An introduction to general biology online

at any point in the plant, it is again changed to glucose and trans-
ported thither. It is 'probable that new leaves and new tissues
generally, are always formed in part from this reserve starch,
and not solely from newly-formed starch.

In dealing with the metabolism of the fern we may safely
assume, as we have done already for the earthworm, a constructive
phase {andbolisiii) and a destructive phase {hatabolism) ; but
these terras represent merely probable events, not known facts.

The Outgo. The outgo, like the income, is of two kinds,
matter and energy, but it cannot be so readily tal)ulated.

The plant suffers annually a great loss both of matter and of
potential energy in the production of spores and in the autumnal
dying-down of the fronds. But matter also leaves the plant
daily as carbon dioxide (in small quantities), water, and oxygen,
both by diffusion through the epidermis and by transpiration
through the stomata. Strictly speaking, the term outgo should
be restricted to the output of matter which has at some time
actually formed a part of the living protoplasm ; hence it does
not apply to the oxygen, which is sinij^ly given off in the manu-



150



THE BIOLOGY OF A PLANT.



facture of starch, or to the bulk of the water of evaporation,
which passes straight through the plant without undergoing any
chemical change. Energy likewise leaves the plant continuously
both as heat and in the doing of mechanical worh^ both of which
are involved in every vital act.

Respiration. It has been remarked that ui the light (i.e.,
when manufacturing starch) Pteris takes in carbon dioxide and
gives off free oxygen. But if the plant be dejDrived of light, as
at night, the reverse is true, and the plant takes in a small
amount of oxygen and gives ofl" a corresponding amount of car-
bon dioxide. This latter process is the true hreathing or resj>i-



PTERIS AQUILINA.


(Balance-Sheet of Nutrition.)


Income.


^

Outgo.


Matter.


Matter.


Foods,


Carbon dioxide.


Inorganic salts.


W^ater,


Carbon dioxide.


Excreted substances.


Water,


Reproductive germs,


Free oxygen.


Leaves, etc..




Free oxygen — from decomposition




of carbon dioxide in light.


Energy.


Energy.


Sunlight absorbed by chlorophyll.


AVork performed.


Potential energy in foods.


Heat.




Potential energy in cast-oflE matters,




reproductive germs, etc.


Balance in favor of the living Pteris :


Matter.


Tissues, protoplasm, starch, cellulose, chlorophyll, etc.


Energy.


Potential energy in organic matters.



ration of the plant, and it must not be confounded with that
taking in of carbon dioxide and gi\dng off of oxygen which is an
incident in the manufacture of starch. Resj^iration goes on in
the light also, probably with greater energy than in darkness,
but it is then largely obscured by the other and more conspicu-
ous process. We have seen that energy is set free in living mat-
ter by a decomposition of its own substance, which is really a
process of oxidation or combustion, where free oxygen plays
an important part (p. 32, Chap. III.) ; hence the absorption of
free oxygen in respiration. Among the products of the combus-
tion, water and carbon dioxide are the most im^^ortant ; and this



ACTION UPON THE ENVIRONMENT. 151

is the origin of tlie carbon dioxide given oft". It will appear
beyond that precisely the same action takes place in the respi-
ration of animals, and that all living things breathe or respire in
essentially the same way.

It was for a long time believed that a leading difference between plants
and animals lay in the fact that the former give off oxygen and absorb
carbon dioxide, while the latter give off carbon dioxide and absorb oxygen.
But it is now known that both give off carbon dioxide and both require
oxygen, and that only the chlorophyll-bearing parts of green plants are en-
dowed with the special function of decomposing carbon dioxide and water
and manufacturing starch — as a result of which they do (but in tlie light
only) give off oxygen as a kind of incidental- or by-product.

Interaction of the Fern and its Environment.

The actions of the environment upon the fern have already
been sufticiently dwelt upon (p. 144). It still remains, however,
to consider the actions of the fern upon the environment.
These are partly physical, but mainly chemical. By pushing
its fronds into the air and slowly thrusting its rhizome, roots, and
branches through the soil, the atmosphere and the earth are alike
displaced. But it is by its chemical activity that it most pro-
foundly affects its environment. Absorbing from the latter
water, salts, carbon dioxide, and other simple substances, as well
as sunlight, it produces with them a remarkable metamorphosis.
It manufactures from them as raw materials organic matter in
the shape of starch, fats, and even proteids. These it gives
back to the environment in some measure during life, and sur-
renders wholly after sudden death. But the most striking fact
is that the fern is on the whole constnictive and capable of pro-
ducing and accumulating compounds rich in energy. In this
respect it is unlike the earthworm (p. lO-t) and is typical of green
plants in general. Thus, wdiile animals are destroyers of ener-
gized compounds, green plants are producers of them. Ani-
mals, therefore, in the long run are absolutely dependent on
plants ; and animals and colorless plants alike upon green plants.
But it must never be forgotten that most plants are enabled to
manufacture organic from inorganic matter by virtue of the
chlorophyll which they contain. Without this they are j^ower-
less in this respect. (See, however, p. 11>T).



152 THE BIOLOGY OF A PLANT.

Physiology of the Tissue- Systems. The ej)idermal tissues
serve as the sole medium of exchange betweeu the mner parts of
the plant and the envnronment ; they are also protective, and in
certain regions are useful for suj^port. The function of repro-
duction also falls upon these tissues, as is shown by the develop-
ment of the sporangia, antheridia, and archegonia.

Tlie Jihro-vascidar tissues serve in part as a supporting
skeleton, for which function their richness in prosencliyma
and their Urm continuity admirably adapt them. An equally
important function, however, is their conductivity., since they
serve for the transportation of the water for evaporation by the
leaf {transpiration)^ and for the movement (through the sieve-
tubes) of the undissolved and indiffusible proteids. T\\q funda-
mental tissues are devoted either to sharing the sjDccial duties
of the other systems, as in the case of the sclerotic j^arenchyma
abutting upon tlie epidermal tissue in the rhizome (p. 119), and
the sclerotic prosenchyma which appears to behave like the fibro-
vascular tissues ; or to nutritive and metabolic functions, as in
the mesophyll (p. 126) and the parenchyma of the rhizojne.

The Physiology of Reproduction. It is not known whether the
brake ever dies of old age. Barring accidents, growth at the
apical buds seems to be unlimited, keeping pace with death of
the hinder parts of the rhizome (p. 111). But whether the indi-
vidual dies or not, ample provision against the death of the race
is made in the act of reproduction. Although reproduction ap-
pears to be useless to the individual, and even entails u^^on it
serious annual losses of matter and energy, yet to tliis function
every part of the plant directly or indirectly contributes. The
reproductive germs are carefully prepared; are provided with a
stock of food sufficient for the earliest stages of development ;
and are endowed w4th the peculiar powers and limitations of
Pteris aquilina.^ which influence their life-history at every step
and are by them transmitted in turn to their descendants. They
are living portions of the parent detached for rej)roductive ^^ur-
poses; they contain a share of protoj^lasm directly descended
from the original protoplasm of the spore from which the parent
came ; and thus they serve to efl" ect that ' ' continuity of the
germ-plasm ' ' to which we have already referred in dealing
with the earthworm. In short, reproduction is the supreme



PLANT AND ANIMAL COMPARED. lo3

function of the plant. If we may paraplirase the words of
Michael Foster, the oosphere is the goal of individual existence,
and life is a cycle, beginning with the oosphere and continually
coming round to it again.

Comparison of the Fern and the Earthworm. To the super-
ficial observer the fern and earthworm seem to have little or
nothing in common, except that both are what we call alive. But
whoever has studied the preceding pages must have perceived
beneath manifold differences of detail a fundamental likeness
between the plant and animal, not oidy in the substantial iden-
tity of the living matter in the two but also in the construction
of their bodies and in the processes by which they come into
existence. Each arises from a single cell which is the result of
the union of two differently-constituted cells, male and female.
In both the primary cell multiplies and forms a mass of cells, at
iirst nearly similar but afterwards dift'erentiated in various di-
rections to enable them to perform different functions, i.e., to
effect a physiological division of labor. In both, the tissues thus
provided are associated more or less closely into distinct organs
and systems, among which the various operations of the body
are distributed. And in both the ultimate o^oal of individual
existence is the production of germ-cells which form the start-
ing-point of new and similar cycles.

This fundamental likeness extends also to most of the actions
(physiology) of the two organisms. Both possess the power of
adapting themselves to the environments in which they live.
Both take in various forms of matter and energy from the en-
vironment, build them up into their own living sul)stance, and
finally break down this substance more or less completely into
simpler compounds by processes of internal combustion, setting
free by this action the energy which maintains their vital ac-
tivity. And, sooner or later, both give back to the environment
the matter and energy which they have taken from it. In other
words, both effect an exchanti^e of matter and of enertj^v with
the environment.

Nevertheless the plant and the animal differ. They differ
widely in form, and the plant is fixed and relatively rigid, while
the animal is flexible and mobile. The body of the plant is
relatively solid; that of the animal contains numerous cavities.



154 THE BIOLOGY OF A PLANT.

The plant absorbs matter directly tlirougli tlie external surface ;
the animal partly throngh the external and partly through an
internal (alimentary) surface. The plant is able to absorb simple
chemical compounds from the air and earth, and kinetic energy
from sunlight; the animal absorbs, for the most part, complex
chemical compounds and makes no nutritive use of the sun's
kinetic energy. By the aid of this energy the plant manufac-
tures starch from simple compounds, carbon dioxide, and water ;
the animal lacks this power. The j^lant can build up proteids
from the nitrogenous and other compounds of its food ; the animal
absolutely requires proteids in its food. And by manufacturing
proteids within its living substance, the plant is relieved of the
necessity of carrying on a process of digestion in order to render
them diffusible for entrance into the body.

Still, great as these differences appear to be at first sight,
all of them, with a single exception, fade away upon closer ex-
amination. This exception is the power of wiaking foods.
Plants and animals differ in form because their mode of life
differs ; but a wider study of biology reveals the existence of in-
numerable animals (corals, sponges, hydroids, etc.) which have
a close superficial resemblance to plants, and of many plants
which resemble animals, not only in form, but also in possessing
the power of active locomotion. The stomach of the worm, as
shown by its development, is really a part of the general outer
surface which is folded into the body ; and the animal, like the
plant, therefore, really absorbs its income over its whole surface
— oxygen through the general outer surface, other food-matters
through the infolded alimentary surface.

In like manner it is easy to show^ that not one of the differ-
ences between the plant and animal is fundamentally impor-
tant save the jpower of inaMiig foods. The worm must have
complex ready-made food including proteid matter. So must
the fern ; but the fern is able to mamfacture this complex food
out of very simple compounds. In terms of energy, the worm
requires ready-made food rich in potential energy; the fern,
aided by the sun's energy, can manufacture food from matters
devoid of energy.

Hence it appears, broadly speaking, that the fern by the aid
of solar energy is constructive, and stores up energy ; the earth-



FOOD OF PLANTS AND ANIMALS. 155

worm is destructive, and dissipates energy. And this difference
becomes of immense importance in view of the fact that the
fern is typical in this respect of all green plants, as the earth-
worm is typical of all animals.

It will hereafter api)ear that even this difference, great as it
is, is partly bridged over by colorless plants like yeast, nionlds,
bacteria, etc., which have no chlorophyll, are therefore unable
to use the energy of light, and hence must have energized fo(>d.
But these organisms do not, like animals, require proteid food,
being able to extract all needful energy from the simpler fats,
carbohydrates, and even from certain salts. When we consider
that the distinctive peculiarities of animals can thus be reduces I
to the sole characteristic of dependence on proteid food, we can-
not doubt that the differences between plants and animals are of
immeasurably less importance than their fundamental likeness.



It has been the object of the foregoing chapters to give the
student a general conception of organisms, whether vegetal or
animal ; of their structure, growth, and mode of action ; of their
position in the world of matter and energy, and of their relations
to lifeless things. With this preliminary knowledge as a basis,
the student is prepared to take up the progressive study of other
organisms, selected as convenient types or examples. It is con-
venient to begin with low and simple forms of life and work
gradually upwards; and it is especially desirable to do so be
cause there is reason to believe that this course corresponds
broadly with the path of actual evolution.



CHAPTER XL

THE UNICELLULAR ORGANISMS.

It lias been shown in the foregoing pages that the complex
body of an adult fern or earthworm, or of any of the higher
forms of life, originates from a single cell of microscopic size.
This cell — the fertilized ovum or oosphcre — gives rise by divi-
sion to new cells which in their turn divide, generation after
generation, until a full-grown hody is formed, composed of
myriads of cells. But the process of cell-division does not in
this case go as far as complete o,^- separation^ and the cells do
not acquire a complete individuality. They do, it is true, ac-
quire a certain independence of structure and function; and
their individual characteristics may even dej)art widely from
those of neighboring cells (differentiation). Nevertheless they
remain closely united by either material or physiological bonds to
form one body. The body is not, however, to be regarded as
merely an assemblage of independent individual cells. The hody
is the individual ^ its more or less perfect division into cells is
only a basis for the physiological division of labor; of which
cell-differentiation is the outward exjDression.

All this is true, howe^'er, only in the higher types. At the
bottom of the scale of life there is a vast multitude of forms in
which the body consists, not of many cells but of only one, and is
therefore comparable in structure not to the adult fern or earth-
worm, but to the germ-cells from which these arise. Such forms
are known as unicellular organisms, in contradistinction to the
Tnulticellular. Like other cells the unicellular organisms multi-
ply by division, but division is followed sooner or later by com-
plete separation ; the daughter-cells become entirely distinct and
independent individuals, and do not remain permanently asso-
ciated. In them a true multicellular body, therefore, is never
formed ; the cell is the individual^ and the hody is unicelhdar.

156



THE UNICELLULAR BODY. 157

Nevertheless the one-celled organism perforins all of the
characteristic operations of life. A single mass of protoplasm, a
single cell, unites in itself the performance of all the various
elementary functions which in the multicellular forms are distrib-
uted among many cells, differentiated into divers tissues and
organs. The unicellular forms are therefore in a physiological
sense as truly ' ' organisms ' ' as the multicellular forms ; and in
many cases the unicellular body shows a very considerable degree
of differentiation among its parts. But the unicellular forms
are organisms reduced to their lowest terms ; they present us with
the problems of life in their most rudimentary form. Hence
they may afford a kind of key to the more elaborate organization
of the higher types.

We shall find among unicellular forms representatives both
of animals and of plants, and to a detailed examination of some
of these we may now proceed.



//



CHAPTER XII.

UNICELLULAR ANIMALS {Protozoa).

A. Amoeba.

(Tlie Proteus Animalcule.)

General Account. Amoeba is a minute organism occasionally
found in stagnant water, in the sediment at the bottom of ponds
and ditches, on the surface of water-plants, in damp earth, in
organic infusions of various kinds — almost anywhere, in short, in
the presence of moisture, organic matter, and other favorable
conditions. There are many species of Amoeba, some living in
salt water, others in fresh. One of the largest and commonest
fresh-water forms is A7noeba Proteus, which forms the subject
of this account.*

Am^oeba occurs in an active or m^otile state, and a quiescent or
encysted state. When active the body consists (Fig. 84) of a
minute naked mass of protoplasm which in the case of large
specimens is barely visible to the naked eye — i.e., half a milli-
metre ("5^0 inch) or less in length. This mass creeps, or rather
flows, actively about by the continual protrusion of lobes or proc-
esses of its own substance, known as jpseudopodia. These may
be put forth from any part of the surface and again merged into
the general mass; the body therefore continually changes its
shape, and hence the name ' ' Proteus. ' '

When the body is well extended the protoplasm is seen to
consist of a clear peripheral substance, the ectoplasm, and a cen-
tral substance, the entoplasm, filled with coarse granules which
give the body a highly characteristic granular appearance some-
times described as a ''gray color." Within the ectoplasm the
more fluid entoplasm freely flows, as if confined in a tube or

* Other common forms are the smaller A. radiosa and A. verrucosa. The
large A. {Pelomyxa) mllosa and A {Dinamceba) mirabilis are not infrequent.
See Leidy, Fresh-water Rhizopods of North America.

158



THE PROTEUS ANIMALCULE.



159



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V:;:?vi''2.6;i^r:.V<j:rr-f







A




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• "■•



.•—•».,



v.







.«»'■



V — , — '.



rvri/- f.v i



^ — •'






^.-s



,,•



yf



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Fig. H.—Amccba Proteus, from life X 300, The arrows indicate the direction of the
protoplasmic currents; ?», nucleus; c.v, contractile vacuole; f.v, food-vacuole ;
w.v, water-vacuole. A shows the texture of the protoplasm. B is an outline of
the same individual four minutes later ; the upward currents at the right of Fig.
A have stopped, reversed, and the main flow is now towards the left.



160 UNICELLULAR ANIMALS.

sac, but tlie two substances are not separated hj any definite
boundary-line, and pass imperceptibly into one another. The
external boundary of the body is formed by the outermost limit
of the ectoplasm. There is no membrane, and the body is
quite naked. ^N^evertheless the protoplasmic mass shows no
tendency to mix with the surrounding water, and perfectly main-
tains its integrity ; it is an individual.

The formation of a pseudopod begins by the bulging out of
the ectoplasm to form a rounded prominence at some point on
the surface. Into its interior a sudden gush of entoplasm then
takes place and a steady outward stream ensues, the entoplasm
pushing the ectoplasm before it, and the substance of the body
flowing into the pseudopod. The whole substance of the body
may thus flow onward into the pseudopod, which meanwhile forms
new pseudopods, and so the entire animal advances in the direction
of the flow ; or, the pseudopod after attaining a certain size may
be withdrav\Ti into the body by reverse (centripetal) currents, the
main body having meanwhile flowed onward in another direction.

As a rule, the new pseudopodia are put forth near one end
of the body (hence called " anterior "), and the general direction
of advance is therefore fairly constant, not vague and indefinite,
as is often stated. The direction of flow fluctuates, however,
about a certain mean, being continually diverted this way or
that by the formation of new pseudopodia. Those which do not
form directly in the line of march either merge little by little
with the advancing ones, or are withdrawn by reversed currents
into the body. In the latter case they often leave shrivelled
wart-like remnants, and a group of similar warts is usually
found near the "j)osterior" end of the body (Fig. 84, j^)-
Definite changes in the general du'ection of advance are effected
by the diversion of the main current into lateral pseudopodia.

Amoeba feeds upon minute plants and animals or other or-
ganic particles. There is no mouth, and food -matters are bodily
ingulfed (at no definite point) by the proto]3lasm which closes
up beyond them.* The indigestible remains are passed out in

* This mode of cellular alimentation is of frequent occurrence in some cells
of multicellular, as well as in unicellular, animals. Cells exhibiting it are
known as phagocytes (eating-cells), and the process is referred to as phagocytosis.
It is obviously only a prelude to intra-cellular digestion.



ENCYSTED STATE OF AMCEBA.



161



an equally primitive fashion, nsually at some point near the
' ' posterior ' ' end. Besides solid food-stuffs uhnaiha takes in a
certain quantity of water (along with minute quantities of inor-
ganic salts dissolved in it), and it also breathes, by taking in
(mainly by ditfusion) tlie free oxygen dissolved in the water and
giving off carbon dioxide.

Such is Ammha in its active phase. The quiescent or en-
cysted state is entered upon under conditions not thoroughly
understood, but probably of an unfavorable nature, such as the











V -^o; -St .•.■••• .^rr^si




Fig. 85.— vl, Amccba dividing by fission, nucleus not seen (after Leidy). C, Amccha
after a full meal consisting of a large diatom (dt). (After Leidy). Letters as in
Fig. 84. D, Encysted Amoeba, containing food-matters (after Howes).

lack of food, drying up of ponds, and the like. The pseudo-
podia are withdrawn, movement ceases, the body becomes
spherical and surrounds itself with a tough membrane (cell- wall)
(Fig. 85, D). The animal takes no food and all of its activities
are nearly suspended. It is like an animal asleep or hibernating,
and in this state it may long remain. Protected by its mem-
brane it is able to resist desiccation, and upon the evaporation of
the surrounding water it may, as a particle of " dust," be trans-
ported by the winds, even to a great distance. When again
placed under favorable conditions the protoplasm bursts its
envelope, crawls forth from it, and reassumes its active phase.



162 UNICELLULAR ANIMALS.

Structure. Lying in the entoplasm, usually near the pos-
terior extremity, is a nucleus {n^ Fig. 84), having the form of
a bi-concave disk and largely made up of coarse granules of
cTiTomatin (cf. p. 23). Amoeba is therefore at once a single
cell and a unicellular organism, morphologically equivalent to a
single tissue-cell of a higher animal or to the germ-cell from
which every multicellular form arises. The hody of Ammha is