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corkscrew shape, and bear upon the finer spirals numerous ex-
tremely active cilia (p. 31), by
which they are driven swiftly
through the water.

The Archegonia^ or female



Fio. 73. (After Hofmeister.) —
Later stage in the development
of an antheridium of Pterix ser-
riilata. p, peripheral cell; c,
central cell from which the
spermatozoid mother - cells
arise.





Fig. 74. (After Luerssen.)— Bursting of
the antheridium and escape of the
spermatozoids. a», antheridium; m.e,
spermatozoid mother-cells; sp, sper-
matozoids.



Fig. 75. (After Strasburger.)— Mature
archegonium, showing the oosphero
(o), the neck (x), and mucus (»») is-
suing from the mouth of the canal.



138



THE BIOLOGY OF A PLANT.



organs (Figs. TO, 75), described for the first time by Siiminski
in 1864, likewise arise from single superficial cells of the pro-
thallium. They are situated almost exclusively upon the cushion
near its anterior or apical extremity, and hence at the bottom of
the anterior depression (Fig. 70). Since they appear later than
the antheridia, they are not likely to be fertilized by spermato-
zoids descended from the same spore. This phenomenon of
maturation of one set of sexual organs of a bisexual individual
before the rij^ening of the other set is a common feature among
plants, and is known as dichogamy. Tliere is reason to believe
that important advantages are gained by thus securing cross-fer-
tilization and preventing self-fertilization or ' ' breeding in and



m



55



In the development of the archegonium the original cell enlarges, be-
comes somewhat dome-shaped, and divides by transverse partitions into
three cells : a proximal, im-
bedded in the tissue of the
prothallium, a middle, and a
distal dome-shaped cell (Fig.
76). The fate of the proximal
cell is unimportant. The dis-
tal cell gives rise by division
to a chimney-like structure,
the neck (Figs. 75, 77), which



B.




A,



Fig. 76.— Diagram to iUustrate
the origin ot an archegonium.
A^ an early stage; B, a later
stage; A, a, the original epi-
dermal cell enlarged ; B, a, the
basal cell; b, the central or
canal cell ; c, the neck-cell.




Fig. 77. (After Strasburger.)— Developing arche-
gonia of Pteris serrulata. A, young stage ; J3,
older ; n, neck ; c, canal ; o, oosphere.



encloses a row of cells (canal-cells) derived from the original middle cell
(Figs. 75, 77). These afterwards become transformed into a mucilaginous
substance filling a canal leading through the neck from the outside to the
oosphere (Fig. 77), which also arises from the original "middle" cell at its



FERTILIZATION AND DEVELOPMENT. 139

proximal end. Theoospbere is the all-important /e/TiaZe germ-cell to which
the " neck-" and "canal-cells" are merely accessory.




.^7^.



Fertilization or Impregnation. Fertilization, or the sexual
act, is perfonned as follows : Sper-
matozoids in vast numbers are at-
tracted to the mouths of the arche-
gonia and there become entangled
in the nmcilage (Fig. 78). In
favorable cases one or more work
their w^ay down the mucilaginous
canal, and at length one penetrates
and fuses with the oosphere.

It is known that one spermatozoid is

enough to fertilize the oosphere, and ^ r,o , k ^. c. -l.

^ ^ ' Fig. 78. (After Strasburger.) —

probably one only penetrates it ; but sev- Mouth of an archegonium of Rc-

eral are often seen in the mucilaginous ri<i sermiata, crowded with sper-

canal. It has been shown that the muci- i^^^tozoids striving to effect an en-

lage contains a small amount (about 0.3^)

of malic acid, which probably acts both as an attraction to the spermato-
zoids and as a stimulus to their movements. Pfeffer has proved that
capillary tubes containing a trace of a malate in solution are as attractive
to the spermatozoids as is the mucilage in the central canal, and phe-
nomena of this kind (chemiotaxts) have recently been shown to be common
and highly important.

The entrance of the spermatozoid into the ovum and its
fusion with it mark an important epoch in the life-history of the
fern. The oosphere is from this instant a new and very difter-
ent thing, viz. , an e?nhri/o, and is known as the oospore. It is
now the first stage of the asexual generation, though it is still
maintained for some time at the expense of the sexual generation
or odpliore (p. 130).

Growth of the Embryo. The oospore, or one-celled embryonic
sporophore (p. 130), now rapidly becomes multicelhihir by di-
viding first into hemispheres, then into quadrants, etc. (Fig. SO ;
compare Fig. 14). The first plane of division is approximately
a prolongation of the long axis of the arcliegonium (Fig. S(")).
The second is nearly at riglit angles to it, so tliat the quadrants
may be descril)ed as anterior and posterior to the first ph\no.
The fate of the quadrant-cells is of special importance. The



140



THE BIOLOGY OF A PLANT.



lower anterior quadrant as it undergoes further division grows
out into i\\Q first root j the upper anterior quadrant in like man-
ner gives rise to the rhizome and \h.Q fii'st leaf. The mass of
cells derived from the two posterior quadrants remains connected
with the prothallium as an organ for the absorption of nutri-
ment from the latter, and is inappropriately called iYiQfoot.



ewu



A. '





Fig. 79. Fig. 80.

Fig. 79. (After Hofmeister.)— Development of the embryo. A^ section showing the
closed neck (n) and the planes of quadrant division of the oospore or embryo {cm).
The fore end of the prothallium is to the right. B and C, stages of the embryo
later than A^ showing the beginnings of apical growth ; /, foot ; Z, leaf ; r, root ;
r?j, rhizome.

Fig. 80. (From Luerssen, after Kienitz-Gerloff.)— Development of the embryo of
Pteris serrulata. The figures are optical sections taken vertically in the antero-
posterior axis of the prothallium, passing through the long axis of the neck of
the archegonium : except C and D, which are taken at right angles to the others.
A, a, and p are the anterior and posterior segments of the oospore after this has
divided into hemispheres. The former (a) forms the stem, the latter (p) the root.
F shows in a late stage the division of the quadrants, r going to form the root, s
the stem or rhizome, I the leaf, and / the foot ; 1\ l, and s soon take on apical
growth as indicated in H and I.

In Pteris serrulata the development is slightly different. The lower
anterior cell becomes the first leaf ; the upper anterior becomes the first
portion of the rhizome, the lower posterior becomes the primary root, and
the upper posterior remains as the 'â– 'â– foot.''''

The several parts now enter upon rapid growth accompanied
by continued cell -multiplication, until a stage is reached repre-



GROWTH AND DIFFERENTIATION.



141



sented in (7, Fig. 79. A stage somewhat later than this, ^vith
its attacliment to the prothallium, is sliown in Fig. 81. After
tliis the leaf grows upwards into the air, the root downwards
into the earth, and the young fern begins to shift for itself.
Eventually it reaches a condition shown in Figs. 82 and 83.
The prothalHuni remains connected
wdth the young fern for some time,
and may readily be found in this
condition attached to ilower-j)ots in
hot-houses, etc. But sooner or
later it falls off, and the young fern
enters upon an entirely independent
existence. The appearance of the
plant and the shape of the leaf do
not always at first resemble those
of the adult fern; growth is also
more rapid at first, several leaves
(7-12) being developed successively in the first year (p. 112).
Differentiation of the Tissues. In the earliest stashes the tissue
is nearly or quite homogeneous, i.e., meristemic. But very
early in development, as tlie leaf turns upwards and the root




tt



Fig. 81. (After Hofmeister.)— Young
embryo of Pteris aquiU)ia, showing
its attachment to the prothallium
by the foot ; I, leaf ; /, foot ; i\ flrsi
root.




Fig. 83. (After Sachs.)— Older embryo of maidenhair-fern (AiJiautum) attached to
the prothallium. Seen in section. Z, leaf; r, first root; rh, beginning of the
rhizome ; p, prothallium ; rz, rhizoids ; ar, archegonia.

downwards, changes take place, which lead directly to a differ-
entiation into tlie three great systems of tissue — epidermal, fibro-
vascular, and fundamental. Tlie epidermal and fundamental
systems take on almost at once the pecuharities which have al-



142



THE BIOLOGY OF A PLANT.



ready been noted in the adult, p. IIT. The fibro-vascular system
of tissues is differentiated a little later. Different as the tissues
of the three systems are, it is plain from their mode of origin
that all are fundamentally of the same nature because of their
descent from the same ancestral cell; hence every cell in the
plant partakes more or less completely of the nature of every
other cell. The resemblances are primary and fundamental, the

differences secondary and derived.
And what is true of the fern in this
respect is equally true of all other
many-celled organisms.

Course of the Fibro-vascular Bundles.

Certain features of the disposition and
course of the fibro-vascular bundles in the
embryo and in the adult may conveniently
be studied at this point. From the point
of junction of the bundles of the first leaf
and first root (Figs. 79, 81, 82) is developed
one central bundle traversing the young
rhizome and sending branches into the new
leaves and roots until 7-9 leaves have been
formed. After this time the rhizome
forks, and the course of the fibro-vascular
bundles in each fork is henceforwards com-
Ftg. 83. (After Sachs.)— Young pQ^^d. A lateral depression appears in
maidenhair-fern (Adiantum) at- \ ^

tachedtotheprothallinm,p. ?, the central bundle of each stem, rapidly

leaf; 1, 2, the first and second increases in depth, and soon divides the
^^^^^- bundle into two, one upper and one lower,

which are best recognized in old specimens (Fig. 48). When the forked
shoots have reached a length of about three inches, these bundles send out
at a small angle towards the periphery thinner, forked branches which
soon unite again to form a network near the epidermis. The uppermost
of these branches, which passes in the median line above the axile bundles,
is usually somewhat more fully developed, and almost as broad as the lat-
ter. This structure is generally retained in the mature rhizome (Fig.
48, x). The number of peripheral bundles may be as great as twelve in the
cross-section. They anastomose in the vicinity of the place of insertion of
each frond, and thus form a hollow, cylindrical network, having elongated
meshes ; but no connecting branches between them and the two axile
bundles are found anywhere in the rhizome. The latter follow an en-
tirely isolated course within the creeping stem ; * branches from them




* See, however, De Bary, Comp. Anat. Phanerogams and Ferns, p. 295.
Oxford, 1884.



EXCEPTIONAL MODES OF DEVELOPMENT. 143

enter the leaves, and it is only inside the leaf -stalk that these ramifications
are met by branches from the peripheral network. The bundles of the
roots arise only from the peripheral bundles, but those of leaves, as already
said, receive branches from both axillary and peripheral bundles. Two
thick brown plates {sclerotic prosenchyma) lie between the inner and
outer systems of bundles, and are only separated from one another at the
sides by a narrow band of parenchyma. They are often joined on one side
or even on both, in the latter case forming a tube which separates the
two systems of bundles. (Hofmeister.)

Apogamy. Apospory. In rare cases, e.g., in Pteris cretica^ the ordi-
nary alternation of generations in the life-cycle of ferns is abbreviated by
the omission of the sexual process, and the immediate vegetative outgrowth
of the sporophore from the prothallium (apogamy). In other cases there
is an omission of the spore stage, and immediate vegetative development
of the oophore from the frond {apospory). (cf. Farlow, Quart. Journ.
Mic. Science, 1874; De Bary, Botan. Zeitimg, 1878; Druery, etc., Journ,
Royal Mic. Soc, 1885, pp. 99 and 491.)



CHAPTEK X.

THE BIOLOGY OF A PLANT {CoJitinued),
The Physiology of the Fern.

The brake, like the eartliworm, is a limited portion of organ-
ized matter occupying a definite position in space and time. It
is bounded on all sides by material j^articles, some of wliich may
be living, but most of which are lifeless. The aerial portion is
immersed in and pressed upon by an invisible fluid, the atmos-
phere, while the underground portion is sunk in a denser
medium, the earth, which likewise acts upon it. At the same
time the fern reacts upon the air and the earth, maintaining
during its life an equilibrium which is disturbed and finally gives
way as the life of the plant draws to a close.

The Fern and its Environment. Those portions of space,
earth, and air which are nearest to the brake constitute its imme-
diate environment. But in a wider and truer sense the environ-
ment includes the whole universe outside the plant. To perceive
the truth of this it is only necessary to obseiwe how profoundly
and directly the plant is affected by rays of liglit which travel to
it from the sun over a distance of many millions of miles, or
how extremely sensitive it is to the alternations of day and night
or of summer and winter. The plant is fitted to make certain
exchanges with its environment, drawing from it certain forms
of matter and energy, and returning to it matter and energy in
other forms. Its whole life is an unconscious struggle to wrest
from the environment the means of subsistence ; death and decay
mark its final and unconditional surrender.

Adaptation of the Organism to its Environment. We can dis-
tinguish in Pteris as clearly as in Liimhincus the adaptation of
the organism to its environment. The aerial part of Pteris
must be fitted to make exchanges with, and maintain its life in,
the atmosphere, while the underground part must be similarly
^ ' adapted ' ' to the soil in wliich it lives.

144



ADAPTATION TO TRE ENVIRONMENT. 145

The aerial part displays admirable adaptation in its stalk, w liich
rises to a point of vantage for procuring air and light, and in its
broadly spreading top, which is covered by a skin, tough and
impervious, to prevent undue evaporation and consequent desic-
cation, yet translucent, to allow the sun's rays to reacli the
starch-making tissue within. The rhizome also, witli its pointed
terminal buds, its elongated roots, armed with Ijoring tips, and
its thick, Heshy parenchyma for the storage of food, is admirably
adapted to its own special surroundings. In order to realize
this, we have only to imagine the fern to be inverted, the aerial
portion being planted in the earth, and the underground portion
lifted into the air and exposed to the winds and sunsliine. Under
these circumstances the want of adaptation of tlie parts to their
respective environments would speedily become apparent.

Yet different as these parts now are, tliey have originally
sprung from the same cell. More recently they were barely dis-
tinguishable in a mass of tissue, part of which turned upwards
into the air, while another part turned downwards into the earth.
But as development went on, the aerial and underground parts
were progressively diiferentiated, thus becoming more and more
perfectly adapted to the peculiar conditions by which each is
surrounded.

Thus it appears that the harmony between every part of the
plant and its environment is brought about, as in the animal, by a
gradual process in the history of each individual. We can here
clearly see also the functional adaptation of the ])lant to chang-
ing external conditions. The environment of Pteris changes
periodically with the regular alternation of summer and winter,
and the plant also undergoes a corresponding periodic change of
structure in order to maintain its adaptation to the environment.
During the summer the aerial part is fully developed, and, as a
result of its activity, starch is accumulated in the rhizome. At
the approach of winter the aerial part dies, and the plant is re-
duced to the underground part safely buried in the soil. During
the winter and spring the starch is gradually consumed, and the
aerial part is put forth again as the aerial environment becomes
once more favorable to it. The plant, therefore, like the animal,
possesses a certain plasticity which enables it to adapt itself to
gradually changing conditions of the enviromnent.



146 THE BIOLOGY OF A PLANT.

A little consideration will show that every function or action of living
things may be regarded as contributing to the same great end, viz., har-
mony with the environment ; and from this point of view life itself has
been defined as ' ' the continuous adjustment of internal relations to ex-
ternal relations^ *

Nutrition. The fern does work. In pushing its stem
through the soil, in lifting its leaves into the air, in moving
food-matters from point to point, in building new tissue, in the
process of reproduction, and in all other forms of vital action,
the plant expends energy. Here, as in the annual, the imme-
diate source of energy is the living protoplasm, which, as it
lives, breaks down into simpler compounds. Hence the need of
an income to supply the power of doing work.

The Income. The income of the fern, like that of the earth-
worm, is of two kinds, viz. , matter and energy, but unlike that
of the worm it is not chiefly an income of foods ^ hut only of the
raw 'materials of food. Matter enters the plant in the liquid or
gaseous form by diffusion^ both from the soil through the roots
(liquids), and from the atmosj)here through the leaves (gases).
We have here the direct absorption into the body j)roj)er of food-
stuffs precisely as the earthworm takes in water and oxygen.
Energy enters the plant, to a small extent, as the potential energy
of food-stuffs, but comes in principally as the kinetic energy of
sunlight absorbed in the leaves. The table on p. 147 shows the
precise nature and the more important sources of the income.

Of the substances, the solids (salts, etc.) must be dissolved
in water before they can be taken in. Water and dissolved salts
continually pass by diffusion from the soil into the roots, where
together they constitute the sap. The sap travels throughout
the whole plant, the main though not the only cause of move-
ment being the constant transpiration (evaporation) of watery
vapor from the leaves, especially through the stomata. The
gaseous matters (carbon dioxide, oxygen, nitrogen) enter the
plant mainly by diffusion from the atmosphere, are dissolved by
the sap in the leaves and elsewhere, and thus may pass to every
portion of the plant.

The Manufacture of Foods — especially Starch. Pteris owes
its power of absorbing the energy of sunlight to the chlorophyll-

* Spencer, Principles of Biology, vol. i. p. 80. N. Y., Appleton, 1881.



INCOME OF THE PLANT.



147



hGclies or (Jiromatophores j for plants wliicli, like fungi, etc., are
devoid of chlorophyll are unable thus to acquire enei'gy. Enter-
ing the chlorophyll-bodies, the kinetic energy of sunlight is ap-
plied to the decomposition of carbon dioxide (COJ and water
(H^O). Alter passing through manifold but imperfectly known
processes, the elements of these substances linally reappear as
starch (CjIIj^OJ often m the form of granules imbedded in the
chloro^^hy 11 -bodies, and free oxygen, most of which is returned



INCOME OF PTERIS.



Matter.


Whence Derived.


Carhon.


Mainly from the atmosphere as carbon dioxide (COq), but per-
haps partly from, dissolved organic matters (food).


Hydrogen.


Mainly from the soil as water (H2O), but perhaps partly from
organic foods.


Oxygen.


Mainly from the soil as water (H2O) and frona the air as free
oxygen.


Nitrogen.


Mainly from the soil * as nitrates or ammonium compounds, or
organic foods.


Sxdphur.


Mainly from the soil as sulphates.


Other elements.


Mainly from the soil as various salts.


Energy.




Kinetic.


Mainly from the sunlight through the leaves.


Potential.


Perhaps to a limited extent in food materials via the roots.



to the atmosphere. Thus the leaf of Pteris in the light is con-
tinually absorbing carbon dioxide and giving forth free oxygen.

Carbon dioxide and water contain no potential energy, since
the affinities of their constituent elements are completely sat-
isfied. Starch, however, contains potential energy, since the
molecule is relatively unstable, i.e., capable of decomposition
into simpler, stabler molecules in which stronger affinities are

* It lias been generally believed that plants are unable to make use of free
atmospheric nitrogen, but recent investigations have disproved this view for
certain species.



148 THE BIOLOGY OF A PLANT.

satisfied. And this is due to tlie fact that in the manufacture
of starch in the chlorophyll-bodies the kinetic energy of sunlight
a was expended in lifting the atoms into position of vantage,'
thus endowing them with energy of position. In this way some
of the radiant and kinetic energy of the sun comes to be stored
up as potential energy in the starch. In short, Pteris^ like all
green plants, is able by co-operation with sunlight to use sunple
raw materials (carbon dioxide, water, oxygen, etc.) poor in en-
ergy or devoid of it, and out of them to manufacture food ^ i.e.,
complex compounds rich in available potential energy. We
shall see hereafter that this power is possessed by green plants
alone ; all other organisms being de23endent for energy upon the
potential energy of ready-made food. This must in the first
instance be provided for them by green plants ; and hence with-
out chlorophyll-l)earing plants animals (and colorless plants as
well) apparently could not long exist.

The plant absorbs also a small amount of kinetic energy, in-
dependently of the sunlight, in the form of heat ; this, however,
is probably not a source of vital energy, but only contributes to
the maintenance of the body temperature.

Circulation of Foods. It is chiefly in the green (chloroj^hyll-
bearing) parts of the plants, and in the presence of sunlight, that
food-manufacture goes on. Somehow, then, the water absorbed
by the roots must be transported to the leaves, and the starch
made in the leaves must be conveyed to the subterranean tissues.
Exactly how these transfers of material are effected is uncertain,
but there is reason to believe that they take place mainly by the
slow processes of diffusion. It is certain that no distinct organs
of circulation or distribution, such as the blood-vessels of the
earthworm, exist in the fern.

Metabolism. Starch, as has just been seen, is first formed in
the chlorophyll-bodies. But the formation of starch, all-imj)or-
tant as it is, is after all only the manufacture of food as a j^re-
liminary to the real processes of nutrition. These processes must
take place everywhere in ordinary protoplasm; for it is here
that oxidations occur and the need for a renewal of matter and
energy consequently arises (cf. pp. 32 and 33). Sooner or later
the starch grains are changed into a kind of sugar {glucose,
CgHj^Og), which, unlike starch, dissolves in the sap, and may



OUTGO OF THE PLANT. 149

tlms be easily transported to all parts of the plant. Wherever
there is need for new protoplasm, whether to repair previous
waste or to supply materials for growth, after absorption into
the cells the elements of the starch (or glucose) are, by the liv-
ing protoplasm, in some unknown way combined with nitrogen
and sulphur (probably also with salts, water, etc.), to form proteid
matter. The particles of this newly-formed coni])ound are incor-
porated into the protoplasm (by " intus-susception," p. 4) and, in
some way at present shrouded in mystery, are endowed w4th the
properties of life. We do not know how long they may remain
in the living state, but sooner or later they are oxidized, and, as a
result of tJie oxidation, that energy is set free which enables the
fern to do work and prolong its existence. The oxidized prod-
ucts are afterwards eliminated (excreted) from the cells.

If a larger quantity of starch is formed in the chlorophyll
bodies than is immediately needed by the protoplasm for pur-
poses of repair or growth, it may be re-converted into starch
after journeying as glucose through the plant, and be laid down
as ' ' reserve starch * ' in the parenchyma of the rhizome, or else-
where. Apparently, when this reserve supply is iinally needed