W. T. (William Thompson) Sedgwick.

An introduction to general biology online

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complete. The conjugating cells show a sexual differentiation,
one being like the ovum, large and fixed, the other like the
spermatozoon, small and motile.

As in Paramoscium the macronuclei entirely disappear, fusion takes
place between derivatives of the micronuclei, and from the resulting body
both macronuclei and micronuclei are derived.

Euglena and Other Simpler Infusoria. Besides forms like
Parammciiim and Yorticella which bear numerous cilia, there
are many Infusoria which possess only one large lash orflagellum.
Of these Euglena^ which is sometimes found in stagnant water,
sewage-polluted pools, etc., is one of the most interesting, inas-
much as it contains chlorophyll, possesses an " eye-spot " of red
pigment, and under certain conditions exliibits amoebiform

Compound or " Colonial '' Forms. In a number of forms,
closely related to Yorticella, the individuals (" zooids ") formed
by fission do not immediately separate, but remain for a time
united to form a ' ' colony ' ' which may contain hundreds of
zooids. Zoothammo7i, a common species, thus forms a beautiful
tree-like organism, consisting of a single central stalk with nu-
merous branching offshoots from its summit, each twig terminat-
ing in a zooid. The entire system of branches is traversed by a
continuous contractile axis. Carchesiurrh is similar, but the axis
is interrupted at the beginning of each branch. In Epistylis
the entire axis is non-contractile.

Such colonial forms are of high interest as indicating the
manner in which true multicellular forms may have arisen.
From the latter, however, they differ not only in the fact that
the association of the cells is not permanent, but in the absence
of any division of labor among the units.

Physiology. Most Infusoria are true animals, agreeing with
Ainoeba in the essential features of their nutrition, and having
the power to digest not only proteids, but also carbohydrates and
fats. ParmnoeciuTn and Yorticella are herbivorous forms,
feeding upon minute plants, and especially upon the bacteria-


Other forms are omnivorous (e.g., Stentor^ Bursarla)^ feeding
both on vegetable and on animal food. Others still are car-
nivorous and lead a predatory life, often attacking herljivorous
forms much larger than themselves, precisely as is the case with
carnivores among the mammalia. Thus the unicellular \v()rl<l
reproduces in miniature the essential biological relations of
higher types.

It is a remarkable fact that some species of Infusoria (e.g.,
Parainoacitiin hursaria^ Yorticella viridis) contain numerous
chlorophyll-bodies embedded in the entoj^lasm. Much discus-
sion has arisen as to whether these bodies are to be regarded as
an integral part of the animal, i.e., differentiated out of its own
protoplasm, or as minute plants living ' ' symbiotically " (i.e. as
mess-mates) within the animal. In the forme:* case (which is
the most probable) the animal w^ould to a certain extent be
nourished after the fashion of a green plant (cf. p. lis).

It will now be clear to any one wdio has carefully considered
the phenomena described in the foregoing pages that the uni-
cellular animals are "organisms" by right, and not merely by
courtesy. In some of the Infusoria, for example, differentia-
tion within the single cell may go so far as to give rise to primi-
tive sense-organs (as in the case of the eye-spot of Euglena) ; a
rudimentary oesophagus and definite mouth (as in Parammciiuri
and Yorticella) ; organs of locomotion (cilia, flagelld) ; organs
of excretion (contractile vacuoles) etc. , etc.



A. Protococcus.

{Protococcus, Pleurococcus, Chlorococcus, Hcematococcus, etc.)

Unicellular plants, like unicellular animals, are very com-
mon, although as individuals mostly invisible on account of their
microscopic size. In the mass, however, they are often visible
either as suspended or floating matter, causing ' ' turbidity ' ' in
liquids (yeast, bacteria, diatoms, desinids, etc.) or discolorations
on tree-trunks, earth, stones, roofs, and flower-pots. {Pro-
tococcus, Glmocapsa, etc.).

Under the term Protococcus {jiporo^, first, kokko^, herry)
we may for our present purposes include a number of the simplest
spherical forms, generally green in color and of uncertain afiin-
ities in classifl cation, but very sunilar in structure, living for the
most part in quiet waters or on moist earth, stones, tree-trunks,
or old roofs, or in water-butts, roof-gutters, and the like.
Sometimes the color which they exhibit is yellowish-green,
sometimes bluish-green, and sometimes, though less often, reddish,
according to the species.

One of the commonest and most conspicuous is a species
often seen on the shady side of old tree-trunks where, when
abundant, it forms a greenish dust-like coating or discoloration,
scarcely visible when dry but becoming a rich bright green dur-
ing prolonged rains or after warm showers. If pieces of bark
covered with this form of Protococcus are moistened, the green-
ish coating may be observed at any time. It is granular in tex-
ture and after moistening is easily loosened by a camePs-hair

Morphology. Microscopical examination shows that the par-
ticles detached consist of rounded yellowish-green cells occurring
either singly or in groups of two, three, four, or even more.



Each single cell is a complete individual, capable of carrying on
an independent life. It fairly represents the green plant (such
as Pteris) reduced to its lowest terms. (Fig. 92.)

Like Amoeha and the Infusoria Protococcus^ at least in some
species, occurs both in a motile or active state in which it moves
about, and a quiescent or non-inotile state analogous to the en-
cysted state of the unicellular animals. In the latter the motile
or active state is the usual or dominant condition and tlie en-
cysted state is rarely assumed. In Protococcus^ on the other
hand, the motile state is rare, and the ordinary activities of tlie
plant are carried on in the non-motile state.

Structure. In structure Protococcus is a nearly typical cell
(p. 22). It consists essentially of an approximately spherical
mass of protoplasm enclosed within a thin woody layer of cellu-
lose (cell-wall or cell-membrane), and contains a single nucleus.
It also includes one or more chlorojphyll-hodies (chromatophores)
(p. 126) by virtue of which it is able to manufacture its own
foods, very much after the fashion of the green cells of Pteris.

In those forms which possess a motile stage the latter con-
sists of a spherical, egg-shaped or pear-shaped cell having cliro-
matophores and a membrane through which two flagella protrude.
In the oval forms these are placed near the narrowed end of the
cell, and in all cases they are locomotor organs and propel the
cell swiftly through the water. (Fig. 92).

Reproduction. The ordinary method of reproduction in the
unicellular plants, as in the unicellular animals, is by cell-division.
In Protococcus the sphere becomes divided by a partition into
two cells which eventually separate completely one from the
other. Yery often, however, the separation being incomplete
or postponed until after each daughter-cell has in turn become
divided, groups or aggregates of cells arise which suggest the
first steps in the formation of tissue in the devel(>}unent of higher
forms. In the end, however, separation is total and complete,
and each cell is therefore not a unit in a body, but is itself a
body and an individual (see p. 156). (Fig. 92.)

The daughter-cells thus produced are the young, or offsiiring,
which have the power to grow and ultimately to divide in their
turn. Under favorable circumstances generation may thus fol-
low generation in quick succession. Each young cell is actually



Fig. 93. — Protococcus (Pleurococcus) from the bark of an elm tree, in active vegeta-
tion and showing aggregation into masses of cells. A, Pleurococcus in the dried
condition. JB, Ascococcus (?), showing endogenous division into two cells and (Q
into four. D, E, F, motile forms of Protococcus (after Cohn).


one half of the parent cell and contains a moiety of whatever
that contained. Here, therefore, as in Amaha^ the problems
of heredity, uncomplicated by the occurrence of sex, are reduced
to their lowest terms.

In some kinds of Protococciis the quiescent cells, under
special circumstances, which are not well understood, t^ive rise
to the motile forms {zoospores) referred to above. Cilia, or
rather flagella, are formed, and the protoplasmic mass with its
included chromatophores swims actively about in the water.
After a time these motile cells may come to rest, lose their fla-
gella and divide into two or more daughter-cells, each of which
in its turn may become a motile cell and repeat the process, or,
under other conditions, develop into the ordinary quiescent cell.

In some species of Protococciis in which there is a motile
stage another form of reproduction, a kind of rudimentary
gamogenesis, has been observed. In this process two of the
motile cells (gametes) meet, fuse (conjugation)^ lose their flagella,
become encysted (see p. 161), and ultimately give rise to the
ordinary cells of Protococciis^ both non-motile and motile.
This process, however, has not yet been observed in the species
under consideration.

Physiology. Our actual knowledge of the physiology of
Protococciis is very small. But the study of comparative plant
physiology gives every reason to believe that the essential phys-
iological operations of this" simple plant are fundamentally of
the same character as in the higher green plants, such as Pteris.

Nutrition. The income of Protococciis^ when growing in
its natural habitat on tree-branches, moist bricks, and the like,
is difficult to determine. But as it is able to live also in ordi-
nary rain-water, we are able to set down its probable income
under those conditions with some deo^ree of accuraev. There
is do doubt that it absorbs water and carl)on dioxide by dif-
fusion through the cellulose wall, and that these substances
are used in the manufacture of starch, which, if stored up,
makes its appearance in the form of small graTiulos within the
chromatophores. This process takes place only in the light and
through the agency of the chlorophyll, and is attended by a
setting free of oxygen precisely as in Pteris. Nitrogen is prob-
ably derived from nitrates or ammoniacal compounds, minute



quantities of which are dissolved in the water, and other neces-
sary salts (sulphates, chlorides, j)hosphates, etc.) as well as free
oxygen are procured from the same source. These substances
may be derived from dust blown or washed by the rain into the
water, or from the walls of the vessel. To the process of starch-
making, attended by the absorption of CO^ and HjO and the
liberation of O, the term "assimilation" is generally given.
Like other plants, moreover, Protococcus probably breathes
by absorbing free oxygen and setting free CO, (respiration).

The income and outgo of Protococcus may then be displayed
by the following diagram :





Nitrates or

Ammonia Comp.


ctlier Salts



Urea (?)

other Salts


It should be understood that this only represents the broad
outlines of the process and under the simplest conditions. It is
quite possible that under other conditions Protococcus may use
more complex foods. The facts remain, however, (1) that
Protococcus is dependent on the energy of light ; (2) that its
action is on the whole constructive, resulting in the formation of
complex compounds (carbohydrates, proteids) out of simpler
ones. In these respects it shows a complete contrast to Amosha,
which is on the whole destructive, breaking down complex com-
pounds into simpler ones, and is indej)endent of light, since it
derives energy from the potential energy of its food. The
relations between Protococcus and Amoeha are therefore an
epitome of the relations between Pteris and Luiiibricus^ and
between green plants and animals generally.

The Fundamental Physiological Properties of Plants. In con-
sidering the physiology of Amoeba we found it possible to re-



duce its vital activities to a few fundamental pliysi(jl(jgical proper-
ties, namely, contractility, irritability, metaljijlism, grcjwtli and
reproduction, common to all animals. A little rejection will
show that the same properties are manifested also by Pmtn-
eoccus. Contraction and irritability are difficult to witness iu
the quiescent stage of Protococcus^ but obvious enougli in tlie
rarer motile forms. Metabolism, growth and reproduction, on
the other hand, are evident accompaniments of normal life, even
in the quiescent condition. And precisely as Protococcus diifers
from Amaiba in respect to contractility and irritability, of wliicli
it possesses relatively little, so plants in general differ in tliese
respects from animals in general. Animals are eminently con-
tractile and irritable, while plants are but feebly specialized in
these directions. On the other hand, as we have already seen
in comparing Pteris with Lumhricus (p. 154), and as we see
once more in comj)aring Protococcus with Amoeba^ in respect to
metabolism, the green plant is pre-eminently constructive, while
the animal is preeminently destructive, of organic matter.

In their modes of nutrition, as stated above. Amoeba,
and Protococcus represent two physiological extremes. AVe
pass now to the study of Yeasts and Bacteria, which are plants
destiUite of chlorophyll and in a certain sense may be regarded
as occupying a middle ground between these extremes.

Other Forms. There are innumerable species of uniceUular green
plants. A vast group of peculiar brownish forms covered with transparent
^lass-like cells composed of siliceous material is known as the Diato-
macecE or diatoms. In these the chlorophyll is masked by a brown pig-
ment, but is nevertheless present. Another group is that known as the
Desmidice or desmids. These often have the individual cells peculiarly
constricted in the middle so that at first sight the two halves appear to be
two separate cells. More closely resembling Protococcus in many respects
are some members of the Cyanophycew or "blue-green algiv," among
which Ghro'ococcus and Glceocapsa differ from Protococcus chiefly, in the
former case, in having a blue-green instead of a yellow-green pigment,
and, in the latter, not only in this respect, but also in the fact that the
single cells are widely separated by transparent mucilage.



B. Yeast.


Under tlie general name of yeast are included some of the
simplest forms of vegetal life. Some yeasts are ''wild," liv-
ing upon fermenting fruits or in fruit juices, and commonly

Fig. 93.— Yeast-cells. Brewer's (top) yeast actively vegetating. The large internal
vacuoles and the small fat-drops are shown, as are also buds, in various stages of
development, and the cell-wall. Nuclei not visible. (Highly magnified.)

oecurring in the air ; others are ' ' domesticated, ' ' or cultivated,
such as those regularly employed in bremng and in baking.

If a bit of "yeast-cake " (either "comj^ressed "or " dried"
yeast) is mixed with water, a milky fluid is obtained which

closely resembles the so-called baker's or brewer's yeast.




Microscopical examination proves that the milky appearance
of liquid yeasts is due chieliy to the presence of myriads of
minute egg-shaped suspended bodies, and tliat pressed yeast is
almost wholly a mass of similar forms. These are the cells of
yeast ; which is therefore essentially a mass of unicellular organ-
isms. For reasons which will soon appear yeast is universally

Fig. 94.— Yeast-ceUs. Brewer's (bottom) yeast showing structure— protoplasm, cell-
walls, vacuoles, fat-drops. (Nuclei not shown.)

regarded as a plant, and the single cell is often sj^oken of as the
yeast -plant.

Morphology. The particular yeasts which we shall consider are
the common cultivated forms of com-
merce. The cells of an ordinary cake
of pressed yeast are spherical, sphe-
roidal, or egg-shaped in form, and con-
sist of a mass of protoplasm enclosed
within a well-deiined cell-wall. Bv
ap2)ropriate treatment the latter may
be shown to consist of cellulose ; and
it is distinctly thicker in old or resting Fm. n5.-Spore9 of Yeast (As-

^ 'J ^ ° cospores). Four spores in a cell

cells than in young ones or those vig- of brewer's yeast (^atc/KiromyftViJ

orously growing. Within the granular «<^'''*^'''"""')'
protoplasm {cyto2)lasm) are usually a number of vacuoles (con-
taining sap) and minute shining dots (probably fat-droplets\ but



no chlorophyll is present and no starch. Until recently the yeast-
cell was snj^posed to be destitute of a nncleiis, but it isnow kno\Mi
that each cell probably jDossesses a large and characteristic nuclens.
This, however, can be demonstrated only by s]3ecial reagents and
is rarely or never seen in the living cell (Fig. 96).

Reproduction. The ordinary mode of reproduction of yeast
is by a modification of cell-division called hudding. Under

Fig, 96.— The Nuclei of Yeast-cells and the Process of Budding. (Drawn by J. H.
Emerton from specimens prepared by S. C. Keith, Jr.) The upper left-hand figure
shows the nucleus in a specimen treated with Delafl eld's hsematoxylin. The
other figures in the upper row and those in the lower (from left to right) show
cells in successive stages of budding, together with the appearance, position, and
movements of the nucleus. It will be observed that the bud is formed before the
nucleus divides. (Iron-haematoxylin method.)

favorable circumstances in actively growing yeast a local bulging
of the wall takes place, usually near, but not precisely at, one
pole of the cell. Protoplasm presses into this dilatation or
'' bud " and extends it still further. At this time we have still
but one cell, although it now consists of two unequal parts and
the separation of a daughter-cell is clearly foreshadowed. Event-
ually the connection between the two parts is severed and the
daughter-cell or ' ' bud ' ' is detached from the original or j^arent-
cell ; but detachment may or may not occur until after the bud



has begun to produce dangliter-cells in its turn, and nmi-t' than
one bud may be borne by either or b<jth parent- or dauglitcr-
cells. In very rapid growth the connection may persist between
the cells even during the formation of several generations of
buds; but this is unusual, and in cases where a number of cells
remain apparently united together forming tree-like forms there
is often no real connection, the cells separating readily on agita-

Endospores (Ascospores). Some yeasts in additicjn to the
method of reproduction by budding exhibit another mode known

Fig. 97.— Spores of Yeast (Ascospores). Three- and two-celled stage of spore for-
mation in S. cereviske.

as endogenous division or ascospore formation. Under certain
circumstances not yet entirely understood there are formed
within the yeast-cell two, three, or four rounded shining spores.
These become surrounded by thick walls and thus give rise
eventually to a group of daughter-cells within the original cellu-
lose sac. To the latter the term ascus (sac) has been applied,
and to its contained daughter-cells the term ascosjjores.

It is not yet allowed by all botanists that this terminology, wliich im-
plies a relationship of yeasts to the Ascoraycetous fungi, is sound ; but it
is commonly used.

Each ascospore is capable under favorable circumstances c»f
sprouting and starting a new series of generations of ordinary
yeast-cells.. It should be particularly observed that the eudi.-
spores of yeast are reproductive bodies, and that the process of
their formation is one of multiplication — not merely one of de-
fence or protection, as is the case with the so-called "spores ''
of bacteria described beyond (p. 11»4).


Physiology. Like all other organisms the yeast-plant occu-
pies a definite j)osition in space and time; it possesses an en-
vironment with which it must be in harmony if it is to live,
from which it derives an income, and to which it contributes an
outgo of matter and energy ; it manufactures its own substance
from foods {anaholism)^ and like all living things it wastes by
oxidation of its substance {kataljolism). It is not obviously con-
tractile or irritable, but it is highly metabolic and reproductive.

Yeast and its Environment. Yeast is an aquatic form, and,
as might be supposed, cultivated yeast thrives best in its usual
habitat, the juices of fruits, such as apples or grapes, and the
watery extracts of sprouted seeds, such as barley, corn, and
rye (wort, mash, etc.). It lives, however, more or less success-
fully in many other places (such as the dough of bread), and can
even endure mucli dryness, as is shown by the commercial
' ' dried - yeast. ' ' It appears to prefer a temperature from
20° to 30° C. ; it is usually killed by boiling, but if dried, it can
endui'e high temperatures. Its action is inhibited by very low
temperatures, but like most living things it endures low temper-
atures better than high. It is killed by many poisons (anti-

Income. Owing to its industrial imj^ortance yeast has been
l^erhaps more thoroughly studied in respect to its nutrition than
any other unicellular organism. And yet it is impossible to
give accurate statistics of its normal income and outgo. It is
believed that the ordinary income of a yeast- cell living in wort
(the watery extract of sprouted barley-grains) consists of a^ dis-
solved oxygen j J, nitrogenous Z>6>(^z^<§ allied to proteids, but diffusi-
ble and able to j)ass through the cellulose wall ; <?, carhohydrates,
esj)eciaUy sugary matters / and 6?, salts of various kinds.

It Avas supposed for a long time by Pasteur and others tliat
yeast could dispense with free (dissolved) oxygen in its dietary.
It now appears that this faculty is temporary only, and that if
yeast is to thrive it must, like all other living things, be sup-
plied, at least occasionally, with free oxygen.

Metabolism. Out of the income of foods just described yeast
is able to build up its own peculiar 23rotoi3lasm {anabolism)^ and,
doubtless, to lay down the droplets of fat which often appear in
it. There is good reason to believe that its substance also breaks


down, witli tlie productiiji of carbon dioxide, water, and nitro-
genous waste (kataholisM), and the concomitant liberation of
energy. The work to De done by the yeast-cell is ])]ainly
limited. The manufacture of new and of surplus protoj)lasni
and the protrusion of buds require work, partly chemical,
partly mechanical; but most of the liberated energy pr(>ba])ly
appears as heat. In point of fact, great activity of yeast is
accompanied by a rise of temperature, as may beproved by
placing a thermometer in "rising" dough or fermenting fruit-

Outgo. Barring the outgo of energy already mentioned, and
the probable excretion of carbon dioxide and nitrogenous waste,
but little can be said concerning the outgo of a yeast-cell. The
ordinary excretions are so masked by the presence of foreign
matters in the liquids which yeast inhabits that little is known of
the real course of events. To the consideration of conditions
which entail these difficulties we may now pass, merely pausing
to caution the student against the suj^position that the evolution
of carbon dioxide in fermentations represents to any great ex-
tent the normal respiration of the yeast cells.

Mineral Nutrients of Yeast. It has been shown (pp. 14S, ISl)
that Pteris and Protococcus^ inasmuch as they possess chlorophyll
can live upon simple inorganic matters such as C0„, II^C ), and
nitrates, out of which they are able to manufacture for them-
selves energized foods such as starch. Yeast is unal)le to do
this, as might be supposed from the fact that it is destitute of
chlorophyll. And yet yeast does not require proteid ready-
made as all true animals do, for experiments have shown that it
can live and grow in a liquid containing only mineral matters

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Online LibraryW. T. (William Thompson) SedgwickAn introduction to general biology → online text (page 16 of 20)