Of milk, especially, much has been written and said as a

disease-transmitting medium, and with every good reason, and, if the

statement of a Continental authority may be accepted that each time we

eat a slice of bread and butter we devour a number of bacteria equal to

the population of Europe, we have sure grounds for seeking for further

information as to what these bacteria are and what they are doing. And

similarly so with cheese, which teems with millions of these minute

organisms.

“Some few years ago it was found that the peculiar aroma of butter

was due to a bacterium. There are two species of bacteria, one of

which develops an exquisite flavour and aroma, but the butter keeps

badly, while the other develops less aroma, but the butter keeps

better. In America, however, they have isolated and distributed pure

cultures of a particular butter bacillus which develops the famous

‘June’ flavour, hitherto only met with in the butter made in a certain

district during a short season of the year. This fine-flavoured butter

is now constantly manufactured in a hundred American dairies; and the

manufacture of pure butter with a constant flavour has become a matter

of certainty.

“Properly considered, the manufacture of cheese is a form of

‘microscopic gardening’ even more complex and more horticultural in

nature than the brewing of beer. From the first moment, when the

cheesemaker guards and cools his milk, till his stock is ready he

is doing his best to keep down the growth of micro-organisms rushing

about to take possession of his milk. He therefore coagulates it with

rennet--an enzyme of animals, but also, as we have seen, common in

plants--and the curd thus prepared is simply treated as a medium, on

which he grows certain fungi and bacteria, with every needed precaution

for favouring their development, and protecting them against the

inroads of other pests and against unsuitable temperature, moisture,

and access of light. Having succeeded in growing the right kind of

plants on his curd, his art then demands that he shall stop their

growth at the critical moment, and his cheese is ready for market.

“Furthermore, the particular flavour and peculiar odours of cheeses,

as Camembert, Stilton, and Roquefort, have to be obtained, and this is

secured, for instance, by cultivating a certain fungus, Penicillium, on

bread, and purposely adding it to Roquefort. This is found to destroy

the lactic and other acids, and so enables certain bacteria in the

cheese to set to work and further change the medium; whereas in another

kind of cheese the object is to prevent this fungus paving the way for

these bacteria. Another kind of bacillus has been discovered which

gives a peculiar clover aroma to certain cheeses.

“It is thought that more definite results will be obtained by the

investigation of the manufacture of the vegetable cheeses of China

and Japan, which are made by exposing the beans of the leguminous

plant, _Glycine_--termed soja-beans--to bacterial fermentations in

warm cellars with or without certain mould-fungi. Several kinds of

bean-cheeses are made in this way, known by special names. They

all depend upon the peculiar decompositions of the tissues of the

cotyledons of the soja-beans, which contain 35 to 40 per cent. of

proteids and quantities of fatty matter. The softened beans are first

rendered mouldy, and the interpenetrating hyphæ render the contents

accessible to certain bacteria, which peptonise and otherwise alter

them. There is the further question of the manufacture of vinegar

by fermentation, of the preparation of soy from a brine extract of

mouldy and fermented soja-beans, of bread-making, and other equally

interesting manufactures.”

Results of De Bary’s Investigations in Parasitism.

“When the idea of parasitism was rendered definite by the fundamental

distinction drawn by De Bary between a _parasite_ and a _saprophyte_,

it soon became evident that some further distinction must be made

between _obligate facultative_ parasites and saprophytes respectively.

De Bary, when he proposed these terms for adoption, was clearly alive

to the existence of transitions which we now know to be numerous

and so gradual in character that we can no longer define any such

physiological groups. Twenty years ago penicillium and mucor would have

been regarded as _saprophytes_ of the most obligate type, but we now

know that under certain circumstances these fungi can become parasites,

and the borderland between facultative parasites and saprophytes on the

one hand, and between the former and true parasites on the other, can

no longer be recognised.”

In 1866 the germ of an idea was sown which has taken root and extended.

De Bary pointed out that in the case of lichens we have either a fungus

parasite on an algæ, or else certain organisms hitherto accepted as

algæ are merely incomplete forms.

“In 1879 the same observer definitely launched the new hypothesis

of _symbiosis_. The word itself is due to Frank, who, in a valuable

paper on the biology of the thallus of certain lichens, very clearly

set forth the existence of various stages of life in common among

all the lower forms of plants. The details of these matters are now

principally of historical interest. We now know that lichens are dual

organisms, composed of various algæ, symbiotic with Ascomycetes, with

Basidiomycetes, and, as Massee has shown, even with Gastromycetes.

The soil contains also bacterio-lichens. Hence arose a new biological

idea--that a fungus may be in such nicely-balanced relationship with

the host from which it derives its sustenance, that it may be attended

with nearly equal advantage to both.

“In the humus of forests we find the roots of beeches and other

Cupuliferæ (willows, pines, and so forth) clothed with a dense mantle

of hyphæ, and swollen into fleshlike masses of mycorhiza. In similar

soils, and in moorlands, which abound in the slowly decomposing

root-fibres and other vegetable remains so characteristic of these

soils, the roots of orchids, heaths, gentians, &c., are similarly

provided with fungi, the hyphæ of which penetrate further into the

tissues, and even send haustoria into the living cells, but without

injuring them. As observations multiplied it became clear that the

mycorhiza, or fungus-root, was not to be dismissed as a mere case of

roots affected by parasites, but that a symbiotic union, comparable

to that of the lichens, exists, and we must assume that both tree and

fungus derive benefit from the connection.

[Illustration: Fig. 280.--Fine Section through Truffle.

_a._ Asci filled with spores; _b._ Mycelia, × 250.]

“Frank stated, as the result of his experimental research, that

seedling forest-trees cannot be grown in sterilised soil, where their

roots are prevented from forming mycorhiza; and he concluded that the

fungus conveys organic materials to the roots, which it obtains by

breaking down the leaf-mould and decaying plant remains, together with

water and minerals from the soil, and plays the especial part of a

nitrogen-catching apparatus. In return for this import service the root

pays a tax to the fungus by sparing it certain of its tissue contents.

It is a curious fact then that the mycorhiza is only formed where

humus or vegetable mould abounds.”

These instructive investigations offer an intelligible explanation of

the growth of that well-known subterranean fungus, the truffle (_Tuber

cibarium_), the microscopic appearances of a section of which formed

the subject of a paper I contributed to “The Popular Science Review”

some years ago (1862). The fungus, as will be seen by the fine section

cut through a truffle, Fig. 280, consists of flocculent filaments,

which in the first instance cover the ground at the fall of the leaf

in autumn, under oak or beech trees, the hyphæ of which penetrate

the ground, through the humid soil to the _root-hairs_ of the tree.

Filaments (mycelia) are again given off which terminate in asci or sacs

filled with minute spores of about 1/2500th of an inch in size, while

the interspaces are filled up by mycelia, that become consolidated into

a firm nut-like body.

What happens, then, is this: Trees and plants with normal roots and

root-hairs, when growing in ordinary soil, can adapt their roots

to life in a soil heavily charged with humus only by contracting

symbiotic association with the fungus and paying the tax demanded by

the latter in return for its supplies and services. If this adaptation

is impossible, and no other suitable variation is evolved, such trees

cannot grow in such soils. The physiological relations of the root

to the fungus must be different in details in the case of non-green,

purely saprophytic, plants, Neottia, Monotropa, &c., and in that of

green plants like Erica, Fagus, and Pinus. It is, however, a well-known

fact that ordinary green plants cannot utilize vegetable débris

directly, and forest trees do so in appearance only, for the fungi,

yeasts and bacteria there are actively decomposing the leaves and other

remains. A class of pseudo-symbiotic organisms are, however, being

brought into the foreground, where the combined action of two symbionts

results in the death of or injury to a third plant, each symbiont

alone proving harmless. Some time ago Vuillemin showed that a disease

in olives results from the invasion of a bacillus (_B. oleæ_), which

can, however, only obtain its way into the tissues through the passages

driven by the hyphæ of a fungus (Chætophoma). The resulting injury is a

sort of burr. This observer also observed the same bacillus and fungus

in the canker burrs of the ash.

Among many similar cases well worth further attention are the invasion

of potato-tubers by bacteria, these making their way down the decaying

hyphæ of pioneer fungi. Professor Marshall Ward has seen tomatoes

infected by similar means, and other facts show that many bacteria

which quicken the rotting of wood are thus led into the tissues by

fungi.

Probably no subject in the whole domain of cryptogamic botany has

wider bearings on agricultural science than the study of the flora

and changes on and in manure and soil. Nitrifying bacteria play a

very important part by providing plant life with a most necessary

food. They occur in the soil, and two kinds have been described--the

one kind converting ammonia into nitrous acid, and the other changing

nitrous into nitric acid. We are principally indebted to Winogradsky

for our knowledge of these bacteria; he furnishes instances of the

bearing of bacteriological work on this department of science, and

explains, not only the origin of nitre-beds and deposits, but also

the way the ammonia compounds fixed by the soil in the neighbourhood

of the root-hairs are nitrified, and so rendered directly available

to plant life. The investigations of other observers show that the

nitrifying organism is a much more highly-developed and complex form

than had been suspected; that it can be grown on various media, and

that it exhibits considerable polymorphism--_i.e._, it can be made to

branch out and show other characteristics of a true fungus. “I have,”

writes Professor Ward, “for some time insisted on the fact that river

water contains reduced forms of bacteria--_i.e._, forms so altered by

exposure to light, changes of temperature, and the low nutritive value

of the water, that it is only after prolonged culture in richer food

media that their true nature becomes apparent.” Strutzer and Hartleb

show that the morphological form of the nitrifying organism can be

profoundly altered by just such variations of the conditions described

by Ward, and that it occurs as a branched mycelial form; as bacilli

or bacteria; or as cocci of various dimensions, according to the

conditions.

“These observations, and others made on variations in form

(polymorphism) in other fungi and bacteria, open out a vast field for

further work, and must lead to advancement in our knowledge of these

puzzling organisms; they also help us to explain many inconsistencies

in the existing systems of classification of the so-called ‘species’ of

bacteria as determined by test-tube culture.”

=Algæ.=--The algals have a special charm for microscopists. I am free

to confess my interest in these organisms, and for several reasons. In

this humid climate of ours they are accessible during the greater part

of the year; they can be found in any damp soil, in bog, moss, and in

water--indeed, wherever the conditions for their existence seem to be

at all favourable for development. Should the soil dry up for a time,

when the rain returns algæ are seen to spring into life and give forth

their dormant spores, which once more resume the circle of formation

and propagation. In the earliest stage of development the spore or

spore cell is so very small when in a desiccated state, that any number

may be carried about by the slightest breath of air and borne away to

a great distance. To all such organisms I originally gave the name of

Ærozoa; now recognised as ærobic and anærobic organisms (Fig. 281).

[Illustration: Fig. 281.--Ærobic Spores × 200.

1. Ærobic fungi caught over a sewer; 2. Fragments of Penicillium

spores; 3. Ærobic fungi taken in the time of the cholera visitation,

1854.]

With reference to the ærobic bacteria I have only to add that in

addition to the simple mode of taking them on glass slides smeared

over with glycerine, special forms of æroscopes are now in use for the

purpose, consisting of a small cylinder in which a current of air is

produced by an aspirator and diffused through a glass vessel containing

a sterilised fluid. These are in constant use in all bacteriological

laboratories. The results obtained are transferred to sterilised flasks

or tubes as those shown in a former chapter.

Miquel, who has given considerable attention to the subject of ærobic

and anærobic bacteria, reckons that the number of spores that find

their way into the human system by respiration, even should health be

perfectly sound, may be estimated at 300,000 a day.

One of the most commonly met with forms of micro-organisms is

_Leptothrix buccalis_. It chiefly finds its nutritive material in the

interstices of the teeth, and is composed of short rods and tufted

stems of vigorous growth, to which the name of _Bacillus subtilis_ has

been given (Fig. 282). Among numerous other fungoid bodies discovered

in the mouth, Sarcinæ have been found. See Plate IX., No. 7.

[Illustration: Fig. 282.--Section of the Mucous Membrane of the Mouth,

× 350.

Showing: _a._ The denser connective tissue; _b._ Teased out tissue;

_c._ Muscular fibre; _d._ _Leptothrix buccalis_, together with minute

forms of bacteria and micrococci; _e._ Ascomycetes and starch granules.]

The Beggiatoa, a sewage fungus, found by me in the river Lea water of

1884 growing in great profusion, consists chiefly of mycelial threads

and a number of globular, highly refractive bodies, and may be regarded

as evidence of the presence in the water of an abnormal amount of

sulphates which set free a gas, sulphuretted hydrogen, of a dangerous

and offensive character. Another curious body closely allied to

_Beggiatoa alba_ is Cladothrix; this assumes a whitish pellicle on the

surface of putrefying liquids.

These saprophytes obtain nourishment from organic matter; nevertheless

they are not true parasites in the first stage of their existence,

during which they live freely in the water or in damp soil; they,

however, become pathogenic parasites when they penetrate into the

tissues of animals, and necessarily live at the expense of their host.

[Illustration: FUNGI, ALGÆ, LICHENS, ETC.

Tuffen West, del. Edmund Evans.

PLATE I.]

Bacteria, as I have said, were for a long time classed with fungi under

the name of Schizomycetes. But the more recent researches into their

organisation, and more especially into their mode of reproduction,

show that they rather more resemble a group of algæ devoid of

chlorophyll. Zopf asserts that the same species of algals may at one

time be presented in the form of a plant living freely in water, or

in damp ground, in association with chlorophyllaceous protoplasm,

and at another in the form of a bacterium devoid of green colouring

matter, and receiving nourishment from organic substances previously

elaborated by plants or animals, thus accommodating itself, according

to circumstances, to two very different modes of existence.

That widely-distributed single-cell plant, the _Palmoglœa macrococca_

of Kützing, that spreads itself as a green slime over damp stones,

walls, and other bodies, affords an example. If a small portion be

scraped off and placed on a slip of glass, and examined with a half

or a quarter-inch power, it will be seen to consist of a number of

ovoid cells, having a transparent structureless envelope, nearly

filled by granular matter of a greenish colour. At certain periods

this mass divides into two parts, and ultimately the cell becomes two.

Sometimes the cells are united end to end, just as we see them united

in the actively-growing yeast plant; but in this case the growth is

accelerated, apparently, by cold and damp. Another plant belonging to

the same species, the _Protococcus pluvialis_, is found in every pool

of water, the spores of which must be always floating in the air, since

they appear after every shower of rain.

_Protococcus pluvialis_ is furnished with motile organs--two

or more vibratile flagella passing through perforations in the

cell-wall--whereby, at certain stages, they move rapidly about. The

flagella are distinctly seen on the application of the smallest drop

of iodine. The more remarkable of the several forms presented by

the plant is that of naked spores, termed by Flotow _Hæmatococcus

porphyrocephalus_. These minute bodies are usually seen to consist

of green, red, and colourless granules in equal proportions, and

occupying different portions of the cell. They seem to have some share

in the after subdivision of the cell (Fig. 283). There are also

_still_-cells, which sub-divide into two, while the motile cells

divide into four or eight. It is not quite clear what becomes of the

motile zoospores, B, but as they have been seen to become encysted,

they doubtless have a special function, or become _still_-cells under

certain circumstances.

It appears that both longitudinal and transverse division of the

primordial cell takes place; and that the vibratile flagella of the

parent cell retain to the last their function and their motion after

the primordial cell has become detached and transformed into an

independent secondary cell (Fig. 283, G).

[Illustration: Fig. 283.--Cell Development. (_Protococcus pluvialis_.)

_Protococcus pluvialis_, Kützing. _Hæmatococcus pluvialis_, Flotow.

_Chlamidococcus versatilis_, A. Braun. _Chlamidococcus pluvialis_,

Flotow and Braun.

A. Division of a simple cell into two, each primordial vesicle having

developed a cellulose envelope; B. Zoospores, having escaped from a

cell; C. Division of an encysted cell into segments; D. Division of

another cell, with vibratile flagella projecting through cell-wall; E.

An encysted flagellate cell; F. Division of an encysted nucleated cell

into four parts, with vibratile filaments projecting; G. Fission of a

young cell.]

The most striking of the vital phenomena presented by Protococcus is

that of periodicity. Certain forms--for instance, encysted zoospores,

of a certain colour, appear in a given infusion, at first exclusively,

then they gradually diminish, become more and more rare, and finally

disappear altogether. After some time their number again increases,

and this may be repeated. Thus, a cell which at one time presented

only still forms at another contained only motile ones. The same may

be said with respect to segmentation. If a number of motile cells be

transferred from a larger vessel into a smaller one, in the course of

a few hours most of them will have subsided to the bottom, and in the

course of the day observed to be on the point of sub-division. On the

following morning division will have become completed; on the next day

the bottom of the vessel will be found covered with a new generation

of self-dividing cells, which, again, will produce another generation.

This regularity, however, is not always observed. The influence of

every change in the external conditions of life upon the plant is very

remarkable. It is only necessary to pour water from a smaller into a

larger or shallower vessel to at once induce segmentation of cells.

The same phenomenon occurs in other algals; thus Vaucheria almost

always develops zoospores at whatever time of year they may be brought

from their natural habitat into a warm room. Light is conducive to

the manifestation of vital action in the motile spores; they usually

collect in great numbers on the surface of the water, and at that part

exposed to the strongest light.

But in the act of propagation, on the contrary, and when about to pass

into the still condition, the motile Protococcus cell seems to shun

light, and falls to the bottom of the vessel. Too strong sunlight,

as when concentrated by a lens, quickly kills the young zoospores.

A temperature of undue elevation is injurious to the development of

their vital activity and the formation of the zoospores. Frost destroys

motile, but not still zoospores.[55]

_Stephanosphæra pluvialis_ is a conspicuous variety of the fresh-water

algals, described by Cohn. It consists of a cell containing eight

primordial cells filled with chlorophyll, uniformly arranged (see Plate

I., No. 24 _d_). The globular mother-cell rotates, somewhat in the

same way as the volvox, by vibratile flagella, two of which are seen

projecting from each cell and piercing the transparent outer cell wall.

Every cell divides first into two, then four, and lastly eight cells,

each one of which again divides into a number of micro-gonidia, which

have a motion within the mother-cell, and ultimately escape from it.

Under certain circumstances each of the eight young cells is observed

to change places in the interior of the cell; eventually they escape,

lose their flagella, form a thicker membrane as at _b_, and for a time

remain motionless, and sink to the bottom of the vessel in which they

are contained. If the vessel is permitted to become thoroughly dry, and

then again has water poured into it, motile cells reappear; from which

circumstance it is probable that these represent the resting spores of

the plant. When in the condition of greatest activity its division into

eight is perfected during the night, and early in the morning light the

young cells escape and pass through similar changes. It is calculated

that in eight days, under favourable circumstances, 16,777,216 families

may be formed from one resting-cell of Stephanosphæra. In certain of

the cells, and at particular periods, remarkable amœboid bodies (Plate

I., No. 24 _c_) make their appearance. There is a marked difference

between Stephanosphæra and Chlamydococcus, for while in the latter the

individual portions of a primordial cell separate entirely from one

another, each developing its own enveloping membrane, and ultimately

escaping as a unicellular individual; in the former, on the other hand,

the eight portions remain for a time living in companionship.

_Volvocineæ._--A fresh-water unicellular plant of singular beauty

and interest to the microscopist is the _Volvox globator_ (Plate I.,

No. 15). No. 16 represents a portion of another cell, with brownish

amœboid bodies enclosed in the protoplasmic web. It is common to our

fresh-water pools, and attains a diameter of about 1/20th or 1/30th

of an inch. Its movement is peculiar, a continued roll onwards, or a

rotation like that of a top; at other times it glides along smoothly.

When examined under a sufficiently high power, it is seen to be a

hollow sphere, studded with green spots, and traversed by green threads

connecting each of the spots or spores with the maternal cell. From

each of the spores proceed two long flagella, lashing filaments, which

keep the globular body on the move. After a time the sphere bursts,

and the contained sporules issue forth and speedily pass through a

similar stage of development. These interesting cells were long taken

to be animal bodies. Ehrenberg described them as _Monads_, possessing a

mouth, stomach, and an eye.

The setting free of the young volvox is essentially a process of

cell division, occurring during the warmer periods of the year, and,

as Professor Cohn shows, is a considerable advance upon the simpler

conjugation of two smaller cells in desmids; it more closely resembles

that which prevails among the higher algæ and a large number of

cryptogams. As autumn advances the volvox spherules usually cease

to multiply by the formation of zoosporanges, and certain of their

ordinary cells begin to undergo changes by which they are converted,

some into male or sperm-cells, others into germ-cells, but the greater

number appear to remain sterile. Both kinds of cells at first so

nearly resemble each other that it is only when the sperm cells begin

to undergo sub-division that they are seen to be about three times

the size of the sterile cells. Then the primary cell resolves itself

into a cluster of peculiar secondary cells, each consisting of an

elongated body containing an orange-coloured endochrome and a pair of

long flagella, as seen in the antherozoids of the higher cryptogams.

As the sperm-cells approach maturity the clusters may be seen to

move within them; the bundles then separate and show an independent

active movement while still within the cavity of the primary cell, and

finally escape through a rupture in the cell-wall, rapidly diffusing

themselves as they pass through the cavity. The germ-cells continue

to increase in size without undergoing sub-division, at first showing

large vacuoles in their protoplasm, but subsequently becoming filled

with a darker coloured endochrome. The form of the cell also changes

from its flask-like shape to the globular, and at the same time seems

to acquire a firmer envelope. Over this the swarming antherozoids

diffuse themselves and penetrate the substance to the interior,

and are then lost to view. The product of this fusion, Cohn tells

us, is a reproductive cell, or “oospore,” which speedily becomes

enveloped in another membrane with a thicker external coat, beset with

conical-pointed processes; and now the chlorophyll of the young cell

gives place, as in Palmoglæ, to starch and reddish or orange-coloured,

and a more highly refractive, fluid. As many as forty of such oospores

have been counted in a single sphere of volvox, which then acquires

the peculiar appearance observed by Ehrenberg, and described by him

under the name of _Volvox stellatus_. The further history of this

wonderful spheroid unicellular plant has been traced out by Kirchner,

who found that their germination commences in the early months of the

year--in February--with the liberation of the spherical endospore

from its envelope and its division into four cells. A remarkable

phenomenon has been observed by Dr. Braxton Hicks--the conversion

of an ordinary volvox cell into a moving mass of protoplasm that

bears a striking resemblance to the well-known amœba. “Towards the

end of the autumn the endochrome mass of the volvox increases to

nearly double its ordinary size, but instead of undergoing the usual

sub-division so as to produce a macrogonidium, it loses its colour

and regularity of form, and becomes an irregular mass of colourless

protoplasm, containing a number of brownish granules.” The final change

and the ultimate destination of these curious amœboid bodies have

not been satisfactorily made out, but from other observations on the

protoplasmic contents of the cells of the roots of mosses, which in the

course of two hours become changed into ciliated bodies, it is believed

that this is the mode in which these fragile structures are enabled to

retain life and to resist all the external conditions, such as damp,

dryness, and the alternations of heat and cold.

It would be quite impossible to deny the great similarity there

is between the structure of volvox and that of the motile cell of

_Protococcus pluvialis_. The influence of reagents will sometimes

cause the connecting processes of the young cells, as in Protococcus,

to be drawn back into the central mass, and the connecting threads

are sometimes seen as double lines, or tubular prolongations of the

membrane. At other times they appear to be connected by star-like

prolongations to the parent cell (Plate I., No. 15), presenting an

almost identical appearance with _Pediastrum pertusum_. Another body

designated by Ehrenberg _Sphærosira volvox_ is an ordinary volvox in

a different stage of development; its only features of dissimilarity

being that a large proportion of the green cells, instead of being

single, are double or quadruple, and that the groups of flagellate

cells form by their aggregation discoid bodies, each furnished with a

single flagellum. These clusters separate themselves from the parent

cell, and swim off freely under the forms which have been designated

Uvella and Syncrypta by Ehrenberg. Mr. Henry Carter, F.R.S., who made

a careful investigation of unicellular plants, described Sphærosira as

the male, or spermatic form of _volvox_.

Among other organisms closely allied to volvox and included in

Volvocineæ, affording the microscopist many interesting transitional

forms in their various modes of fructification, are the Eudorina,

still-water organisms that pass through a similar process of

reproduction as the volvox. In the _Pandorina morum_, its reproduction

is curiously intermediate between the lower and the higher types; as

within each cell is a mulberry-like mass, composed of cells possessing

a definite number of swarm spores, sixteen usually, which rupture the

mother cell, and swim off furnished with a pair of flagella. A similar

process takes place in some of the Confervaceæ and other fresh-water

algæ. The Palmella, again, consist of (Plate I., No. 21) minute

organisms of very simple structure, which grow either on damp surfaces

or in fresh water. The stonework of some of our churches is often

seen to be covered with a species of Palmella, that take the form of

an indefinite slimy film. The “red snow” of Arctic or Alpine regions,

considered to be a species of Protococcus, is frequently placed among

Palmella. A more characteristic form of the _P. cruenta_ is the

_Hæmatococcus sanguinis_, the whole mass of which is sub-divided by

partitions enclosing a larger or smaller number of cells, which diffuse

their granular contents through the gelatinous mass in which their

several changes take place. The albuminoid envelope of these masses

is seen to contain parasitic growths, which have given rise to some

discussion, especially when their filaments are observed to radiate in

various directions.

The _Oscillariaceæ_ constitute a genus of Confervaceæ which have

always had very great interest for the microscopist in consequence

of their very remarkable animal-like movements, and from which they

derive their generic name. For more than a century these Bacillaria

have excited the curiosity of all observers without any one having

derived more than an approximate idea of their remarkable rhythmical

movements. The frustule consists of a number of very fine short threads

attached together by a gelatinous sheath, in one species all of equal

length. Their backward and forward movement is of a most singular

character; the only other conferva in which I have seen a motion of

a similar kind is the Schizonema. In this species the frustules are

packed together in regular series, the front and side views being

always in the same direction. These several bodies move along within

the filamentous sheath without leaving their respective places. On

carefully following the movement, it is seen at first much extended,

and then more compressed, while the frustules become more linear in

their arrangement, and present a closer resemblance to _Bacillaria

paradoxa_, augmented by the circumstance that the frustules are

seen to move in both directions. A frustule of Schizonema can move

independently of the sheath, and so will a detached frustule of

bacillaria. This peculiar and exceptionally anomalous phenomenon

as that of the movements of bacillaria can hardly be confined to a

solitary species. The movements of the frustules are much accelerated

by warmth and light. The longer filaments of other minute species only

slightly exhibit any motion of the kind, but have peculiar undulating

motions.

[Illustration: Fig. 284.--Confervaceæ.

1. _Volvox globator_; 2. A section of volvox, showing the flagellate

margin of the cell; 3. A portion more highly magnified, to show the

young volvocina, with their nuclei and thread-like attachments; 4.

Spirogyra, near which are spores in different stages of development; 5.

_Conferva floccosa_; 6. _Stigeoclonium protensum_, jointed filaments

and single zoospores; 7. _Staurocarpus gracilis_, conjugating filaments

and spores.]

Confervaceæ are a genus of algals. The species consist of unbranched

filaments composed of cylindrical or moniliform cells, with starch

granules. Many are vesicular, and all multiply by zoospores generated

in the interior of the plant at the expense of the granular matter.

They are, for the most part, found in fresh water attached or floating,

some in salt water, and a few in both, in colour usually green, but

occasionally olive, violet, and red. The Confervaceæ proper are often

divided into four families: 1. _Hydrodictidæ_; 2. _Zygnemidæ_; 3.

_Confervidæ_; 4. _Chætophoridæ_. To the microscopist all the plants

of this genera are extremely interesting as subjects for the study of

cell multiplication. The process usually takes place in the terminal

cell, the first step in which is the division of the endochrome, and

then follows a sort of hour-glass contraction across the cavity of the

parent cell, whereby it is divided into two equal parts. This is better

seen in some of the desmids than in Fig. 284, Nos. 4, 5, and 6. Some

species are characterised by a different mode of reproduction; these

possess a number of nuclei, and multiply by zoospores of two kinds,

the largest of which have either two or four cilia, which germinate

directly the smaller are biciliated; conjugation has been seen to take

place in a few instances.

Allied to the Confervaceæ is an interesting plant, _Sphæroplea

annulina_, which has received careful attention from Cohn. The oospores

of this plant are the product of a process partaking of a sexual

nature, and when mature are filled with reddish fat vesicles which

divide by segmentation.

The _Ædogoniaceæ_ also closely resemble Confervaceæ in habits of

life, but differ in some particulars, especially so in the mode of

reproduction (only a single large zoospore being set free from each

cell) and by the almost complete fission of the cell-wall or one of

the rings which serve as a hinge. The zoospores are the largest known

among algals, and each is described as having a red eye-spot. The

_Chætophoraceæ_ form an interesting group of confervoid plants, and are

usually found in running streams, as they prefer pure water. One of the

characteristics of the group is that the extremities of the branches

are prolonged into an acute-shaped termination, as represented in Fig.

284, No. 6. A very pretty object under the microscope is _Draparnaldia

glomerata_, belonging to this species. It consists of an axis composed

of a row of cells, and at regular intervals whorls of slender

prolongations, containing chlorophyll or endochrome of a deeper green;

these attain to an extraordinary length.

The _Batrachospermæ_ bear a strong resemblance to frog-spawn, from

which they derive their name, and are chiefly a marine group of

algals allied to the Rhodespermeæ or red seaweeds. The late Dr. A.

Hassall first described them; they have since received more careful

attention from M. Sirodot. They are reddish-green, extremely flexible,

and nothing can surpass the grace of their movements in water; but

when removed from their element they lose all form, and resemble a

jelly-like substance without a trace of organisation; but if allowed to

remain quiet they regain their original shape.

The presence of the cell-membrane will be best demonstrated by breaking

up the filaments, either by moving the thin glass cover, or by cutting

through a mass of them in all directions with a fine dissecting knife.

On now examining the slide, in most instances many detached empty

pieces of the cell-membrane, with its striæ, will be seen, as well as

filaments partly deprived of protoplasm. On the application of iodine

all these appearances become more distinguishable in consequence of

the filament turning red or brown, while the empty cells remain either

unaffected, or present a slight yellowish tint, as is frequently the

case with cellulose when old.

[Illustration: Fig. 285.--_Mesoglia vermicularis._]

With regard to the contents of the cell, the endochrome is coloured in

the Oscillatoriæ, and is distinguishable by circular bands or rings

around the axis of the cylindrical filament. Iodine stains them brown

or red, and syrup and dilute sulphuric acid produce a beautiful rose

colour. As to their mode of propagation, nothing positive is known. If

kept for some time they gradually lose their green colour; a portion

of the mass, becoming brown, sinks to the bottom of the vessel, and

presents a granular layer.

_Mesoglia vermicularis_ (Fig. 285) consists of strings of cells

cohering and held together by their membranous covering. In the lowly

organised plant Vaucheria (Plate I., No. 23, _V. sessilis_)--so

named after its discoverer Vaucher, a German botanist--a genus of

Siphonaceæ, we have an example of true processes of sexual generation.

The branching filaments are often seen to bear at their sides peculiar

globular bodies or oval protuberances, nipple-shaped buddings-out of

the cell-wall, filled with a dark-coloured endochrome and distributed

in pairs, one of which curves round to meet the other, when conjugation

is seen to take place. Near these bodies others are found with

pointed projections, which have been described as “horns,” but these,

Pringshelm says, are “antherids which produce antherozoids in their

interior,” while the capsule-like bodies constituting the oospores

become, when fertilised, a new generation, which swarm out through a

cavity or aperture in the parent cell-wall.

The fruit of fresh-water and most olive-green algals is enclosed

in spherical cavities under the epidermis of the frond, termed

conceptacles, and may be either male or female. The zoids are

bottle-shaped and have flagella; the transparent vesicle in which

they are contained is itself enclosed in a second of similar form. In

monœcious and diœcious algals the female conceptacles are distinguished

from the male by their olive colour. The spores, when developed, are

borne on a pedicle emanating from the inner wall of the conceptacle.

They rupture the outer wall at its apex; at first the spore appears

simple, but soon after a series of changes takes place, consisting in a

splitting up of the endochrome into six or eight masses of spheroidal

bodies. A budding-out occurs in a few hours’ time, and ultimately

elongates into a cylindrical thread. The Vaucheria present a double

mode of reproduction, and their fronds consist of branching tubes

resembling in their general character that of the Bryophyta, from which

indeed they differ only in respect of the arrangement of their green

contents. In that most remarkable plant _Saprolegnia ferox_, which is

structurally so closely allied to Vaucheria, though separated from

them by the absence of green colouring matter, a corresponding analogy

in the processes of development takes place. In the formation of its

zoospores, an intermediate step is presented between that of the algæ

and a class of plants formally placed among fungi.

_The Ulvaceæ._--The typical form of seaweeds is the _Ulva lactuca_,

well known from its fronds of dark-green “laver” on every coast

throughout the world. Ulvæ are seen to differ but little from the

preceding group of fresh-water algals. The specific difference is that

the cells, when multiplied by binary subdivision, not only remain in

firm connection with each other but possess a more regular arrangement.

The frond plane of the algal is either more simple or lobed, and is

formed of a double layer of cells closely packed together and producing

zoospores. The whole group is chiefly distinguished from Porphyra by

their green colour, the latter being roseate or purple. Ulvæ are mostly

marine, with one or two exceptions. One species (_U. thermalis_) grows

in the hot springs of Gastein, in a temperature of about 117° Fahr.

The development of Ulvæ is seen in Fig. 286. The isolated cells, A,

resemble in some points those of the Protococcus; these give rise to

successive subdivisions determining the clusters seen at B and C, and

by their aggregation to the confervoid filament shown at D. These

filaments increase in length and breadth by successive additions, and

finally take the form of fronds, or rows of cells.

[Illustration: Fig. 286.--Successive Stages of Development of Ulvæ.

A. Isolated spores; B and C. Clusters of cells; D. Cells in the

filamentous stage.]

[Illustration: Fig. 287.--_Sphacelaria cirrhosa_, with spores borne at

the sides of the branchlets.]

The marine greenish-olive algæ present a general appearance which might

at first sight be mistaken for plants of a higher order of cryptogams.

Their fronds have no longer the form of a filament, but assume that

of a membranous expansion of cells. The cells in which zoospores are

found have an increased quantity of coloured protoplasm accumulated

towards one point of the cell-wall; while the zoospores are observed to

converge with their apices towards the same point. In some algæ, which

seem to be closely related in form and structure to the Bryophyta, we

notice this important difference, that the zoospores are developed in

an organ specially destined for the purpose, presenting peculiarities

of form and distinguishing it from other parts of the branching tubular

frond. In the genus Derbesia distinct spore cases develop, a young

branch of which, when destined to become a spore case, instead of

elongating indefinitely, begins, after having arrived at a certain

length, to swell out into an ovoid vesicle, in the cavity of which a

considerable accumulation of protoplasm takes place. This is separated

from the rest of the plant, and becomes an opaque mass, surrounded by a

distinct membrane. After a time a division of the mass takes place,

and a number of pyriform zoospores, each of which is furnished with

flagella, are set free.

[Illustration: DESMIDIACEÆ, DIATOMACEÆ, ALGÆ.

Tuffen West, del. Edmund Evans.

PLATE II.]

[Illustration: Fig. 288.--_Cutleria dichotoma._ Section of lacinia of a

frond, showing the stalked eight-chambered oosporanges growing on tufts

with intercalated filaments. Magnified 50 diameters.]

In _Cutleria_ (Fig. 288) we have a special feature of interest with

two kinds of organs, seemingly opposed to each other with regard to

their reproductive functions. The sporangia not only differ from

those of other species, but the frond consists of olive-coloured

irregularly-divided flagella, on each side of which tufts (_sori_)

consisting of the reproductive organs, intermixed with hair-like

bodies, are scattered. The zoospores are divided by transverse

partitions into four cavities, each of which is again bisected by

a longitudinal median septum. When first thrown off they are in

appearance so much like the spores of Puccinia that they may be

mistaken for them, although so very much larger than those of other

olive-coloured algæ.

_Florideæ_, the red algæ (Plate II.), present many varieties of

structure, although less appears to be known of their reproductive

processes than of lower forms of cryptogamic plants. These are,

however, of three kinds. The first, to which the term polyspore has

been applied, is that of a gelatinous or membranous pericarp or

conceptacle, in which an indefinite number of zoospores are contained.

This organ may be either at the summit or base of a branch, or it may

be concealed in or below the cortical layer of the stem. In some cases

a number of spore-bearing filaments emanate from a kind of membrane

at the base of a spheroidal cellular perisporangium, by the rupture

of which the zoospores formed from the endochrome of the filaments

make their escape. Other changes have been observed; however, they all

agree in one particular, namely, that the zoospore is developed in the

interior of a cell, the wall of which forms its perispore, and the

internal protoplasmic membrane endochrome, the zoospore itself, for

the escape of which the perispore opens out at its apex.

[Illustration: Fig. 289.--_Dasya kutzingiana_, with seed vessel and two

rows of tetraspores. Magnified 50 diameters.]

The second form is more simple, and consists of a globular or ovoid

cell, containing a central granular mass; this ultimately divides into

four quadrate-shaped spores; these, on attaining maturity, escape by

rupture of the cell-wall. Another organ, called a tetraspore, takes

its origin in the cortical layer. The tetraspores are arranged either

in an isolated manner along the branches, or in numbers together; in

some instances the branches that contain them are so modified in form

they look like special organs, and have been called stichidia; as,

for example, in Dasya (Fig. 289). Of the third kind of reproductive

organ a difference of opinion exists as to the signification of their

antheridia; although always produced in precisely the same situations

as the tetraspores and polyspores, they are agglomerations of little

colourless cells, either united in a bunch, as in Griffithsia, or

enclosed in a transparent cylinder, as in Polysiphonia, or covering a

kind of expanded disc of peculiar form, as in Laurencia. According to

competent observers, the cells contain spermatozoids. Nägeli describes

the spermatozoid as a spiral fibre, which, as it escapes, lengthens

itself in the form of a screw. Thuret, on the contrary, says the

contents are granular, and offer no trace of a spiral filament, but

are expelled from the cells by a slow motion. The antheridia appear in

their most simple form in Callithamnion (Plate II., Nos. 32 and 34),

being reduced to a small mass of cells composed by numerous little

bunches which are sessile on the bifurcations of the terminal branches.

The spores are simpler structures than the tetraspores, and mostly

occupy a more important position. They are not scattered through the

frond, but grouped in definite masses, and generally enclosed in a

special capsule or conceptacle, which may be mistaken for a tetraspore

case. The simplest form of the spore fruit consists of spherical

masses of spores attached to the wall of the frond, or imbedded in

its substance, without a proper conceptacle; such a fruit is called

a _favellidium_, and occurs in Halymenia; the same name is applied to

the fruits of similar structures not perfectly immersed, as those of

Gigartina, Gelidium, &c., where they form tubercular swellings on the

lobes. In some, the tubercles present a pore at the summit, through

which the spores emerge forth. In other cases, as in Ceramium (Plate

II., Nos. 27 and 37), the spores occupy a more conspicuous place; a

characteristic species is Delessaria (Plate II., No. 39), the coccidium

either occurring on lateral branches, or is sessile on the face of the

frond, when it consists of a case filled with angular-shaped spores

attached to the wall of the case. The general external appearance of

the red algæ is very varied, but it seems to attain to its deepest

colouring in the Red Sea, which, it is said, is entirely due to the

peculiarly vivid red seaweed. They are all exquisite objects for the

microscope, as may be surmised from the few varieties presented in

Plate II. The Florideæ of the warmer seas exhibit most elegantly formed

fronds, as will be seen on reference to the “Phycologia Australica” of

the late Dr. William Harvey, F.R.S.

The Characeæ may be placed among the highest of the algals, if only

for the complexity of their reproductive organs, which certainly

offer a contrast in their simplicity of structure. _Chara vulgaris_,

stonewort, is a simple fresh-water plant, preferring still freshwater

ponds or slow-moving rivers running over a chalky soil. It thus derives

the calcareous matter found in the axis of the plant, together with a

small portion of silica. Its filaments (or branches, as some botanists

prefer to call them) are given off in whorls. The Characeæ are a small

family of acrogens, consisting of only two or three at most. They are

monœcious and diœcious, the two kinds of fruit being often placed close

together. They may easily be grown in a tall glass jar for observation.

All that is necessary is to put the jar occasionally under the house

tap and let the water run slowly over the top for a short time, thus

renewing the contents without disturbing the plant. The hard water

supplied to London suits chara better than softer water. Both chara and

nitella are objects of great interest to microscopists, since in the

former the important fact of vegetable circulation was first observed.

A portion of the plant of the natural size is shown in Fig. 290, No. 1.

Characeæ.

[Illustration: Fig. 290.--Diagrammatic sketch of Chara.

1. A stem of _Chara vulgaris_, natural size; 2. Magnified view (arrows

indicating the course taken by the chlorophyll); 3. A limb, with buds

protruding; 4. Portion of a leaf of _Vallisneria spiralis_, showing

cyclosis of chlorophyll granules.]

Each plant is composed of an assemblage of long tubiform cells placed

end to end, with fixed intervals, around which the branchlets are

disposed with great regularity. In nitella the stem and branches are

composed of simple cells, which sometimes attain to several inches in

length. Each _node_, or zone, from which the branches spring, consists

of a single plate, or layer, of small cells, which are a continuation

of the cortical layer of the internode (Fig. 290, No. 3) as an

outgrowth. Each cell is partially filled with chlorophyll granules,

and it is these that are seen under the microscope taking the course

shown by the arrows (Fig. 290, No. 2). The rate of movement of the

granules is accelerated by moderate warmth and retarded by cold. It

is in viewing the circulation in water plants that the warm stage of

the microscope is brought into use. Borne along with the protoplasmic

stream are a number of solid particles consisting of starch granules

and other matters. The method of viewing the circulation is by

cutting sections off a portion of the plant with a very sharp knife,

and arranging them in a growing cell with a few drops of water, and

covering over with a thin cover-glass.

[Illustration: Fig. 291.--The Fructification of _Chara fragilis_.

A. Portion of filament containing “antheroids”; B. A group of

antheridial filaments, composed of a series of cells, within each of

which antherozoids are formed; C. The escape of mature antherozoids,

with whip-like prolongations, about to swim off; D. Antherid supported

on flask-shaped pedicle; E. Nucule enlarging, and seen to contain

oospores; F. Spores and elaters of Equisetum; G. Spores surrounded by

elaters of Equisetum.]

The reproductive process of Chara is effected by two sets of bodies,

both of which are placed at the base of the branches (Fig. 291, E and

D) either on the same or different plants, one set known as globules

or _antherids_, and the other as nucules, containing the oospores or

_archegones_. These are often of a bright red colour, and have covering

plates, or shields (B and E), curiously marked, and the central portion

is composed of a number of filaments rolled up (as in E) or free (as

seen at B), projecting out from the centre of the sphere. The antherid

is supported on a short flask-shaped pedicle, which projects into the

interior. At the apex of each of the eight manubria is a roundish

hyaline cell, termed a capitulum, and at its apex again six smaller

or secondary capitula. The long whip-shaped filaments are divided by

transverse septa into a hundred or more compartments, every one of

which is filled with an antherozoid (as at A), consisting of a spiral

thread of protoplasm packed into two or three coils; these escape and

become free (as seen at C), each having two long fine flagella. The

young antherozoid swims off with a lashing action, and the whole field

appears for a time filled with life. They swim about freely, but their

motion gradually ceases, and soon they arrive at a state of inaction.

_Nitella_ appears to have a somewhat different mode of fructification

to that of its congener. It puts forth a long filamentous branch

from one of its joints, which, on reaching the surface of the water,

terminates in a whitish fruit-like cluster. It is even a more delicate

and less robust algal than chara, and every care should be taken to

imitate the still water in which it grows. It delights in shady woods

and in calcareous open pools.

Similar care is requisite with regard to Vallisneria; and a more equal

temperature is better suited to the growth of this aquatic plant. It

should be planted in the middle of the jar or aquarium, about two

inches deep in mould, closely pressed down, then gently fill the jar

with water. When the water requires changing, a small portion only

should be run off at a time. It appears to thrive in proportion to the

frequency of changing the water, and taking care that the water added

rather increases the temperature than lowers it.

The natural habitat of the _Frog-bit_, another water-plant of much

interest, is found on the surface of ponds and ditches; in the autumn

its seeds fall, and become buried in the mud at the bottom during

the winter; in the spring these plants rise to the surface, produce

flowers, and grow throughout the summer. Chara may be found in many

places around London, and in the upper reaches of the Thames.

_Anacharis alsinastrum._--This remarkable plant is so unlike any other

water-plant that it may be at once recognised by its leaves growing _in

threes_ round a slender stem. It is also known as “Waterthyme,” from a

resemblance it bears to that plant.

The colour of the plant is deep green; the leaves are nearly half an

inch long, by an eighth wide, egg-shaped at the point, with serrated

edges. Its powers of increase are prodigious, as every fragment is

capable of becoming an independent plant, producing roots and stems,

and extending itself indefinitely in every direction. The specific

gravity of it is so nearly that of water, that it is more disposed to

sink than float. A small branch of the plant is represented, with a

hydra attached to it, in a subsequent chapter.

The special cells in which the circulation is most readily seen are the

elongated cells around the margin of the leaf and those of the midrib.

On examining the leaf with polarised light, the cells are observed to

contain a large proportion of silica, and present a very interesting

appearance. A bright band of light encircles the leaf, and traverses

its centre. In fact, the leaf is set, as it were, in a framework of

silica. By boiling the leaf for a short time in equal parts of nitric

acid and water, a portion of the vegetable tissue is destroyed, and the

silica rendered more distinct, without changing the form of the leaf.

It is necessary to make a thin section or strip from the leaf of

Vallisneria for the purpose of exhibiting the circulation in the cells,

as shown in Fig. 290, No. 4. Among the cell granules, a few of a more

transparent character than the rest, are seen to have a nucleolus

within.

The phenomenon of cell cyclosis occurs in other plants beside those

growing in water. The leaf of the common plantain or dock, Plantago,

furnishes a good example, the movement being seen both in the cells of

the plant and hairs of the cuticle torn from the midrib.

_Cell-division._--In order to study the process of cell-division the

hairs on the stamens of Tradescantia should be taken. Remove one from

a bud on a warm day and let a drop of a one per cent. sugar solution

fall upon it, and cover it with a thin glass cover. Place it for a

short time in a _moist-chamber_ (Fig. 256), and then examine it with

a magnifying power of 500 diameters. The nucleus of the cell will

be seen, near its terminal position, to gradually elongate in the

direction of the longer axis of the cell and become more granular,

while the protoplasm moves towards the extreme end; the nucleus at

the same time will present a striated appearance, with the fibrilla

arranged parallel to the longer axis of the nucleus, and at length

approach each other at the poles. A nuclear spindle will now be

produced, and the fibres ruptured in the equatorial plane, so that

two nuclei will be found in place of the one. The best preparations

of nuclei are obtained by making thin longitudinal sections of

actively-growing plants (young rootlets of Pinus, for example), and

staining them with hæmatoxylin in the manner described in a former

chapter.

Desmidiaceæ and Diatomaceæ.

The two groups of Desmidiaceæ and Diatomaceæ differ so little in

their general characters that they may be spoken of as members or

representative families of microscopic and unicellular algæ alike in

their remarkable beauty and bilateral symmetry, and of such peculiar

interest as to call for special notice. Desmids differ from diatoms

chiefly in colour, in lacking a non-silicious skeleton, and in their

generative process, which for the most part consists in the conjugation

of two similar cells. Diatoms, on the other hand, have dense silicious

skeletons and a general absence of green colouring matter. Ralfs, in

his systematic monograph, enumerates twenty genera of desmids. The

limiting membrane is alike firm and flexible, since it exhibits some

elasticity and resistance to pressure, and is not readily decomposable.

Traces of silica are found in only a few of the desmids, while the

frustule of the diatom is chiefly composed of this substance; both

have an external membranous covering, so transparent and homogeneous

in structure as to be in danger of being entirely overlooked, unless

some staining material is used, together with a high-power objective

possessing considerable penetration. In some species, however, the

mucous covering is more clearly defined, as in Staurastrum and

_Didymoprium Grevelli_. Openings occur in the outer membrane of other

species, as the Closterium.

[Illustration: PLATE X.

DESMIDIACEÆ.]

Many species of desmids have a power of motion, the cause of which must

be due either to cilia or a flagellate organ. This, however, is denied

by some observers, who regard their movements as due to an exudation

of the mucilaginous contents of the cell, to exosmose, or diffusion,

neither of which hypotheses will at all help us to understand the

gliding movements of the Oscillariæ or the sharp jerky movement of the

Schizonema. The movements of desmids are especially exerted when in

the act of dividing, and by sunlight, towards which they are always

observed to move. The force with which some diatoms move about is very

great, and this can only be satisfactorily explained by admitting a

specialised organ.

The appearance of the Desmidiaceæ (Plate X.) is much modified by

their eminences, depressions, and processes, as well as that of the

surface, the margin of the fronds, and the depth and width of the

central constriction. The surfaces may be dotted over irregularly,

the dots themselves being elevated or depressed points in their

structural character. The margins of some have a dentate appearance,

as in Cosmarium. In the elongated forms, such as Penium, the puncta

are disposed in lines parallel to the length. In several these lines

are either elevations or furrows, it is not always easy to say which;

they are peculiar, however, to the elongated forms of Closterium.

When the lines are fine they produce a striation of the surface, but

in order to discover this the fronds should be viewed when empty and

by a fairly good power. The modification of surface in several genera

seems to be due, not to mere simple appendages, but to expansion of the

limiting membrance into thickened processes, and which may terminate

in spines, as in Xanthidium and Staurastrum (Plate X., Nos. 8-19 and

22). A general distribution over the surface is characteristic of the

former, but in Euastrum the surfaces are very irregular, and therefore

described as “swellings or inflations.” Micrasterias has its margin

deeply incised into lobes, which in some have a radiating arrangement;

when the lobes on the margin are small they constitute crenations or

dentations. The fronds of _Euastrum binatum_ are bicrenate on the

sides, as are those of Desmidium and Hyalotheca and other species.

Another variety of margin exists, known as undulating or wavy, while

the general concavity or convexity of the margins furnish other

specific characteristics.

_Pediastreæ_ (Plate X., Nos. 24-29).--The members of this family

formerly included the Micrasterias and Arthrodesmius of Ehrenberg.

From their arrangement of cells in determinate numbers and definite

forms, it has been thought by some observers that they should be

removed from the desmids to a special or sub-family. The points

of difference consist in the firmness of the outer covering, in

the frequent interruptions on the margin of the cells, and in the

protrusion of “horns,” or rather a notch more or less deep. It is true

that the cells are not made up of two symmetrical halves, and that

they are in aggregation, which is not (except in the Scenedesmus, a

genus that distinctly connects this group with desmids) in linear

series, but in the form of discoidal fronds. They, however, divide

into 8, 16, or 32 gonidia, and these move about for some time before

the formation of a new frond. It was Nägeli who first instituted a

sub-genus of Pediastrum, under the designation of Anomopedium, the

chief characteristic of which is the absence of bilobed peripheral

cells. In Cœlastrum the cells are hexangular, the central ones very

regularly so; in Sorastrum they are wedge-shaped, or triangular, with

rounded-off angles. Viewed laterally the cells appear oblong. The cells

of Pediastrum are considerably compressed, so that when aggregated

they form a flattened tubular structure; in figure they are polygonal,

frequently hexagonal, a shape owing, in all probability, to mutual

lateral pressure during growth. There is a pervading uniformity in

the contents of the cells of the different genera, which consist of

protoplasmic endochrome. At first the colour is pale green, but it

becomes deeper with full maturity, while the decaying cells are seen

to change to a deep reddish-yellow or brown. The protoplasm is also

clear and homogeneous, but in time granules appear, enlarge, and

multiply in number; moreover, each cell presents a single bright green

vesicle, around which are collected clear circular spaces or globules,

recalling those of Closterium, and varying in number from two to six or

more, their position not being regulated by the partition wall as in

Palmellæ, but by the centre of the entire frond. Oil globules are also

contained in the cells; their presence is indicated by the addition of

a drop of tincture of iodine. On one occasion Nägeli saw in _Pediastrum

boryanum_ the endochrome disposed in a radiating manner, an arrangement

which often obtains in algals and in other vegetable cells with a

central nucleus. The cells of Pediastreæ are always united together in

compound fronds, as represented in Plate X., Nos. 24 and 29.[56]

The differences pointed out in no way constitute a claim to remove

Pediastreæ from among Desmidiaceæ, certainly not to rank as a distinct

species.

_Reproduction of Desmidiaceæ._--A true reproductive act is presented

by the conjugation or coupling of two fronds, and by the resulting

development of a sporangium and subsequent interchange of the contents

of the two cells. At another time self-division is frequently seen

to take place in all respects as in the cells of other algæ. The

proceeding is varied in some essential particulars by the form of

the fronds and by other circumstances; as in fission of Euastrum,

for instance (seen in Plate X., Nos. 1, 2, and 12), when the narrow

connecting bands between the two segments of the fronds are rapidly

pushed aside by growth and finally divide. Two modes of conjugation of

fronds are represented in Plate X., Nos. 25 and 33, in Closterium and

Penium. The act of conjugation admits of variations in character, as

shown in Staurastrum and Microsterias; the contents of both fronds are

discharged into a delicate intermediate sac; this gradually thickens

and produces spines (Plate X., Nos. 8 and 19). In Didymoprium the

separate joints unite by a narrow process pushed out from each other,

often of considerable length, through which the endochrome of one

cell is transferred to the other, and thus a sporangium is produced

within one of two cells, just as in the conjugatæ (No. 5). In _Penium

Jennereri_ the conjugation takes a varied form; the fronds do not open

and gape at the suture, but couple by small but distinct cylindrical

tubes (No. 27).

Among those enumerated, the compressed and deeply constricted cells

of Euastrum offer the more favourable opportunities for studying the

manner of their division; for although the frond is really a single

cell, in all its stages it appears like two, the segments being

always distinct, from the earliest stage. The segments, however, are

separated by a connecting link, which is subsequently converted into

two somewhat round hyaline bodies. These bodies gradually increase and

acquire colour, and as they grow the original segments are further

divided, and at length become disconnected, each taking a new segment

to supply the place of that from which it is separated. It is curious

to trace the progressive development of the newer portions, which at

first are devoid of all colour; but as they become larger a faint green

tint is observed, which gradually darkens, and then assumes a granular

appearance. Soon the new segments attain their normal size, while the

covering in some species shows the presence of puncta. In Xanthidium,

Plate X., Nos. 9, 10, and Staurastrum, Nos. 15-18, the spines and

processes make their appearance last, beginning as mere tubercles, and

then lengthening until they attain their perfect form and size, armed

with setæ; but complete separation frequently occurs before growth is

fully completed. This singular process is repeated again and again, so

that the older segments are united successively, as it were, with many

generations. When the cells approach maturity, molecular movements may

be at times noticed in their contents, precisely similar to what Agardh

and others aptly term “swarming.” Meyen describes this granular matter

as starch.[57] Closterium, early in the spring, when freshly secured

and exposed to light, presents a wonderful appearance, these bodies

being kept continually in motion at both ends of the frustule by the

ciliary action within the cell, and the whole frond is seen brilliantly

glittering with active cilia. When a gleam of stronger light is

allowed for a moment to fall on the frond, the rapid undulations of

the cilia produce a series of most delicate prismatic Newton’s rings.

The action and distribution of the cilia, together with the cyclosis

of the granular bodies in the frond, are better seen by the aid of

Wenham’s parabola or a good condenser with a central stop. One of the

wide angular objectives shows the circulation around the marginal

portions of the whole frond. The stream is seen to be running up the

more external portion, internal to which is another stream following a

contrary direction; this action, confined to the space between the mass

of endochrome and the outer portion of the cell-wall, is seen to pass

above or around the space in which cyclosis of the spores is taking

place.

During the summer of 1854, the late Rev. Lord Sidney Godolphin

Osborne and myself became much interested in the remarkable family of

Closteria. Fig. 292 is a highly magnified view of _Closterium lunula_

which I drew by the aid of the camera-lucida at the time. There could

be no doubt about the ciliary action within the frond: it was in

every way similar to that of the branchiæ of the muscle, the same

wavy motion, which gradually became slower as the death of the desmid

drew near. This was brought about earlier when the cell was not kept

supplied with fresh water.

[Illustration: Fig. 292.--_Closterium lunula._]

In diagram A, line _b_ points to a cluster of ovoid bodies; these

are seen at intervals throughout the endochrome within the investing

membrane. These bodies are attached to the membrane by small pedicles,

and are occasionally seen in motion about the spot, from which they

eventually break away, and are carried off, by the circulating fluid,

to the chambers at the extremities of the frond; there they join a

crowd of similar bodies, in constant motion within the chambers, when

the specimen is quite fresh. That the action of these free granules or

spores is “Brownian,” as surmised by some writers, is in my opinion

entirely fallacious. It is doubtless in a measure due to the current

brought about by the ciliary motion of the more fluid contents of the

cell.

The circulation, when made out over the centre of the frond, for

instance at _a_, is in appearance of a wholly different nature from

that seen at the edges. In the latter the matter circulated is that

of granules, passing each other in distinct lines, but in opposite

directions; in the circulation as seen at _a_, the streams are broad,

tortuous, of far greater body, and passing with much less rapidity. To

see the centre circulation, use a Gillett’s illuminator and a 1/8th or

a 1/10th immersion; work the fine adjustment so as to bring the centre

of the frond into focus, then almost lose it by raising the objective;

after this, with great care, work the milled head until the darker body

of the endochrome is clearly brought out.

At B is an enlarged sketch of one extremity of the frond. The arrows

within the chamber pointing to _b_ denote the direction of a strong

current of fluid, which can be occasionally followed throughout. It is

acted upon by cilia at the edges of the chamber, the greater impetus

appearing to come from the centre of the endochrome. The fluid is here

acting in positive jets, that is, with an almost arterial action; and

according to the strength with which it is propelled at the time, the

loose floating bodies are sent to a greater or less distance from the

end of the frustule; the fluid is thus impelled from a centre, and kept

in activity by the lateral cilia, that create a rapid current and give

a turning motion to the free bodies. The line--_a_, in this diagram,

denotes the outline of the membrane which encloses the endochrome; on

both sides cilia can be seen. The circulation exterior to it passes and

repasses in opposite directions, in three or four distinct courses;

these, when they arrive at--_c_, seem to encounter a stream making

its way towards an aperture at the apex of the chamber; then they

appear to be driven back again by a stronger force. Some, however, do

occasionally enter the chamber, but very rarely will one of the bodies

escape into the outer current, and should it do so, is carried about

until it becomes adherent to the side wall of the frond.

With regard to the propagation of the _C. lunula_, I have never seen

anything like conjugation; but I have repeatedly seen self-division

(shown at D _a_ _a_). This act is chiefly the work of one half of

the frond. Having watched for some time, one half is seen to remain

passive, while the other has a lateral motion from side to side, as if

moving on an axis at the point of juncture; the motion increases, is

more active, until at last with a jerk one segment separates itself

from the other, as seen at E. It will be noticed that each end of the

segment is perfectly closed before separation finally takes place;

there is, however, only one perfect chamber, that belonging to the

extremity of the original entire frond. The circulation continues

for some time previous to and after subdivision, in both fronds, and

by almost imperceptible degrees increases in volume. From the end of

the endochrome symptoms of elongation of the frond take place, the

semi-lunar form gradually changes, elongates, and is more defined,

until it takes the form and outline of the fully-formed frustule at

the extremity. The obtuse end--_b_ of the other portion of frond is at

the same time elongating and contracting, and in a few hours from the

division of the one segment from the other the appearance of each half

is that of a nearly perfect frustule, the chamber at the new end is

complete, the globular circulation exterior to it becomes affected by

the circulation from within the said chamber, and, shortly afterwards,

some of the free bodies descend, and become exposed to the current

already going on in the chamber. E is a diagram of one end of a _C.

didymotocum_, in which the same process was well marked, and completed

while it was under observation.

It will appear to most observers that if the continuation of the

widely-spread family of Desmidiaceæ was wholly dependent upon

conjugation and subdivision of their frustules, a process requiring

several hours for its completion, the whole species must have long

ago disappeared. It may be presumed then that some other mode of

reproduction must prevail. In the fresh-water algæ the two more general

methods of multiplication are clearly governed by the conditions of

the seasons; the resting-spores securing continuity of life during

the winter, the swarm-spores spreading the plant profusely during the

warmer portion of the year, when rapid growth is possible. I therefore

regard the actively swarming bodies seen in continuous motion at the

two extreme portions of the frustule of _Closterium lunula_ as being

either oospores or zoospores, by means of which reproduction takes

place.

=Diatomaceæ=, commonly called brittleworts, Plate XI., are chiefly

composed of two symmetrical valves, narrow and wand-like, navicular,

miniature boat-shaped, hence their name _Navicula_ (little ship).

Hitherto they have excited the deepest interest among microscopists

because of their wonderfully minute structure, and the difficulty

involved in determining their exact nature and formation. Each

individual diatom has a silicious skeleton, spoken of as a frustule,

frond, or cell, having a rectangular or prismatic form, which mostly

obtains in the whole family, the angles of the junction of the two

united valves being, as a rule, acute, and enclosing a yellowish-brown

endochrome. Deeply-notched frustules, like those of the Desmidiaceæ, do

not occur, and the production of spines and tubercles so common in that

family is rare in the Diatomaceæ. Great variety of outline prevails, so

much so that no rule in this respect can be formulated.

The frustules, however, are usually composed of two equal and

similar halves, but exceptions to this are found in the Actinomtheæ,

Cocconcidæ, and one or two other families. The extremities of some

species, _e.g._, Nitzshia and Pleurosigma, are extremely elongated,

forming long, filiform, tubular processes; in Biddulphia and

Rizoselenia, short tubular processes from their margins. Great variety

of outline may prevail in a genus, so considerable indeed that no

accurate definition can be given, the characteristics shading off

through several species until the similarity to an assumed typical

form is much diminished, which may again be modified by accidental

circumstances that surround the development of the silicious frustule.

It must not be forgotten that the figure is greatly modified or

entirely changed by the position of the valves, whether seen in one

position or another, as already explained in connection with “Errors

of Interpretation.” Again, in the genera Navicula, Pinnularia (Plate

II., Nos. 33, 38, and 40), and others, the frustules are in one aspect

boat-shaped, but in the other either oblong with truncated ends, or

prismatic. In the genus Triceratium (Plate XI., No. 10), the difference

of figure is very remarkable as the front or side view is examined.

The sudden change in appearance presented to the eye as the frustule

is seen to roll over is rather peculiar. As a rule, therefore, we must

examine all specimens in every aspect, to accomplish which very shallow

cells should be selected, say of 1/100th of an inch deep, and covered

with glass 1/250th of an inch thick. A good penetrating objective

should be used, and careful illumination obtained. The Diatomaceæ are

perhaps more widely distributed than any other class of infusorial

life; they are found in fresh, salt, and brackish water; many grow

attached to other bodies by a stalk (Plate II., No. 33, Licmophora and

Achnanthidium); while others, as Pleurosigma, No. 40, swim about freely.

[Illustration: PLATE XI.

DIATOMACEÆ, RECENT AND FOSSIL.]

There are a considerable number of Diatomaceæ which, when in the young

state, are enclosed in a muco-gelatinous sheath; while others are

attached by stipes or stalk to algæ. It would be vain, in a limited

space, to attempt a description of this numerous and extensive family.

Nägeli and other observers describe a “mucilaginous pellicle on the

inner layer of the valves,” while, as Menghine observes, “an organic

membrane ought to exist both inside and outside, for the silica could

not become solid except by crystallizing or depositing itself on some

pre-existing substance.” The surface of the frustules is generally

very beautifully sculptured, and the markings assume the appearance

of dots (puncta), stripes (striæ), ribs (costæ), pinnules (pinnæ),

of furrows and fine lines; longitudinal, transverse, and radiating

bands; canals or canaliculi; and of cells or areolæ; whilst all present

striking varieties and modifications in their form, character, and

degree of development. Again, the fine lines or striæ of many frustules

are resolvable into rows of minute dots or perforations, as occur in

_Pleurosigma angulatum_, delineated in the accompanying microphotograph

(Fig. 294), taken for the author purposely to show the markings on this

especially selected test diatom.

[Illustration: Fig. 293.

1. _Pleurosigma attenuatum_; 2. _Pleurosigma angulatum_; 3.

_Pleurosigma Spencerii_. Magnified 450 diameters.]

The nature of the markings on the diatom valves is one of considerable

interest, and attempts have been made to produce them artificially, but

without success.

[Illustration: Fig. 294.--_Pleurosigma angulatum_, magnified 4500

diameters.

(From a microphotograph taken by Zeiss with the 2 mm. aprochromatic

objective, 1·30 numerical aperture, and projection eye-piece, No. 4.)]

Professor Max Schultze devoted a great amount of time to the

investigation of the subject, and has recorded in a voluminous

paper[58] the results of his observations. He says, “Most of the

species of the Diatomaceæ are characterised by the presence on their

outer surface of certain differences of relief, referable either

to elevations or to depressions disposed in rows. The opinions of

microscopists with respect to the nature of these markings are still

somewhat divided. Whilst in the larger forms, and those distinguished

by their coarser dots, the appearance is manifestly due to the

existence of thinner spots in the valve, we cannot so easily explain

the cause of the striation or punctation in _Pleurosigma angulatum_ and

similar finely-marked forms.”

Dr. R. Zeiss some time ago furnished me with a microphotograph of

a frustule magnified 4500 diameters that seemed to confirm Mr. T.

F. Smith’s view of the structure of these valves. Dr. Van Heurck

has also made a study of this diatom, and concludes that the valves

consist of two membranes of thin films, and of an intermediate layer,

the outer being pierced with openings. The outer membrane is, he

believes, “so delicate that it is easily destroyed by acid or by

friction, and the several processes employed in cleaning and preparing

it for microscopical examination. When the openings or apertures of

the internal portion are arranged in alternate rows they assume the

hexagonal form; when in straight rows, the openings are seen to be

square or oblong.” A description hardly in accord with Fig. 294.

Movements of Diatoms.

The late Professor Smith, in his “Synopsis of Diatoms,” refers to their

movements in the following terms: “I am constrained to believe that

the movements observed in the Diatomaceæ are due to forces operating

within the frustule, and are probably connected with the endosmotic

and exosmotic action of the cells. The fluids which are concerned in

these actions must enter, and be emitted through the minute foramina at

the extremities of the silicious valves.” Schultze’s researches, which

were made at a later date, carried this debatable question somewhat

further. He is of opinion “that a sarcode (protoplasmic) substance

envelops the external surface of the diatoms, and its movements are due

to this agent exclusively.” His investigations were mainly confined

to _P. angulatum_, and to the larger _P. attenuatum_ (Fig. 293, 1 and

2), as the transverse markings on the frustule do not impede to so

great an extent the observation of what is going on within. The living

specimen of _P. angulatum_ under the microscope usually has its broad

side turned to view, with one long curved “raphe” uppermost, and the

other in contact with the glass cover (Fig. 293). Within the frustule

the yellow colouring matter, “endochrome,” fills the cavity more or

less completely. In the broader part of the frustule these bands

of endochrome describe one or two complicated windings. It is only

possible in those specimens in which the bands are narrow to properly

trace their foldings, and determine their number. The next objects

which strike the eye on examining a freshly-gathered Pleurosigma are

numerous highly refractive oil-globules. These are not, however, all in

the same place, and one globule appears nearer the observer than the

other; their relative position is best seen when a view of the narrow

side of the frustule can be obtained, so that one raphe is to the left

and the other to the right. The blue-black colour which is assumed by

these globules after treating with acid demonstrates their oleaginous

nature. The middle of the cavity of the frustule is occupied, in the

larger navicula, by two large oil-globules (seen in the diagrammatic

Fig. 295), and by a colourless finely granular mass, whose position

in the body is not so clearly seen in the flat view as in the side

view. Besides the central mass, the conical cavities at either end of

the frustule are seen to enclose granular substance, and two linear

extensions from each of three masses are developed, closely underlying

the raphæ. In the side view, therefore, they appear attached to the

right and left edges of the interior of the frustule. This colourless

granular substance carries in its centre, near the middle part of the

diatom, an imperfectly developed nucleus which is not very easy to see,

but may be demonstrated by the application of an acid. The colourless

substance is protoplasm, and encloses numerous small refractive

particles; this, on adding a drop of a one per cent. solution of

osmic acid, is coloured blue-black, and proves to be fat. It is,

however, exceedingly difficult to determine the exact limitations of

the protoplasm, on account of the highly refractive character of the

silicious skeleton, and the obstruction to the light presented by the

endochrome.

At a short distance the protoplasm reappears, contracted into a

considerable mass, within the terminal ends of the frustule. Schultze

observed in this part of the protoplasm a rapid molecular movement,

“cyclosis,” such as occurs in Closterium, and also a current of the

granules of the protoplasm along the raphe. “_Pleurosigma angulatum

‘crawls,’_ as do all diatoms possessing a raphe, along this line of

suture. To crawl along, it must have a fixed support.” “There is

obviously,” adds Schultze, “but one explanation; it is clear that there

must be a band of protoplasm lying along the raphe, which causes the

particles of colouring matter to adhere, and gives rise to a gliding

movement. For there is but one phenomenon which can be compared with

the gliding motion of foreign bodies on the Diatomaceæ, and that is,

the clinging to and casting off of particles by the pseudopodia of the

rhizopod, as observed, for instance, on placing a living Gromia or

Miliolina in still water with finely-powdered carmine. The nature of

the adhesion and of the motion is in both cases the same. And since,

with diatoms as unicellular organisms, protoplasm forms a large part

of the cell (in many cases two distinctly moving protoplasms), this

implies that the external movements are referable to the movements of

the protoplasm.” It is quite evident to those who have studied the

movements of diatoms that they are surrounded by a sarcode structure

of a more pellucid character than that of Amœba. Six years before

Schultze’s observations were published, I wrote in a third edition

of my book, page 307, “The act of progression favours the notion of

contractile tentacular filaments--_pseudopodia_--as the organs of

locomotion and prehension.”

Since my former observations on the movements of diatoms, I have given

much attention to two forms, _P. angulatum_ and _Pinnularia_. The

powers used were Hartnack’s No. 8, and Gunlack’s 1/16-inch immersion;

Gillett’s condenser illumination, with lamp flame edge turned to mirror

and bull’s-eye lens; a perforated slide with a square of thin glass

·006 cemented to it, and a cover-glass of ·005. So far as I could

satisfy myself, no terminal space, as in the Closteria, could be seen,

otherwise the course of the gemmules is as freely traced as in that

form. They are more minute than the _Closterium lunula_ granules, more

steadily or slowly seen to pass up and down one half the frustule

towards the extremity, one half of the current seeming to turn round

upon its axis and descending towards the other. The granules were

thickly scattered at the apex, but gradually became fewer, and the

ascending and descending current tapered away towards the central

nodule, which became more filled up or closed in.

[Illustration: Fig. 295.--Outline sketches of Pinnulariæ, showing

vesicles.]

[Illustration: Fig. 296.--_Gomphonema constrictum._ (From a

microphotograph.)]