even when leaves are en- 
tirely hairless the calyx may be hairy. 
Sometimes this is true of the petals, 
and often of the filaments. In other 
words (to generalize these facts), hairi- 
ness begins at the growing point, and 
persists there the longest. It may never 
exist elsewhere on the plant to any 
considerable extent, but it is likely to 
exist upon the part last developed, 
whether leaf or flower. The hairs 
may drop off from the older parts, 
but they are not often absent from the 
younger parts. This matter also has 
an important connection with physi- 
ological facts, which I shall mention 
hereafter. 
One of the most obvious facts 
connected with the distribution of 
hairs is that they are found more fre- 
quently and more abundantly upon 
the under side than upon the upper 
side of the leaves, the sepals or the 
petals. This is a very general rule, 
though there are exceptions to it 
which seem to indicate that the rela- 
tive hairiness of the two surfaces of 
the leaf depends upon the position in 
which the leaf grows. I am inclined 
to the opinion that when the leaf 
naturally assumes such a position 
that one side receives a much larger 
amount of light than the other, that 
side will be the least hairy, if not en- 
tirely destitute of hairs, while the sha- 
ded side will be the one most favorable 
to hairiness. In plants whose leaves 
grow more or less perpendicularly, 
the difference between the two sides 
is apt to be less marked. 
Heliotropism may affect the rela- 
tive hairiness of the two sides of the 
leaf, either in causing a greater differ- 
ence by presenting one side more 
constantly to the light, or in caus- 
ing less difference by presenting both 8 
sides equally. It is said that the 
“ compass plant ” of our western 
prairies (Silphium laciniatum), always 
presents its leaf in a plane with the 
north and south meridian, and both 
sides of its leaf are alike; but, 
aside from its heliotropism, this may 
perhaps be due merely to the leaf’s 
tending to a perpendicular position. 
In the Bayberry (Myrica cerifera) 
we find that the upper side of the 
leaf is the more hairy, while the 
glands follow the general rule in being 
more abundant on the under side. 
But, notwithstanding these occa- 
sional exceptions (which will doubt- 
less be found to be explainable in 
accordance with the general theory), 
it is a rule that the under side of the 
leaf, and that which corresponds to 
the under side in homologous parts, 
is the hairy side. If both sides are 
hairy this side will be the more so. 
This, as I have already said, I am 
inclined to refer to the influence of 
light as a general principle, though 
the fact that the hairiness of sepals 
and petals is still on the outer side 
(or the side corresponding to the 
lower side of the leaf), does not seem 
to be directly explainable on this 
theory, but seems to be indirectly 
connected with it, through the mere 
homology of the parts. 
Another very obvious fact as to 
distribution, is that hairs are most 
abundant upon the mid-rib and the 
larger fibro-vascular veins. When 
they do not exist at all upon 
other parts, they are often abun- 
dant upon these vessels, and the 
mid-rib is seldom absolutely hairless. 
It is one of the most striking points 
in connection with this subject, that 
hairs commonly spring from the sur- 
faces of fibro-vascular bundles, or are 
more or less directly connected with 
them. In most plants this character- 
istic is plainly seen, but in many it is 
obscured by the great profusion of 
hairs ; and in some, though not hid- 
den, it is not marked. I believe, 
however, that there is an essential 
relation between hairs and the fibro- 
vascular system, which I will refer to 
more specifically by and by. 
What I have said as to the distri- 
bution of hairs, is generally true also 
of glands. Indeed, this is to be ex- 
pected from the fact that hairs and 
glands are only different develop- 
ments of the same organ, as it has 
been already stated. 
Glands exist under every form, 
from a mere epidermal cavity con- 
taining an essential oil, as in Mag- 
nolia glauca (the White Bay), or Hy- 
pericum parviflorum, to the complexly 
specialized, secreting and absorbing 
tentacles of the Droseraceae. Between 
these extremes we find a mere orifice 
emitting a drop of resin or gum—as in 
Gaylussacia—the half-external, half- 
internal, multicellular, rosette-like 
gland of Rhododendron ferrugineum, 
the sessile, but quite external gland of 
Myrica or Callicarpa, the glass fun- 
nel of Chenopodium, the pedicellate, 
spreading, flower-like gland of Rho- 
dodendron Nuttallii; the short, gland- 
ular hair of the tomato or Rhodora 
or of Pluchea ; and the long, stinging 
hair of Urtica. As has been inti- 
mated already, the hairs of Croton 
caramba and the glands of Carya 
glabra are so nearly alike in appear- 
ance that they may be regarded as 
connecting links. 
One of the characteristics of all 
epidermal appendages is the tendency 
to a deposition in them of silica or 
of lime, particularly of silica. In the 
Diatomacese the siliceous frustule is 
the largest part of the whole organ- 
ism ; and in the Equisetacese and the 
Graminese the deposit of silica in the 
epidermis and cuticle is of no small 
amount or importance. The peculiar 
form of the deposit in and upon 
many kinds of hairs is a distinguish- 
ing mark, and in many it is so general 
and abundant that an indestructible 
cast of the hair remains with the cuti- 
cle, after maceration or burning of 
the leaf on which it is found. 
This is particularly the case with the 
large and striking stellate hairs of the 
Deutzias. In these hairs as well as in 9 
many others (as for example : those of 
Onosmodium, Lithospermum, Bentha- 
?nia and Astragalus), the silica is dis- 
posed in apparently granular masses, 
distributed over the surface of the hair, 
giving it the appearance of the larger 
and coarser spicules of some of the 
Gorgonidae. 
Under a high power these grains 
have sometimes seemed to me more 
like irregular truncate cones, or like 
the deposits of silica or lime which 
form on a larger scale around the 
mouths of geysers and hot springs, 
and I have therefore imagined that 
perhaps they were actually forma- 
tions about orifices from the interior 
of the hairs, though I have never been 
able to demonstrate, to my own satis- 
faction,any suchopeningthrough them. 
Before passing to another division 
of my subject, I must say a few 
words about stomata, which also are 
epidermal structures. As a general 
rule they are found upon the same 
parts as the hairs and glands, and in 
the same relative proportions. That 
is to say, they abound upon the leaves 
and homologous parts, and on young 
and growing stems and branches; 
and they are found in most abun- 
dance on the under side of leaves. 
Unlike hairs, however, they never 
appear upon the mid-rib and veins 
but are always found directly over 
intercellular spaces, of which they are 
the means of communication with the 
external air. Of course they are use- 
less, and therefore exist but as strag- 
glers, if at all, on parts not exposed 
to the air. They are therefore not 
usually found on true roots or sub- 
merged parts of aquatic plants. Like 
hairs, their relative number upon the 
upper and upon the lower side of the 
leaf depend in great measure upon 
the natural position of the leaf with 
reference to the light. 
I will now pass to the physiological 
portion pf my subject. 
It is not to be doubted that such 
highly developed and specialized 
organs as plant-hairs have an im- 
portant physiological office in the 
vegetable economy. Neither is it to 
be supposed that so large a part 
of the mere bulk of the plant is 
practically useless or only ornament- 
al. The fact that the hairs of 
plants constitute a very large propor- 
tion of their substance, argues that 
they are not accidental or merely in- 
cidental growths, but that they must 
be exceedingly important organs to 
warrant such an expenditure of ma- 
terial and energy as is necessary for 
their production. We all appreciate 
the extraordinary quantity of mater- 
ial used to form the seed-hairs which 
constitute the commercial article, 
cotton ; and all have observed 
the also lavish use of material made 
to produce the seed-hairs of the 
milk-weed and the thistle. These 
seed-hairs serve a useful purpose in 
the distribution of the seeds to a dis- 
tance, thus disseminating the species 
to which they belong ; but aside from 
this I believe that, in common with 
other plant-hairs, they serve an im- 
portant physiological purpose before 
the seeds upon which they grow at- 
tain maturity. 
It seems to me that light is 
thrown upon the office of plant- 
hairs in general, by what we know 
of the physiological uses of root- 
hairs and the tentacular hairs of 
the insectivorous plants. Such hairs 
are primarily absorbing organs, tak- 
ing up for the use of the plant not 
only nitrogen and nitrogenous com- 
pounds, but probably also oxygen. 
It is generally agreed that there are 
but two principal physiological pro- 
cesses going on in the plant, namely, 
assimilation (the elaboration of or- 
ganic compounds out of inorganic), 
and metastasis (the transmutation of 
one organic compound into another). 
Assimilation is that process by which 
carbon-dioxide and water are decom- 
posed and their elements formed in- 
to what are known as the carbo-hy- 
drates, of which starch is the most 
important. In this process oxygen 
is liberated and exhaled. Assimilation 
can take place only in the chloro- 10 
phyll-bearing cells, in the presence of 
light and a certain degree of heat. 
The materials produced by this pro- 
cess are the only materials which are 
suitable for use as food for the pro- 
toplasmic basis of life in the plant, 
which is the seat of growth and re- 
production. The process by which 
these carbo-hydrates are appropri- 
ated to the specific purposes of the 
plant, are transported from one part 
to another, and transmuted into ac- 
tive, living, propagating and then per- 
manent vegetable tissue, is what is 
called metastasis. Inorganic sub- 
stances cannot be directly appropri- 
ated by the protoplasm of the plant, 
except perhaps to a very limited ex- 
tent in some special cases. Assimila- 
tion, therefore, is a somewhat doubt- 
ful name for a preparatory process, of 
which metastasis is the ultimate object. 
Metastasis is, therefore, really the as- 
similative process, and corresponds 
to the process to which the name 
assimilation is given in animal physi- 
ology ; while assimilation, so-called 
in vegetable physiology, is more anal- 
agous to the preparatory process of 
digestion in animals. There is greater 
analogy between the vegetal pro- 
cess of metastasis and the animal 
process of assimilation from the fact 
that oxidation, by some sort of res- 
piration or absorption from without, 
is an essential part of each. 
It has already been stated that 
what is termed assimilation, takes 
place only in the chlorophyll-bearing 
cells in the presence of light and heat. 
Therefore, fungi, parasitical plants 
and, generally, plants which are des- 
titute of chlorophyll, are unable to 
perform the assimilative process. 
They must therefore obtain for their 
food ready-made, organic compounds, 
and are, in this respect, like animals. 
Unlike the process of assimilation, 
metastasis takes place in cells which 
do not bear chlorophyll, or else in the 
chlorophyll-cells in the absence of 
light ; and while the assimilative pro- 
cess is promoted by light and warmth, 
the metastatic process is promoted 
by darkness and probably by a 
lower temperature. What aids the 
one process retards the other; 
so that the assimilative process is 
active in the day-time when the met- 
astatic process is checked, and the 
latter process is active at night when 
assimilation is checked. Day-time is 
therefore the period of preparation, 
and night time the period of 
growth in plants. In the day-time 
they take up and decompose the 
compounds from which the carbo- 
hydrates are formed, at the same 
time exhaling oxygen, and at 
night they inspire oxygen for the pur- 
poses of internal change and growth, 
at that time exhaling the products 
of oxidation, which consist mostly 
of carbon-dioxide. Now this fact is 
to be particularly noted, that the pro- 
toplasm of the plant is its life-bearing 
element, and that the process of meta- 
stasis is a process directly connected 
with the preservation, augmentation, 
modification and reproduction of this 
life-bearing element; while the pro- 
cess called assimilation is ..almost 
purely chemical, and has for its re- 
sult merely the production of appro- 
priate material for the metastatic 
process. 
It has been shown that the rotation 
of protoplasm in the plant-cell is not 
aided by light, if, indeed, it does not 
take place more readily in the dark. 
It has also been shown that it cannot 
take place without a supply of oxy- 
gen. It is also a familiar fact that 
the rotation of protoplasm is common 
in plant-hairs, in which it maybe seen 
under the microscope. A favorite 
object for this purpose used to be the 
stinging hair of the nettle, but the 
cyclosis is easily seen in many other 
hairs, particularly in the segmented 
hairs on the stamens of Tradescantia 
and in the leaf-hairs of the pansy or of 
the holly-hock. The facts I have just 
referred to serve to connect the hairs 
with the protoplasmic portion of the 
plant and, through it, with the pro- 
cess of metastasis. 
As there are two essential physio- 11 
logical processes in plant-life, so there 
are two principal sets of organs for 
the effecting of these processes, name- 
ly, the chlorophyll-cells with their 
appendages, and the circulatory sys- 
tem with its appendages. The for- 
mer system is apparently much the 
simpler, and consists of little be- 
sides the chlorophyll-cells themselves 
(which are parenchymal cells), with 
the stomata, their means of com- 
munication with the external air. 
But the circulatory system begins 
in the roots, follows the trunk, 
branches, stems and petioles, and 
spreads out in a finely ramified net- 
work of veins in the leaf, the latter 
being merely the extension of the 
fibro-vascular bundles from the peti- 
ole into the leaf. 
Circulation in the plant is depen- 
dent upon evaporation from its 
surfaces. The leaves promote evap- 
oration, and the lost moisture is 
made good mainly by absorption of 
water through the roots. There are 
reasons for supposing that the sto- 
mata are the chief organs of evap- 
oration, especially in those plants 
which have a dense and imper- 
vious cuticle. But there are also 
good reasons for believing that the 
stomata have other offices besides 
merely facilitating evaporation. There 
can be but little doubt that they are 
also the means of communication be- 
tween the external atmosphere and 
the internal chlorophyll-bearing cells 
through which the supply of materials 
is obtained for the process of assim- 
ilation already described. Such a 
relation between the stomata and the 
chlorophyll-cells is at once suggested 
by the fact that the stomata are invari- 
ably directly over intercellular cavi- 
ties ; and it is confirmed by the fact 
that they open in the presence of light 
and close in darkness, being ready 
for duty only at a time when the 
assimilative process is active. If the 
function of evaporation were their 
sole office, one would suppose that 
they would be most open when evap- 
oration was least active (in the shade), 
and more closed when evaporation 
was likely to be too rapid (in the sun- 
light), acting, in this respect, like the 
governor on an engine. 
While there is much evidence that 
the stomata are organs of the assimila- 
tive process, there is quite as much 
evidence that they are not connected 
with the metastatic process. Of this 
negative evidence we recall at once 
the fact that the stomata are never 
found upon the fibro-vascular veins of 
the leaf. Other negative evidence 
will suggest itself as the converse of 
postive evidence which will hereafter 
be mentioned as showing that the 
hairs are connected with, and are 
organs of, the metastatic process. 
Several recent observers of plant- 
hairs have attempted to account 
for their existence ; but no one, as 
far as I know, has done so upon 
purely physiological grounds. To 
my mind, however, the evidence that 
hairs are primarily organs of the 
metastatic process, strengthens with 
each new fact that comes to my know- 
ledge. This theory is not affected 
by the fact that not all plants 
in all stages of their existence 
are hairy, because, probably no pro- 
cess in vegetable physiology is per- 
formed in precisely the same manner, 
and by exactly the same means 
throughout the whole kingdom. Nor 
is the theory weakened because hairs 
often serve other purposes than strict- 
ly physiological ones, for nothing is 
more common than for an organ, or 
part, to become modified or special- 
ized for an office for which it was 
not primarily intended. Thus it is 
by no means strange if hairs, while 
essentially organs of the physiological 
process of metastasis, should be util- 
ized for purposes of protection against 
cold or dryness, as has been sug- 
gested by some of the writers to 
whom I have alluded. I have no 
doubt that they are in most in- 
stances correct in their inference 
that great hairiness or wooliness may 
be a protection against sudden 
changes of temperature, or a check 12 
upon too rapid evaporation. Never- 
theless, I cannot find that unusual 
pubescence is generally associated 
with a warm, or a cold, or a dry cli- 
mate, although this last relation is 
the most plausible. It seems to be a 
fact, however, that the hairiness of a 
plant bears some proportion to the 
general sterility of the region in 
which the plant grows. In confirma- 
tion of this I am informed that, in the 
sterile regions of our western territor- 
ies, not only do the more hairy spe- 
cies and genera prevail, but those 
which are hairy elsewhere are especi- 
ally so there. 
Mr. Charles Darwin is plainly of 
the opinion that the saprophytic 
habit of the so-called insectivorous 
land plants is the result of a struggle 
against sterility of soil, but not par- 
ticularly against dryness ; for while 
these plants usually inhabit sandy or 
barren regions, they quite as com- 
monly grow in very wet places. Such 
being the case, it seems altogether 
natural that we should find a general 
increase of pubescence in the plants 
of a desert region ; for what is the 
cause of a most remarkable specializ- 
ation in the case of the insectivorous 
plants, may readily be conceived to 
be the cause of a less remarkable 
modification in the general flora of a 
district. A further fact, strengthen- 
ing this idea, has also been noticed 
by Mr. Darwin, in his work on in- 
sectivorous plants, viz. : that the high 
degree of specialization in the gland- 
ular hairs of these plants is associated 
with smallness of the roots, the latter 
being in most cases barely of sufficient 
size to collect water from the soil. In 
some genera, indeed, as, for instance, 
in Dioncea, the plant does not seem to 
depend upon the soil at all, for this 
plant may be grown epiphytically 
upon moist cotton or sponge. 
It is well known that roots and foli- 
age-branches are homologous parts, so 
that in some plants they may easily 
be made to exchange places and 
functions, the branches when planted 
in the soil running to roots, and the 
roots when turned upwards developing 
into foliage. It is therefore not 
astonishing that, in their natural po- 
sition the roots and their appen- 
dages and the leaves and their 
appendages should maintain a more 
or less steady equilibrium. When, 
for any reason, the plant does not 
find use for the natural root func- 
tions, we should expect it to transfer 
them to the leaf if possible; and 
where the supply of oxygen and ni- 
trogen compounds is cut off at the 
root—as it is in a desert region—it is 
quite in accordance with analogy that 
the leaf organs should be unusually 
developed, for the purpose of in- 
creasing the supply of oxygen or of 
nitrogenous compounds through the 
leaf. This will be particularly the case 
when the assimilative process is not 
active, and when the plant must 
therefore, rely more upon obtaining 
organic compounds ready-made than 
upon making them itself. 
Aside from the question of the abun- 
dance or scarcity of food, the plant’s 
means of appropriating its food-ele- 
ments must be in proportion to the re- 
quirements of growth. That is to say, 
other things being equal, a rapidly 
growing plant, or part of a plant, is 
more likely to be hairy than is a slow- 
ly growing plant or part. On this 
principle, as I have already men- 
tioned, the newest part of a plant, 
whether leaf-bud or flower-bud, is 
usually the most pubescent part. 
It is not easy to account for all the 
differences among genera, as to hairi- 
ness or smoothness, but I have no 
doubt that ultimately it will be ascer- 
tained pretty nearly why each differ- 
ent genus is or is not hairy ; for I do 
not think there can be a doubt that 
every such difference has a reason, 
founded in general laws, in the main 
physiological. 
In the insectivorous plants some of 
the hairs are adapted to a use not 
strictly physiological, namely, the 
alluring and capturing of prey. For 
this reason they have become most 
developed and modified on the upper 13 
instead of the lower side of the leaf. 
It is not unusual, however, to find 
organs of every kind which have be- 
come specialized for peculiar pur- 
poses ; and an instance, somewhat in 
the line of the alluring bait of some 
of the insectivorous plants, is the 
change which a simple gland, such as 
we find on many leaves and petals, is 
supposed to have undergone, in order 
to produce the nectary of the flower. 
Where hairs have not been modi- 
fied for unusual purposes, we can 
see reasons why they should grow 
most abundantly and luxuriantly 
on the under side of the leaf. In 
the first place, the hairs, being organs 
of the metastatic process, naturally 
grow, under ordinary circumstances, 
where that process is best promoted, 
namely, in the shade, where also, for 
the same reason, are the veins with 
which the hairs are connected. But 
aside from this, it seems to me not 
merely a fancy that, as the upper 
side of the leaf is the one where 
the chlorophyll-cells most naturally 
abound, it should be free from any or- 
gans which would cut off the full sup- 
ply of light and heat which is necessary 
to the operation of their functions. 
Moreover, since the stomata open in 
the light, and evaporation is has- 
tened by heat, the under side of the 
leaf seems to be the better place for 
them, for there they are less liable to 
unrestrained action which would be 
disturbing and dangerous to the plant. 
Besides this, we can see that it is an 
economical arrangement that hairs 
and stomata should exist together, 
chiefly for the reason already referred 
to, that the hairs probably help to 
retain moisture and check too rapid 
evaporation. 
After this hasty and necessarily 
imperfect summary of the more gen- 
eral facts known as to external epi- 
dermal organs, I wish now to call 
attention briefly to some facts con- 
cerning analogous internal organs. 
These are particularly the internal 
hairs of some water plants, of the 
genera Nymphcea and Nuphar. 
If we examine with the microscope 
a section of the leaf, or the petiole, of 
a plant from either of these genera, 
we are at once struck by certain 
abundant, thickened, branching, uni- 
cellular structures scattered through 
the parenchyma and projecting into 
the intercellular spaces. These struc- 
tures have long attracted the attention 
of microscopists, but not until recent- 
ly have they been distinctly recog- 
nized as internal hairs. The stan- 
dard text-books of the microscope, 
such as Carpenter’s and the Micro- 
graphic Dictionary, refer rather 
doubtfully to the resemblance be- 
tween these bodies and the stellate 
external hairs of Deutzia and Aly- 
ssum, but they generally shyly avoid 
calling them plainly internal hairs. 
Some writers speak of them as “ stel- 
late parenchymal cells ; ” and on a 
purchased slide which I own, they 
are described as “ stellate raphides.” 
But out of the confusion and uncer- 
tainty which has prevailed with re- 
gard to these structures, there has 
gradually crystallized a clear and 
definite recognition of the fact that 
they really are epidermal organs, ex- 
actly analogous to the external hairs 
of terrestrial plants, such as we have 
been considering. 
In the first place, the mere mor- 
phology of these structures confirms 
the idea of their being truly hairs. 
In their outlines they so closely 
resemble some of the external hairs 
with which we are familiar (for in- 
stance those of the genus Arab's), 
that their analogy is at once suggest- 
ed to the observer. They seem, 
however, to be always unicellular 
growths, and transverse sections 
show them to be hollow. Over their 
surfaces are scattered the grains of 
silex, so characteristic of other hairs, 
and made particularly familiar to us 
by the hairs of the Deutzias. If one 
wishes a striking demonstration of 
the mere resemblance of these inter- 
nal hairs to better known external 
hairs, he has only to split the petiole 
of Nuphar or Nymphcea by tearing, 14 
and view it with the binocular as an 
opaque object. This, of itself, will 
be convincing to most persons ac- 
quainted with the peculiarities of leaf- 
hairs. 
But, aside from configuration and 
other merely morphological consider- 
ations, the mode of distribution of 
these internal hairs is almost precisely 
like that of external hairs. If we 
take a thin section of a leaf of Nym- 
phcea, cut parellel to its surface, and 
examine it as a transparent object 
(still better if we illuminate it with 
polarized light), we shall see that 
these internal hairs, like external 
ones, are associated with the fibro- 
vascular system ; and toward the 
margin of the leaf, where the tissues 
are thin and the veins small, we shall 
find that the hairs exist only upon the 
veins and not between them. This is 
quite in accordance with what we 
know of the habit of all hairs, as I 
have already explained. 
If we take a transverse section of 
the leaf of Nymphcea or Nuphar, cut 
across the mid-rib, we shall be struck 
by another fact connecting these in- 
ternal hairs with the epidermal sys- 
tem. We shall at once notice that from 
the upper side of the leaf—which is, 
under common circumstances, the 
only side exposed to the air, and is 
consequently the only side possessing 
a true epidermis—these hairs spring 
like stumps of trees in an inverted 
field. From end to end of the sec- 
tion they will be observed planted 
close together, their pedicels im- 
bedded in, or forming a part of, the 
epidermal layer, and their branches 
spreading downward and inward 
through the underlying parenchymal 
tissue. This is very marked in 
stained sections, in which the hairs 
take a darker color than the sur- 
rounding tissues ; but in an unstained 
section, polarized light differentiates 
the structures quite as well as the 
elaborate, double-staining process now 
so commonly employed. It is to be 
noted that this arrangement of the 
hairs is confined to the upper or epi- 
dermal side of the leaf, and that no 
similar arrangement is to be seen at 
any other part. The distribution of 
the hairs on the interior of the petiole 
of the water plants is somewhat 
different from their arrangement on 
the interior of the leaf, but it is 
closely analogous to the arrangement 
of hairs upon the exterior of the 
petiole of land-plants. 
I have referred to the fact that 
these internal hairs are good objects 
upon which to use polarized light; 
and this is a point of no small im- 
portance in the argument for their 
being actually hairs. To any one 
acquainted with the behavior of veg- 
etable tissues with the polariscope, 
particularly of vegetable hairs, the 
manner in which these internal struc- 
tures of the Nymphseaceae are affect- 
ed by polarized light is strong con- 
firmation of their claim to be regard- 
ed as epidermal organs and true 
hairs. The way in which they take 
different colors in the process of 
double staining, will be another 
affirmative argument with those fa- 
miliar with that process and its 
effects. Suffice it to say in this connec- 
tion, that these internal hairs behave 
in precisely the way, and take exactly 
the colors, that external hairs do. 
That structures physiologically re- 
ferable to the epidermal system 
should be found growing in the midst 
of the parenchymal system, is not alto- 
gether anomalous, for sections of the 
leaf and petal of Magnolia grandiflora 
reveal an abundance of thickened, 
irregular, unicellular structures scat- 
tered through the parenchyma, which, 
both in their appearance and in their 
mode of distribution, at once suggest 
some sort of similarity to the inter- 
nal hairs of Nyynphcea and Nuphar; 
but which, in my judgement, are parts 
of the glandular system of the Mag- 
nolia. Indeed, any true sunken gland 
may be regarded as an internal epi- 
dermal organ. 
Plants which do not live either en- 
tirely in the water or entirely out of it 
may naturally be expected to occupy, 15 
so far as their organization is con- 
cerned, a position intermediate be- 
tween submerged plants and aerial 
plants. Such is the case with the 
Nymphaeaceae. While not relinquish- 
ing their dependence upon, and their 
connection with, the external atmos- 
phere, they nevertheless provide 
against partial submergence, by an 
increase of their capacity for internal 
interchange of the gases necessary for 
their life and growth. In other words 
the amount of external surface ex- 
posed to the atmosphere being largely 
curtailed, by reason of their partial 
submergence, this loss is compensat- 
ed for by a great increase in the 
amount of internal surface exposed 
to the air and gases contained in 
the intercellular spaces. By this en- 
largement of the intercellular spaces 
the inside of the plant becomes (if I 
may be allowed the paradox) to some 
extent, for physiological purposes 
another outside ; and the practical 
effect is the same as if there were 
less intercellular space, and more sur- 
face exposed to the outer atmos- 
phere. To the same extent as the 
inside becomes practically a part of 
the outside, by reason of its exposure 
to surrounding air and gases, that 
part of the outside which is sub- 
merged becomes practically part of 
the inside, by reason of its exposure 
to the surrounding fluid. 
In plants existing under such pecu- 
liar circumstances, we need not be sur- 
prised to find organs and tissues 
which, in strictly terrestrial plants, are 
external, becoming internal. And so 
there is no a priori reason against the 
existence of internal hairs, or even of a 
whole internal epidermal system, in the 
Nymphaeaceae. But we have no war- 
rant for looking for internal hairs in 
all partially submerged, or wholly 
aquatic, plants, any more than we 
have for expecting to find external 
hiirs upon all terrestrial or aerial 
plants. As a matter of fact, hairs do 
not exist upon many land-plants 
which seem to grow under the same 
circumstances and surroundings as 
others, upon which hairs found ; 
and so, while Nymphcea and Nuphar 
are internally pubescent, Nelubium 
and Bracefiia are internally glabrous. 
It is no easier to account for this dif- 
ference in land-plants than it is in 
water-plants ; but in both cases it is 
doubtless caused by some fundamen- 
tal, physiological difference at present 
unknown. 
That the great enlargement of the 
intercellular spaces in submerged, or 
partly submerged, plants, is for 
the purpose of facilitating the inter- 
nal interchange of gases which, in 
plants growing upon the land, would 
take place externally, is no new 
theory of my own. Sachs, in his 
“Botanical Text-Book,” says :— 
“ A submerged water-plant, for ex- 
ample, which contains chlorophyll, ab- 
sorbs carbon dioxide from without, 
under the influence of sunlight ; and 
at least a portion of the disengaged 
oxygen collects in the cavities. When 
it becomes dark this process ceases ; 
the collected oxygen is now absorbed 
by the fluids of the tissue, and gradu- 
ally transformed into carbon dioxide, 
which can again diffuse back into the 
cavities, but partially also through 
the layers of tissue into the surround- 
ing water.” 
In the light of these facts we can 
perceive how the interior of a water- 
plant may become of more impor- 
tance to it than the exterior, for most 
physiological purposes, and under 
these circumstances it is not strange 
that we find such important organs of 
the metastatic process as the hairs, 
transferred from the exterior to the 
interior, where the amount of surface 
exposed to the interchanging gases is 
many times greater than that exposed 
to the external atmosphere.