This beautiful sight was not confined to one frustule, but was

exhibited in all that were in a healthy condition. I examined several,

and watched them for a long time. The phenomenon described depends

much upon the healthy condition of the frustule at the time; as the

movements of the diatoms became sluggish, the circulation gradually

slackens and then ceases altogether. I also saw a somewhat similar

action in the more active specimens of _P. hippocampus_ and _Navicula

cuspidata_, but the coarser markings and thickness of the wall of these

diatoms seemed to place greater difficulties in the way of observation

than the finer valves of the _P. angulatum_. One thing I believe is

certain, that the circulation described is precisely similar to that

seen in the Closteria, or, on a much larger scale, in Chara and the

leaf of the Anacharis, bearing in mind also that in the Closterium

the cell is divided by a transverse suture, and in _P. angulatum_

by a longitudinal one (Plate II., Nos. 38-40). About the same time

some very lively specimens of the Pinnulariæ were sent to me, and the

movements of these frustules were more closely observed. One or two

of the more active would attack a body relatively larger than itself,

it would also force its way into a mass of granular matter, and then

recede from it with a jerky motion. In more than one instance a cell

of Palmoglæa was seized and carried away by the Pinnularia, the former

at the time being actively engaged in the process of cell division.

Other diatoms present among my specimens were also in an active

condition, and the circulation of granular matter in all was distinctly

visible. In the Pinnulariæ two large colourless vesicles were seen on

either side of the median nodule, each having a central nucleus, as

represented in the accompanying sketch, made while under observation

in two positions. The central portion of each frustule was closely

packed with a rich yellowish-brown coloured endochrome, interspersed

with a few fat globules. The phenomenon of cyclosis was not seen in

any of these diatoms, but I have satisfied myself, by staining, of the

presence of a delicately fine external protoplasmic covering in many

diatoms. That their movements resemble the gliding movements exhibited

by the Amœba can scarcely be doubted. Numerous forms of Diatomaceæ are

found growing on or attached to water-plants or pieces of detached

stalks, which, although generally simple, are sometimes compound,

dividing and subdividing in a beautiful ramous manner. Pinnulariæ,

Nitzschia, &c., are seen adherent by one extremity, about which they

turn or bend themselves as on a hinge. By the process of cell-division,

groups of Synedræ become attached by a point, in a fan-like form.

The fan-like collection of frustules is said to be flabellate, or

radiate. In Licmophora, Achnanthes and other species (Plate II., Nos.

29-33) the double condition of union of frustules and of attachment by

a pedicle are illustrated. When a stipe branches it does so normally

in a dichotomous manner, each new individual being produced by a

secondary pedicle. This regular dichotomy is seen in several genera:

Cocconema and Gomphonema, the latter more perfectly in Fig. 296, from a

microphotograph, in which a branching, or rather longitudinal, rupture

of the pedicle takes place at intervals, and the entire organism

presents a more or less complete flabella, or fan-like cluster, on the

summit of the branches, and imperfect or single frustules irregularly

scattered throughout the whole length of the pedicle.

_Isthmia enervis_ (Fig. 297).--The unicellular frustule of this species

is extremely difficult to define, owing to the large areolations of the

valves; it has a remarkable internal structure. The olive-brown cell

contents are found collected, for the most part, into a central mass,

from which radiating, branched, granular threads extend to and unite

with the periphery. When viewed by a magnifying power of 600 or 700

diameters, these prolongations are seen to be composed of aggregations

of ovate or spindle-shaped corpuscles, held together by protoplasmic

matter. These bodies are sometimes quiescent, but more often travel

slowly to and fro from the central mass. The general aspect under these

conditions so nearly corresponds to the characteristic circulation in

the frustules of unicellular plants and of certain rhizopoda, that it

is difficult to realise that the object when under examination is an

elegant marine diatom.

[Illustration: Fig. 297.--_Isthmia enervis._ Microphotograph.]

There is a large section of diatoms in which the frustules are

diffused throughout a muco-gelatinous envelope in a definite manner.

Histologically this is homologous with the pedicles and connecting

nodules thrown out during the act of self-division, and in some species

(Cocconeis, Fragillaria, &c.) it often persists after that act is

complete.

Diatomaceæ, Recent and Fossil.

[Illustration: Fig. 298.--Fossil Diatoms from Springfield (Barbadoes).

1, Achnanthidium; 2, _Diatoma vulgare_, side view and front view; 3,

Biddulphia; 4, 5, 6, 7, _Amphitetias antediluviana_, front view, with

globular and oval forms; _Gomphonema elongatum_ and _capitatum_.]

_Fossilised Diatomaceæ._--Dr. Gregory was of opinion that a large

number of diatoms separated into species are only transition forms,

and more extended observations have proved that form and outline are

not always to be trusted in this matter. Species-making is a modern

invention, and can hardly apply to the indestructible fossilised forms

of the frustules of Diatomaceæ, with their beautiful sculpturings and

geometrical constructions, which have not been materially changed since

they were first deposited. Startling and almost incredible as the

assertion may appear to some, it is none the less a fact established

beyond all question, that some of the most gigantic mountain-ranges,

as the mighty Andes, towering into space 25,250 feet above the level

of the sea, their base occupying vast areas of land; as also massive

limestone rocks; the sand that covers boundless deserts; and the soil

of many wide-extended plains, are each and all principally composed of

Diatomaceæ. And, as Dr. Buckland once observed: “The remains of such

minute animals have added much more to the mass of materials which

compose the exterior crust of the globe than the bones of elephants,

hippopotami, and whales.”

In 1841 the late Mr. Sollitt, of Hull, discovered the beautiful

longitudinal and transverse _striæ_ (markings) on the _Pleurosigma

hippocampus_. A curved graceful line runs down the shell, in the centre

of which is an expanded oval opening. Near to the central opening the

dots elongate crossways, presenting the appearance of small short bands.

In the vicinity of this town many interesting varieties of Diatomaceæ

have been found, the beauty of the varied forms of which are constantly

under investigation; at the same time some of them are highly useful,

as forming that class of _test objects_ which are better calculated

than many others for determining the excellence and powers of certain

objectives. Mr. Sollitt carefully measured the markings on some of the

frustules and found they ranged between the 1/30000th and 1/130000th of

an inch; the _Pleurosigma strigilis_ having the strongest markings, and

the _Pleurosigma acus_ the finest.

Mr. J. D. Sollitt not only first proposed their use, but he also

furnished the measurements of the lines of the several members of this

family, as follows:--

Amphipleura pellucida, or Acus, 130,000 in the inch, cross lines.

" sigmoidea, 70,000 in the inch.

Navicula rhomboides, 111,000 in the inch, cross lines.

Pleurosigma fasciola, fine shell, 86,000 in the inch, cross lines.

" " strong shell, 64,000 in the inch, cross lines.

" strigosum, 72,000 in the inch, diagonal lines.

" angulatum, 51,000 in the inch, diagonal lines.

" quadratum, 50,000 in the inch, diagonal lines.

" Spencerii, 50,000 in the inch, cross lines.

" attenuatum, 42,000 in the inch, cross lines.

" Balticum, 40,000 in the inch, cross lines.

" formosum, 32,000 in the inch, diagonal lines.

" strigilis, 30,000 in the inch, cross lines.

[Illustration: PLATE XII.

MICRO-PHOTOGRAPH OF TEST DIATOMS.]

Lichenaceæ.

The lichens are a family of autonomous plants, an intermediary group of

algals or cellular cryptogams, drawing their nourishment from the air

through their whole surface medium, and propagating by spores usually

enclosed in asci, and always having green gonidia in their thallus.

Their gonidia, bright coloured globular cells, form layers under the

cortical covering of the thallus, and generally develop in the form

of incrustations, which cover stones, wood, and the bark of trees,

or penetrate into the lamellæ of the epidermis of woody plants. The

gonidia of lichens partake of both the character of vegetative and

reproductive cells.

The thallus in the fructicose group attaches itself by a narrow base,

growing in the form of a miniature shrub. Another group is met with

in a slimy condition--the gelatinous lichens. These species, for the

most part, furnished dyes before the discovery of the coal-tar dyes.

In many of the _Palmella cruenta_, commonly found growing on the walls

and roofs of houses, a colourless acid liquid is found, which, on being

treated with alkali, produces a bright yellow colour; and another,

_Avernia vulpina_, furnishes a brown dye; the _Rocella fuciformis_

and _R. tinctoria_ yield the purple dye substance known as orchil, or

archil, from which the useful blue paper of the chemist for testing

acidity is manufactured. Usnic acid, combined with green and yellow

resins, seems to be more or less a constituent of many lichens.

A vertical section of _Palmella stellata_ is given in Plate I., No.

26, in which the emission of the ripe spores of the lichens is seen

to be not unlike that which takes place in some of the fungi, Pezizæ,

Sphæriæ, &c. If a portion of the thallus be moistened and placed in a

common phial, with the apotheca turned toward one side, in a few hours

the opposite surface of the glass will be found covered with patches of

spores, easily perceptible by their colour; or if placed on a moistened

surface, and one of the usual glass slips laid over it, the latter

will be covered in a short time. As to the powers of dissemination of

these lowly organised plants, an observation led to the conclusion

that the gonidia of lichens have greater powers in this direction

than was formerly supposed. It is found that by placing a clean sheet

of glass in the open air during a fall of snow, and receiving the

melting water in a tube or bottle, quantities of what has been looked

upon as a “unicellular plant” can be taken, the cells of which may be

kept in a dormant condition for a long time during cold weather, but

upon the return of spring warmth and moisture they begin to increase,

by a process of subdivision, into two, four or eight portions; these

soon assume a rounded form, and burst the parent cell-wall open; these

secondary cells then begin to divide and subdivide again, and the

process may go on without much variation for a long time. The phenomena

described may be watched by taking a portion of the bark of a tree on

which Chlorococcus has been deposited, and placing it under a glass to

keep it in a moderately moist atmosphere; the only difference being a

change in colour, caused by the growth of the fibres, as may be seen

on microscopical examination. “And this,” says Dr. Hicks, who first

observed this phenomenon in plant life, “is an instructive point,

because it will be found that the colour varies notably according to

the lichen prevalent in its neighbourhood.”[59] He believes there can

be no doubt that what has been called Chlorococcus is nothing more

than the gonidia of a lichen; and that under suitable conditions,

chiefly drought and warmth, the gonidium often throws out from its

external envelope a small fibre, which, adhering and branching, forms

a “soridium.” “The soridia remain dormant for a very long time, and

do not develop into thalli unless in a favourable situation, in some

cases it may be for years. It will be perceived that the soridium

contains all the elements of a thallus in miniature; in fact, a thallus

does frequently arise from one alone, and the fibres of neighbouring

soridia interlace; thus a thallus is matured very rapidly. This is

one of the causes of the variation of appearance so common in many

species of lichens, more readily seen towards the centre of the parent

thallus. When the gonidia remain attached to the parent thallus, the

circumstances are, of course, more favourable, and they develop into

secondary thalli, attached more or less to the older one, which, in

many instances, decays beneath them. This process being continued

year after year gives an apparent thickness and spongy appearance to

the lichen, and is the principal cause of the various modifications in

the external aspect of the lichens which caused them formerly to be

misunderstood and wrongly classified.”[60]

The erratic lichens are found among the genus Palmella, some of which

grow among boulders of the primary and metamorphic formations, curled

up into a ball, and only fixed to their matrix by a slender thread. The

globular _Lecanora esculenta_ will at times suddenly cover large tracts

of country in Persia and Tartary, where it is eaten by the cattle.

During a scarcity of food a shower of these lichens, Mr. Berkeley tells

us, fell at Erzeroum, and saved the cattle from starvation.[61]

Another group of the Palmella, or Peltigeri, so named from the

target-like discs on their surface, spread their foliaceous fronds

over the ground, and as the fruit is marginal, it gives the thallus

a digitate appearance. These are often spotted over by a little red

fungus. The Lecidinei contains numerous species of the most varied

habits, and always crustaceous, and so closely adherent to the hard

rocks and stones on which they grow, that at length they disintegrate

them. From this low species a higher form arises, with erect branching

stems, and clothed with foliaceous, brightly-coloured scales.

The Coccocarpei is mainly distinguished by having orbicular discs

entirely deprived of the cortical envelope called an excipulum. The

discs spring at once from the medullary stratum, and contain asci and

sporidia similar to those of the minute fungi Sphæriæ. Some of the

lichens are themselves parasitic, and begin existence under the thick

skin of the leaves of tropical plants, and spread encrusting thallus

over their surface, the excipulum and perithecia being black; but

in most cases these are beautifully sculptured, and are interesting

objects for the microscope. Indeed, the sphere-bearing lichens,

with upright stems bearing globular fruit at the extremity of their

branches, are at first indicated by a swelling, but in time the outer

layer bursts and exposes sporidia, which are beautiful objects under

the microscope on account of their spherical form and more or less deep

blue tint. Humble and lowly as lichens may appear to be, they have been

divided into fifty-eight or more genera and 2,500 species. The brothers

Tulasne, De Bary, the Rev. Mr. Berkeley, and others, devoted great

attention to the peculiarities of their structure and natural history.

_Hepaticæ._--An intermediary group of much interest to the microscopist

are the Hepaticæ (liverworts). These are found growing on damp rocks in

the neighbourhood of springs and dripping banks. The scale-moss, the

_Marchantia polymorphia_ (Fig. 299), may be taken as typical of this

little group, with its gemmiparous conceptacles and lobed receptacles,

bearing archegones on transparent glass-like fruit stalks, carrying on

their summits either round shield-like discs or radiating bodies with a

striking resemblance to a wheel without its tyre.

[Illustration: Fig. 299.--_Marchantia polymorphia._]

The liverworts are closely allied to the mosses, and as much difficulty

was experienced in dividing the two, Hooker placed the whole under one

genus, the Jungermannia. More recently, however, they have been divided

into those with a stem and leaves confluent in a frond, Marchantia;

those with stem and leaves distinct, Jungermannia; and those with a

solitary capsule, filiform, bivalved, stalked, with a free central

placentation, Anthocotaceæ. Some botanists have further divided them,

but they are all extensively propagated by gemmæ.

The fronds carry the male organs, or _antherids_, and the disc, in the

first instance, bears the female organs, or _archegones_, and after

a time gives place to the _sporanges_, or spore cases. It is these

bodies which are of so much interest to microscopists; if the plant is

brought into a warm room, they suddenly burst open with some violence

the moment a drop of water is applied to them, and the sporanges are

dispersed in a small cloud of brownish dust. If this dust is examined

under a medium power, it is seen to consist of a number of chain-like

bodies, somewhat like the spring of a small watch; and if the process

of bursting be closely watched, these minute springs will be found

twisting and curling about in every direction. The structure of the

frond itself will be seen to be interesting when cut in the vertical

direction and placed under the microscope.

[Illustration: Fig. 300.--Gemmiparous conceptacle of _Marchantia

polymorphia_, expanding and rising from the surface of a frond. In the

interior are seen gopidial gemmæ already detached by the splitting of

the epiderm.]

The gemmæ of _Marchantia polymorphia_ are produced in elegant

membranous cups, with a toothed margin growing on the upper surface

of the frond, especially in very damp courtyards between the stones,

or near running water, where its lobed fronds are found covering

extensive tracts of moist soil. At the period of fructification the

fronds send up stalks, which carry at their summit round shield-like

radiating discs, which bear upon their surface a number of little

open basket-shaped “conceptacles.” These again expand into singularly

graceful cups (as in Fig. 300), and are found in all stages of

development. When mature, the basket contains a number of little green

round or oblong discs, each composed of two or more layers of cells;

the wall itself being surmounted by a glistening fringe of teeth, whose

edges are themselves regularly fringed with minute outgrowths. The cup

seems to be formed by a development of the superior epidermis, which is

raised up, and finally bursts and spreads out, laying bare the seeds.

The archegones of Marchantia are very curious bodies, while the elater

and spores are even still more so. These are elongated cells, each

containing a double spiral fibre coiled up in the interior. It is the

elasticity of this which tears apart the cell-membrane, and sends

forth the spores with a jerk, and thus assists in their dispersion.

Marchantia is the type of the malloid Hepaticæ.

Musci, Bryophyta.

Mosses are a beautiful class of non-vascular cryptogams. Linnæus

called them _servi_, servants or workmen, as they seem to labour to

produce vegetation in places where soil is not already formed. The

Bryophyta form three natural divisions: the Bryinæ, or true mosses;

the Sphagnaceæ, or peat-mosses; and the Hepaticæ, or liverworts. The

two first are commonly united. In these the sexual organs consist of

antheridia and archegonia, but they are of simpler structure than will

be found in ferns; and the first generation from the spore is asexual.

[Illustration: Fig. 301.--Screw-moss.]

The common or wall screw-moss (Fig. 301) grows almost everywhere, and

if examined closely, is seen to have springing from its base numerous

very slender stems, each terminating in a dark brown case, which

encloses antheroids. If a patch of the moss is gathered when in this

state, and the green part of the base is put into water, the threads of

the fringe will uncoil and disentangle themselves in a most curious and

beautiful manner; from this circumstance the plant takes its popular

name of screw-moss. The leaf usually consists of either a single or a

double layer of cells, having flattened sides, by which they adhere

one to another. The leaf-cells (Fig. 302) of the Sphagnum or bog-moss

exhibit a curious departure from the ordinary type; they are large,

polygonal, and elongated, and contain spiral fibres loosely coiled in

their interior. The young leaf does not differ from the older; both are

evolved by a gradual process of differentiation.

[Illustration: Fig. 302.--Section of leaf of Sphagnum moss, showing

large cells of spiral fibres and connecting apertures.]

Mosses, like liverworts, possess both antheridia and pistillida,

which are engaged in the process of fructification. The fertilized

cell becomes gradually developed into a conical body elevated upon a

footstalk, the walls of the flask-shaped body carrying the higher part

upwards as a _calyptra_ or hood upon its summit, while the lower part

remains to form a kind of collar round the base. These spore-capsules

are closed on their summit by _opercula_ or lids, and their mouths

when laid open are surrounded by a beautiful toothed fringe, termed

the _peristome_. This fringe is shown in Fig. 303, in the centre of a

capsule of Funaria, with its peristome _in situ_. The fringes of teeth

are variously constructed, and are of great service in discriminating

the genera. In _Neckera antipyretica_ the peristome is double, the

inner being composed of teeth united by cross bars, forming a very

pretty trellis. The seed spores are contained in the upper part of the

capsule, where they are clustered round the central pillar, termed the

_columella_; and at the time of maturity, the interior of the capsule

is almost entirely occupied by spores.

[Illustration: Fig. 303.--Mouth of Capsule of Funaria, showing

Peristome.]

[Illustration: Fig. 304.--Hair-moss in Fruit.]

The undulating hair-moss, _Polytrichum undulatum_ (Fig. 304), is found

on moist, shady banks of pools and rivulets. The seed-vessel has a

curious shaggy cap; but in its construction it is very similar to

that of the screw-moss, except that the fringe around its opening is

not twisted. The reproductive organs of mosses are of two kinds; the

capsule containing minute spores, _archegonia_, and the _antheridia_,

or male efflorescence. The capsule, _theca_, or sporangium, is

lateral or terminal, sessile, or on a fruit stalk (_seta_) of various

shapes, indehiscent, or bursting by four valves at the sides, or more

commonly by a deciduous cup, _operculum_. When this falls the mouth

of the capsule becomes exposed. The rim is crowned with tooth-like

or cilia-like appendages in sets of four or multiples of that

number--_peristome_. These are often brightly coloured and hydroscopic.

By simply breathing upon them they suddenly fly open, and are endowed

with motion, that is, if they contain spores. The spores on germination

produce a green confervoid-like mass of threads, from which the young

plant arises.

The Sphagnaceæ, or “bog mosses,” have been separated from true mosses

from the marked differences they present. The stem is more widely

differentiated, and throughout its structure a rapid passage of

fluid takes place. It has the power of absorbing moisture from the

atmosphere, so that if a plant be placed dry in a glass of water with

its rosette of leaves hanging over the edge, it acts like a syphon,

and the water will drop from it until the glass is emptied. As may be

supposed, the leaf is composed of large open cells, and it absorbs

more water than the root. The antherids or male organs of Sphagnaceæ

resemble those of liverworts, rather than those of mosses, both in

form and arrangement; they are grouped in “catkins” at the tips of the

lateral branches, each of the imbricated perigonal leaves enclosing

a single globose antherid on a slender foot-stalk, and surrounded by

long branched paraphyses of cobweb-like tenuity. The female organs,

or archegones, do not differ materially in structure from those of

mosses; they are grouped together in a sheath of deep green leaves at

the end of the shorter lateral branchlets at the side of the rosette

or terminal crown of leaves. The sporange is very uniform in all the

species, and the spores are in groups of fours, as in mosses, around a

hemispherical columella. These plants grow so rapidly that they soon

cover a pool with thin matted bundles of branches, and as they decay

they fall to the bottom, and become the foundation of the future bog or

peat moss.

=Felices.=--Of all the spore-bearing families the ferns are the more

universally known. They constitute an exceedingly numerous genera and

species, and vary from low herbaceous plants of an inch in height

to that of tree ferns, which attain a height of fifty or more feet,

terminating in a graceful coronet of fronds or leaves. Of whatever size

a fern may be, its spores are, for the most part, microscopic, produced

within the sporangium by cell division, and are therefore free and

variously shaped.

The true mode of development of ferns from their spores was that

furnished by Nägeli, who announced the existence of antheridia. On the

spore starting into life it sends out from the cell-wall of its outer

coat a white tubular projection, or root fibre (Fig. 305, A, B, and C),

which passes through the cell-wall of its outer coat. This attracts

sufficient moisture to burst open the outer, and then it begins to

increase by the subdivision of its cells, until the primary green

prothallus D is formed. This falls to the ground, and, being furnished

on its under side with thread-like fibres, fixes itself to the earth,

and thus is developed the rhizome, or root of the future plant. In each

of the antheridia, which are numerous, a cell is formed, chiefly filled

with albuminous matter and free spores, each having attached a flat

ribbon-like filament, or stermatoid, curled in a spiral manner. These

are ultimately set free by the rupture of the cell-wall, and commence

revolving rapidly by the agency of the whip-like appendage at the

larger end.

[Illustration: Fig. 305.--Development of the Globular Antheridium and

Spermatoids of _Pteris serrulata_.

A. Spores; B, C. Early stages of development; D. Prothallus with radial

fibres; _a_, _a_ and _a_, _b_ are stermatoids; and _h_, _h_. Enclosed

antheridia.]

The sporangia, or spore-cases, are, for the most part, globular in

form, and are nearly or quite surrounded by a strong elastic ring,

which in some cases is continued to form a stalk. When the spores

are ripe, this ring, by its elastic force, tears open the sporangia

and gives exit to a quantity of microscopic filaments, curled in

corkscrew-like fashion (Figs. 305 and 307). The ring assumes various

forms; in one group it passes vertically up the back of the sporangium,

and is continued to a point termed the stomata, where the horizontal

bursting takes place. This form is seen in Fig. 306, _a_, _b_. In other

groups it is vertical, as in _c_, _c_; in others transverse, as in _d_;

or apical, as at _e_; and in a few instances it is obsolete, as in _f_.

These are the true ferns, and their systematic arrangement is chiefly

founded on the peculiarity of the sori and sporangia, characters which

become quite intelligible by the aid of the microscope.

[Illustration: Fig. 306.--Sporangia of Polypodiaceous Ferns.

_a_, _b_. Polypodiaceæ; _c._ Cyantheineæ; _d._ Gleichenineæ; _e._

Schizeineæ; _f._ Osmundineæ.]

[Illustration: Fig. 307.--Spores of _Deparia prolifera_.]

The beautiful ringed sporangium of the fern (Fig. 307) when ruptured

gives exit to the dust-like spores; these, examined under a moderate

power, are seen to be sub-globose and pyramidal, the outer coat or

exospore being a coloured hyaline cell with nuclei similar to the

spores of mosses, but in which chlorophyll soon begins to form, and

from this little green embryonic growth the organs of reproduction are

formed.

In all ferns the pistillidia or archegonia are analogous to the ovules

or nascent seeds of flowering plants, and contain, like them, a

germinal vesicle, which becomes fertilized through the agency of the

spiral filaments, and then gradually develops into an embryo plant

possessing a terminal bud. This bud begins at once to unfold and push

out leaves with a circinate vernation, of a very simple form at first,

and growing up beneath the prothallium, coming out at the notch;

single fibrous roots are at the same time sent down into the earth,

the delicate expanded prothallium withers away, and the foundation of

the perfect fern plant is laid. When a fern acquires a considerable

stem, as in a tree fern, it consists of cellular tissue and an external

cortical portion forming fibro-vascular bundles, scalariform ducts, and

woody fibre. Fig. 308, _b_, shows an oblique section of the footstalk

of a fern leaf with its bundle of scalariform ducts.

These observations on ferns have acquired increased interest from

subsequent investigations made on the allied Cryptogams, and on the

processes occurring in the impregnation of the Conifers. Not only

have later researches furnished a satisfactory interpretation of the

archegonia and antheridia of the mosses and liverworts, but they have

made known and co-ordinated the existence of analogous phenomena in

the Equisetaceæ, Lycopodiaceæ, and Rhizocarpeæ, and prove, moreover,

that the bodies described by Dr. Brown in the Conifers under the name

of “corpuscles” are analogous to the _archegonia_ of the Cryptogams;

so that a link is hereby formed between these groups and the higher

flowering plants.

[Illustration: Fig. 308.--_a._ Vertical section of Fern-root, showing

spiral tissue and cells filled with granular bodies; _b._ Section of

Footstalk.]

_Equisetaceæ._--The development of _Horse-tails_ (Fig. 309), the

name by which they are commonly known, corresponds in some respects

with that of ferns. They comprise a little group, and the whole of

their structure is composed in an extraordinary degree by silex, so

that even when the organic portion has been destroyed by prolonged

maceration in strong acid, a consistent skeleton still remains. It

is this flinty material that constitutes their chief interest for

microscopists. A portion of their silicious particles is distributed

in two lines, arranged parallel to the axis of the plant, others are

grouped into oval forms, and connected by a chain as in a necklace. The

form and arrangement of the crystals are better seen under polarised

light. Plate VIII., No. 170, a portion of the epidermis, forms an

extremely beautiful object. Sir David Brewster pointed out that each

silicious particle has a regular axis of double refraction. What is

usually said to be the fructification of the Equisetaceæ forms a

cone or spike-like extremity to the top of the stem (Fig. 309), the

whole resembling a series of spike-like branches (the real stem being

a horizontal rhizome), and a cluster of shield-like discs, each of

which carries a circle of sporanges that open by longitudinal slits to

set free the spores which are attached to it in two pairs of elastic

filaments (shown in Fig. 291, F, G), _elaters_; these are at first

coiled up around the spore in the manner represented at G, but on their

liberation they extend themselves as shown at F. The slightest moisture

will close them up again, and their purpose having been served in the

distribution of the spores, they are no longer required. If a number of

spores be spread out on a glass-slip under the microscope and, while

watching, a bystander breathes upon them, they immediately respond,

are set in motion, presenting a curious appearance, but as soon as

the hydroscopic effect has passed off they return to their previous

condition. These spores can be mounted in a cell with a movable cover,

and made to exhibit the same effect over and over again.

[Illustration: Fig. 309.--Equisetum giganticum.

_a._ Fragment of stem showing mode of branching out; _b._ Cone or

spike of fructification; _c._ Scale detached from cone; _d._ Spore

with elastic filaments; _e._ Vertical section of stem; _f._ Transverse

section showing hexagonal cells.]

The vascular tissue of the Equisetaceæ (Fig. 309, _e_, _f_) shows

them to be of a higher grade than the ferns. More recently discovered

Horse-tails, of Brazil, grow to a gigantic size, but even these are

comparatively small when compared with the Calamites, and other fossil

Equisetaceæ of the coal measures and new red sandstone. They all

require a calcareous flinty soil for growth. A spring water-course

making its way to the sea, as in the Chines of the Isle of Wight, is

very favourable, the author having gathered them more than once in

Bramble Chine.

Nearly allied to ferns is a little group of small aquatic plants, the

Rhizocarpeæ (pepperworts), which either float on the water or creep

along shallow bottoms. These are chiefly curious from having two

kinds of spores produced from separate sporanges; smaller and larger

“microspores” undergoing progressive sub-division without the formation

of a distinct prothallium; each cell giving origin to an antherozoid,

a generative process said to belong exclusively to flowering plants,

corresponding indeed to the pollen grains of higher plants.

Structure of Phanerogamiæ or Flowering Plants.

The two great divisions of the vegetable kingdom are known as

Cryptogamia and Phanerogamia. It does not follow, however, that there

is any abrupt break between the two, as will appear from the context.

Although it is customary to speak of the flowering plants as a higher

grade of life, yet there is an intermediary class of Phanerogamiæ in

which the conspicuous parts of the generative system partake of a

condition closely resembling those of the higher Cryptogamiæ, observed

in Gymnosperms, Coniferæ, and Cycadæ. So it may be said the distinctive

character of the former is that of reproduction by seeds rather than

flowers. The progress of botanical science during the latter half of

the Victorian reign has been quite as remarkable as that of histology;

while the comparative physiology and morphology of plants have

perhaps advanced even more rapidly because the ground was newer. The

consequence is that the specialisation of botanical science has been

brought about con-currently with a more comprehensive nomenclature.

The chief cause in this instance of modern specialisation is utility.

“If we look at the great groups of plants from a broad point of view,

it will be seen that the fungi and the phanerogams occupy public

attention on other grounds than do the algæ, mosses and ferns. Algæ are

especially a physiologist’s group, employed in questions on nutrition,

reproduction, and cell division and growth. The Bryophyta and

Pteridophyta, are, on the other hand, the domain of the morphologist

concerned with such questions as the alternations of generations and

the evolution of the higher plants.

“Fungi and phanerogams, while equally or even more employed by

specialists in morphology and physiology, appeal widely to general

interest, and evidently so on the ground of utility. Without saying

that this enhances the importance of either group, it certainly

attracts scientific attention to them. However, the histology of the

minute cell, in addition to its importance from an academical point of

view, has a special interest for the microscopist.”

It would be impossible to find anything more remarkable in histology

than the detailed agreement in the structure and behaviour of the

nucleus in the higher plants and the higher animals, an agreement which

is conspicuously manifest in those special divisions which take place

during the maturation of the sexual cells.

So with regard to the question of “alternation of generations.” We

have known since the important discoveries of Hofmeister that the

development of a large part of the vegetable kingdom involves a regular

alternation of two distinct generations, the one which is sexual being

constantly succeeded, so far as the normal cycle is concerned, by the

other which is asexual. This alternation is most marked in the mosses

and ferns. In the Bryophyta the ordinary moss or liverwort plant is the

sexual generation of the ovum, which, when fertilised, gives rise to

the moss-fruit, and represents the asexual stage. The latter is once

more seen to form spores from which the sexual plant is again developed.

In the Pteridophyta the alternation is equally regular, but the

relative development of the two generations is totally different,

the sexual form being the insignificant prothallus, while the whole

fern-plant, as we ordinarily know it, is the asexual generation.

The thallus of some of the lower Bryophyta is quite comparable with the

prothallus of a fern, so as regards the sexual generation there is no

difficulty in seeing the relation of the two classes; but when we come

to the asexual generation or sporophyte the case is totally different.

There is no appreciable resemblance between the fruit of any of the

Bryophyta and the plant of any vascular Cryptogam.

“It is now known that in the higher plants a remarkable numerical

change takes place in the constituents of the nucleus of the cell

shortly before fertilisation. In angiospermous plants a reduction of

the chromosomes occurs shortly before differentiation of the sexual

cells. Thus, in the case of the lily, fertilisation is not the simple

fusion of nuclear bodies. These spheres are seen to fuse in pairs, and

then by position to determine the plane of first cleavage of the ovum;

agreeing, in fact, closely with what is observed to take place in the

animal kingdom.”

In the higher grades of plants it will be evident that the several

tissues that compose their bodies are not found in the root, stem, and

leaf without definite order and purpose, but that they are grouped

into systems for the performance of different kinds of work. In all

flowering plants at least three different systems may be clearly

distinguished. These are the epidermal or boundary tissue system,

the fundamental or ground tissue system, and the fibro-vascular or

conducting system. All three systems of tissue originate from meristem

cells, located at the growing point of the stem and root.

Although these systems characterise the higher types of plants, the

elementary tissues (represented in Plate XIII. and in other figures)

enter alike into the several component parts of nearly all plants.

The stem, the branch, and the root, are alike constituted of an outer

coating which affords a mechanical support, and once formed takes no

further share in the economy of the plant, excepting that of assisting

to convey fluid from the roots to the branches and leaves, an action

more of a capillary nature than vital. The nourishment of the plant is

brought about by other material structures, as the pith, the cortex,

the cambium, and so forth, all of which greatly assist in the formative

process. The woody portion of the plants is especially concerned in

furnishing support to the softer pulpy textures, while the tissues

of leaves and flowers are chiefly composed of cells compactly held

together by protoplasmic or albuminoid matter. Water, of course, enters

largely into the constituents of all plants. Beneath the epidermis is

another layer of importance, the parenchymatous, which becomes more

or less solid with the growth of the pith and cellular wall. In the

pulpy substance of some leaves the epidermis presents a thin lamina

of palisade-tissue, the bulk of the mesophyll consisting of spongy

parenchyma or sclerenchymatous fibres (seen in Fig. 310), which also

serve to show the disposition of the several layers about to be brought

under notice.

_Development of the Tissue Systems._--In the growing plant the

embryonic cells soon become differentiated into three primary meristem

layers, known as dermatogen, periblem, and phloem, from which are

developed respectively the primary cortex, epidermis, and the stele or

vascular cylinder. The dermatogen forms the outermost layer of cells at

the growing point, and when present always develops into true epidermal

tissue. In stems the dermatogen is always single-layered, while in

roots it consists of several layers and develops a many-layered

epidermis.

[Illustration: Fig. 310.--Section of Leaf of Piper.

_c._ Cortex; _ep._ Epidermis; _pal._ Palisade-tissue; _scl f._

Sclerenchymatous fibres of pericycle; _o._ Oil gland.]

The periblem occurs immediately beneath the dermatogen, forming a

hollow cylinder of tissue, which surrounds the phloem. From the

periblem is developed the fundamental tissue of the primary cortex.

When no dermatogen is present in the growing-point (stems of vascular

cryptogams) the external layer of the periblem develops cells which

perform epidermal functions. The phloem occupies the centre of the

growing-point, and consists of a solid mass of somewhat elongated

cells. From the phloem are developed the fibro-vascular and fundamental

tissues of the vascular-cylinder or stele.

[Illustration: PLATE XIII.

ELEMENTARY PLANT TISSUES.]

_Epidermal or Boundary Tissue System._--This system constitutes the

external covering of the plant, and is commonly called the epidermis.

It includes, besides the ordinary epidermal cells, the guard-cells

of the stomata and water pores, the plant hairs or trichomes, and

the epidermal or external glands. The epidermal tissues are chiefly

protective in function, serving to prevent excessive evaporation from

the interior tissues of the plant.

[Illustration: Fig. 311.

_a._ Epidermis, reticulated ducts, and conjunctive palisade cells; _b._

Vertical section of alder root, woody layer, and boundary ducts.]

In stems the external layer of cells, whatever its origin, is known as

the epidermis, while in roots it is called the epiblema. The epidermis

usually consists of a single layer of cells, but in some cases it is

two or three-layered, as in the leaves of figs and begonias.

In land plants the epidermis is usually strongly cutinised, while in

submerged plants it is never cutinised. The epidermis of land plants

is also often waxy, the wax occurring on the surface as minute grains,

rods or flakes, constituting the so-called bloom of leaves and fruits,

and giving to them their glaucous appearance. Chlorophyll bodies are

usually absent from the ordinary epidermal cells of land plants, while

they commonly occur in the epidermal cells of aquatic plants.

Ordinary epidermal cells are usually thin-walled and transparent, and

contain a nucleus and colourless watery protoplasm, but are destitute

of both chlorophyll-bodies and starch-grains.

The external layers of the outer walls constitute the cuticle of the

plant, while the internal layers and the radial and inner walls are

composed of cellulose. The cells of the epidermis are always very

compactly arranged, having their walls so closely adherent that the

intercellular spaces are entirely obliterated except at the stomata and

water-pores.

[Illustration: Fig. 312.

1. Vertical section of leaf of _Iris germanica_; _a, a._ Elongated

cells of the epiderm; _b._ Stomata cut through longitudinally; _c,

c._ Green cells of the parenchyma; _d, d._ Colourless tissue of the

interior of the leaf. 2. Portion of leaf torn from its surface; _a._

Elongated cells of the cuticle; _b._ Cells of the stomata; _c._ Cells

of the parenchyma; _d._ Limiting wall of the epidermic cell; _e._

Lacunæ or openings in the parenchyma corresponding to the stomata.]

There are exceptions to this rule, as, for example, in _Cinchona

calisaya_, which shows no trace of epidermis, this being replaced by

a corky layer of tubular cells. Where this occurs in a plant to any

extent, the whole of the outer tissues are displaced, and the bark

consists exclusively of phloem tissues. This, although of constant

occurrence in _C. calisaya_, is not so common in other species, as _C.

succirubia_, the middle structure of which consists of parenchyma in

which appear more or less numerous isolated store-cells, and when these

are absent there is a formation of rhytidoma and displacement of the

tissues containing the store-cells and ducts. The chlorophyll of _C.

succirubia_ is very marked, and its spectrum presents seven distinct

absorption bands.

The epidermal system of plants in general includes other tissues

than those already named, as the guard-cells of the stomata, the

water-pores, plant-hairs or trichomes, and the external or epidermal

glands, all of which are but modifications of ordinary epidermal tissue.

_The Stomata or Breathing Pores_ are apertures in the epidermal which

lie over large intercellular spaces (Fig 312, 2, _b_). These are

usually bordered by two modified epidermal cells, called guard-cells.

Stomata are formed in the following manner: A young epidermal cell

divides into two equal portions by the formation of a septum across its

middle, each half developing into a guard-cell; the septum now splits

lengthwise and separates the guard-cells, leaving an aperture or stoma

between them.

In the higher plants the guard-cells of the stomata are crescent-shaped

and occur in pairs, the concave sides of the cells facing each other

with the aperture between, while in mosses the stomata possesses but a

single annular guard-cell which surrounds the aperture. The guard-cells

of stomata usually contain chlorophyll-bodies in addition to the

ordinary protoplasm. They have the power of increasing or diminishing

the size of the aperture under the influence of light and moisture,

thus regulating the amount of evaporation from the internal tissues of

the plant.

_Water Pores or Water Stomata_ are apertures in the epidermis, similar

in structure to ordinary stomata, but differ from them both in function

and distribution. Water-pores excrete water in the form of drops, and

have their guard-cells fixed and immovable. They always occur at the

ends of vasal bundles, and are found on the margin and at the apex of

leaves.

_Plant Hairs or Trichomes_ are modified epidermal cells prolonged

externally, and may be either unicellular or multicellular. Each hair

consists of a basal portion, or foot, which is embedded among the

ordinary epidermal cells, and an apical portion or body, which is

prolonged externally. Ordinary epidermal hairs are usually thin-walled,

the inner layers of the wall being composed of cellulose, while the

outer layer is more or less strongly cutinised. The walls may become

hardened by deposits of lime-salts or silica. Sometimes the cells

become glandular and secrete oily, resinous, or irritating matters, as

in stinging-nettle hairs (Plate XIII., No. 19), when they are known as

glandular hairs. The development of resin-passages may be observed in

transverse sections of the stem of the ivy (_Hedera helix_) cut from

a young succulent stem, and mounted in glycerine. The resin is seen

scattered through the cortex and pith, and in the soft bast which lies

outside the cambium in various stages of development, starting from a

group of four cells without intercellular spaces.

Root hairs spring from the epiblema and are never cutinised, but are

frequently more or less mucilaginous. The root-hairs are the principal

absorbing organs of the plant, and are confined to the younger

roots, occurring just above their tips. Root-hairs are never present

in aquatic plants, and are absent from the roots of certain of the

Coniferæ. It is a curious fact with regard to bell-heather growing in

higher latitudes, that the plants possess a peculiar root structure as

a protection against droughts. In most of them the sustentation of life

depends upon the formation of a number of long thin filaments on their

roots resembling root-hairs, which penetrate the root, forming nodular

masses within it. These filaments belong to a fungus entirely parasitic

to the root, and yet different from a common parasite, inasmuch as the

plant in this way obtains so much of its nourishment, and when the

fungus is not present, or is removed, the plant can no longer live

on a peaty soil. The leaf-blade of the coarse moorland grass Nardus

is likewise endowed with a singular property--that of rolling up

cylindrically and spreading out again to adapt itself to the dry and

wet weather of the moorlands of Scotland.

[Illustration: Fig. 313.

_a._ Section of the testa of Gourd Seed, showing communicating cells

filled with colouring matter; _b._ Section of stem of Clematis, three

pores separated and more highly magnified; _c._ Transverse section of

same, showing medullary rays.]

_Fundamental or Ground Tissue System._--This system constitutes the

groundwork of plants, and is the system through which the vasal bundles

are distributed. The fundamental tissues are composed largely, though

not wholly, of parenchyma, and are chiefly concerned in the metabolic

work of plant life.

Ground tissue includes, besides ordinary parenchyma, collenchyma,

selerenchymatous parenchyma, fibrous tissue, cork, laticiferous

and glandular tissues. To the fundamental system also belongs the

chlorophyll cells of leaves, the thin-walled cells of the pith and

medullary rays, the cells of the cortex of stems and roots, and most of

the soft cellular tissues in all plants.

The lower plants consist almost entirely of fundamental tissue. In

the herbaceous forms of the higher plants the ground tissues largely

predominate, while in woody plants they are present in much smaller

proportion, the vascular tissues being the most abundant. In aquatic

plants generally, the fundamental tissues constitute the principal

system.

The hypoderma occurs immediately beneath the epidermis, and consists of

several layers of cells. A collenchymatous hypoderma is found in the

stems and petioles of most herbaceous dicotyls, and frequently occurs

next the mid-rib of leaves, where it forms a strengthening tissue. A

sclerenchymatous hypoderma occurs either as a continuous layer beneath

the epidermis, as in the stems of some ferns, _Pteris aquilina_, and in

leaves of the pine; or it may form numerous isolated strands beneath

the epidermis, as in the stems of horsetails and in certain Umbelliferæ.

[Illustration: Fig. 314.

_a._ Tangential section of _Taxus baccata_ (Yew), showing the woody

fibre; _b._ Vertical section of same, spiral fibres, and ducts; _c._

Vertical section of Elm, showing ducts and dotted cells.]

The endodermis is the innermost layer of the extra-stelar fundamental

tissues, and always abuts on the stele or steles. In monocotyls it

marks the boundary between the cortex and the central cylinder, and it

is sometimes spoken of as the nucleus sheath.

In stems the endodermal cells are usually thin-walled and unlignified,

having a suberous thickening band extending round the upper, lower and

lateral surfaces, which in cross-section appears as a black dot on the

radial wall (Fig. 314, _c_.)

According to its position in the stele, the conjunctive tissue is

divided into three principal portions, viz., that portion which invests

the vasal bundles, the pericycle; that portion which lies between the

bundles of the stele, the interfascicular conjunctive tissue; and that

which occupies the centre of the stele, the medullary conjunctive

tissue. The pericycle, formerly called the pericambium, is the

outermost layer of the conjunctive tissue of the stele.

The bundle-sheath of the young stem is more easily recognised than in

the older stem. It is, in fact, a continuous layer of cells, whose

radial walls have a characteristic dark spot on each radial wall. The

bundle-sheath lies immediately outside the vascular bundles, curving

slightly towards the centre of the stem in the spaces between the

bundles. It is more prominent in the stem when very young, as the cells

are then filled with starch granules. This layer of cells will be

readily seen in sections treated with iodine.

In dicotyls and gymnosperms the medullary rays consist essentially

of interfascicular ground tissue. The medullary conjunctive tissue

occupies the centre of the stele, constituting the so-called pith, and

usually consists of parenchymatous cells, but may contain, in addition,

either stone cells, sclerenchyma fibres, laticiferous or glandular

tissues.

_The Fibro-vascular or Conducting Tissue System._--This system

constitutes the fibrous framework of the plant, and is the system by

means of which fluids are conducted from one part of the plant to

another. Its function is partly to give strength and support, but

principally to conduct the crude and elaborated juices to and from

the leaves. It is found only in the higher plants, constituting the

tough and stringy tissues in stems and roots, and the system of veins

in leaves. The fibro-vascular system consists essentially of vascular

tissue (ducts, tracheids, and sieve-tubes), and forms long strands--the

fibro-vascular bundles--which extend vertically through the fundamental

tissues of the plant. The term “fibro-vascular,” as applied to the

conducting system, is not strictly correct, since fibres do not always

accompany the vascular elements, hence this system is often spoken of

as the vascular system, and the bundles as vascular, or more briefly as

vasal bundles.

That the arrangement, and course of the vascular bundles in

dicotyledous stems are connected with those of the leaves is an

obvious fact. It may be seen in sections of Helianthus, but is

more markedly shown in plants with regularly decussate leaves, as

Cerastium, Clematis, &c. Still, the arrangement of the bundles may

differ radically from that of the leaves, and is, to a certain extent,

independent of them. This will be noticed in sections of _Iberis

amara_, where the bundles do not run longitudinally, but in tangential

spirals. These, as Nägeli pointed out, have no direct relation with

the leaves; and he recommends a series of types for investigation, in

which it will be seen how closely the arrangement of the bundles is

connected with the arrangement of the leaves, and the number of bundles

entering the stem from each leaf: _Iberis amara_, leaves alternate,

leaf-trace with one bundle; Lupinus, leaves alternate, leaf-trace with

three bundles; Cerastium, leaves opposite, leaf-trace with one bundle;

Clematis, leaves opposite, leaf-trace with three bundles; Stachys,

leaves opposite, leaf-trace with two bundles.

[Illustration: Fig. 315.

1. Transverse section of the stem of Cedar, showing xylem or wood; 2.

Section of stem of Conifer, the phloem and zones of annual growth; 3.

Section of an Ivory Nut, cells, and radiating pores; 4. Section of the

outer or ligneous portion of same, with radiating cells.]

The connection of the leaf and stem will be best seen by cutting

longitudinal sections through a _young node_ of Helianthus, so as to

include the median plane of the leaf, or of both leaves if opposite

to each other, as they often are; steep them in dilute potash for

twenty-four hours and mount in glycerine. A medium power will serve

for their examination. The course of the vascular bundles will appear

dark through the more transparent parenchyma. The continuity of the

tissues of the stem and petiole if followed will be seen to have no

definite boundary between the two parts; the bundles from the petiole

pass into the stem, and no bundle of the upper internode lies in the

same vertical plane as that which enters from the petiole between two

successive bundles of the vascular ring.

Every complete _vasal bundle_ consists of xylem or wood and phloem or

bast.

The former consists essentially of trachery tissue (ducts and

tracheids), and may contain in addition both wood fibres and wood

parenchyma. The phloem or bast consists essentially of sieve tissue,

and usually contains some ordinary parenchyma. In angiosperms

companion-cells always accompany the sieve-tubes in the phloem, while

in gymnosperms they are absent.

According to the relative positions of the xylem and phloem elements,

there are two principal kinds of conjoint bundles--the collateral and

the concentric. Of these again there are three varieties, but the

experiments with leaves bring out parallel facts; that in ordinary

stems the staining of the wood by an ascending coloured liquid is due,

not to the passage of the coloured liquid up the substance of the wood,

but to the permeability of its ducts and such of its pitted cells as

are united into regular canals; and the facts showing this at the same

time indicate with tolerable clearness the process by which wood is

formed, for what in these cases is seen to take place with dye may be

fairly presumed to take place with sap.

Taking it, then, as a fact that the vessels and ducts are the channels

through which the sap is distributed, the varying permeability of their

walls, and consequent formation of wood, is due to the exposure of the

plant to intermittent mechanical strains, actual or potential, or both,

in this way. If a trunk, a bough, shoot, or a petiole is bent by a gust

of wind, the substance of its convex side is subject to longitudinal

tension, the substance of its concave side being at the same time

compressed. This is the primary mechanical effect. The secondary is

when the tissues of the convex side are stretched, and also produce

lateral compression. In short, the formation of wood is dependent

upon transverse strains, such as are produced in the aerial parts of

upright plants by the action of the wind.

[Illustration: Fig. 316.--Termination of Vascular System.

1.--Absorbent organ from the leaf of _Euphorbia neriifolia_. The

cluster of fibrous cells forming one of the terminations of the

vascular system is here embedded in a solid parenchyma.

2.--A structure of analogous kind from the leaf of _Ficus elastica_.

Here the expanded terminations of the vessels are embedded in the

network parenchyma, the cells of which unite to form envelopes for them.

3.--End view of an absorbent organ from the root of a turnip. It is

taken from the outermost layer of vessels. Its funnel-shaped interior

is drawn as it presents itself when looked at from the outside of this

layer, its narrow end being directed towards the centre of the turnip.

4.--Shows on a larger scale one of these absorbents from the leaf of

_Panax Lessonii_. In this figure is clearly seen the way in which the

cells of the network parenchyma unite into a closely-fitting case for

the spiral cells.

5.--A less-developed absorbent, showing its approximate connection with

a duct. In their simplest forms these structures consist of only two

fenestrated cells, with their ends bent round so as to meet. Such types

occur in the central mass of the turnip, where the vascular system is

relatively imperfect. Besides the comparatively regular forms of these

absorbents, there are forms composed of amorphous masses of fenestrated

cells. It should be added that both the regular and irregular kinds are

very variable in their numbers: in some turnips they are abundant, and

in others scarcely to be found. Possibly their presence depends on the

age of the turnip.

6.--Represents a much more massive absorbent from the same leaf, the

surrounding tissues being omitted.

7.--Similarly represents, without its sheath, an absorbent from the

leaf of _Clusia flava_.

8.--A longitudinal section through the axis of another such organ,

showing its annuli of reticulated cells when cut through. The cellular

tissue which fills the interior is supposed to be removed.]

In concentric bundles one of the elements, either the xylem or the

phloem, occupies the centre, and is more or less surrounded by the

other, as seen in Fig. 310. Meristem tissue is never present, hence

concentric bundles are always closed. They, however, occur in the stems

of most ferns, and are always surrounded by a pericycle and endodermis,

and should be regarded as steles. Concentric bundles with a central

phloem occur in the rhizomes of some monocotyles, as Calamus, Iris,

Convallaria, &c.

[Illustration: Fig. 317.--Vertical section of Sugar-cane Stem showing

parachyma and crystalline cells, × 200 diameters.]

_The Stele, or Vascular Cylinder_, is developed from the phloem of

the growing plant, and consists of one or more vasal bundles imbedded

in fundamental tissue, the whole being enclosed by a pericycle and an

endoderm. The typical _stele_ includes all the tissues evolved by the

endodermis, which, however, forms no part of the vascular cylinder

itself, but merely surrounds it. The pericycle is always the outermost

layer of the tissues of the stele, while the endodermis is the

innermost layer of the extra-stelar tissues.

The arboreus type of stem can be best followed by making sections

of a twig of the elm (_Ulmus campestris_), which will be found to

be cylindrical hirsute, green or brown according to age, the latter

colour being due to the formation of _cork_. Small brown excrescences

are scattered over its surface; these are termed _lenticels_. The cork

will be seen to lie immediately below the epidermis, and to consist

of cubical cells, with little or no cell contents; they are arranged

in radial rows, without intercellular spaces. The walls of these cork

cells will stain yellowish-brown with Schultze’s solution. Treat a

thin section with sulphuric acid and the walls will swell out and

gradually lose their sharpness of outline, with the exception of the

cuticularised outer wall of the epidermis and the _cork_. This material

is occasionally found developed in the twigs of the elm, so that it can

be separated as thick radial plates of tissue.

“By comparing sections of twigs cut of various ages, the following

information may be gleaned: That cork cambium, or _phellogen_, appears

as a layer of cortical cells below the epidermis, and that these divide

parallel to the surface of the stem. The result of successive divisions

in this direction is the formation of secondary tissue, which develops

externally as cork, internally as phelloderm. The true cork cambium

consists of only a single cell in each radial row, from which, by

successive division, all these secondary tissues are derived--_i.e._,

cambium of vascular bundles. As stems grow older, layers of cork appear

successively further and further from the external surface; not only

the cortex, but also the outer portions of the phloem are thus cut off

from physiological connection with the inner tissue. The term _bark_

is applied to tissues thus cut off, together with the cork which forms

the physiological boundary. The stem of Vitis affords a good example of

such successive layers of cork.”

[Illustration: Fig. 318.

1. Laticiferous Tissue; 2. Vertical section of a Leaf of the

India-rubber Tree, with a central gland; 3. Vertical cast of spiral

tubes of Opuntia.]

For the study of _sieve-tubes_ take the vegetable marrow, in which they

are of extraordinary size. Cut transverse sections of the stem and

stain with eosin, and mount them in glycerine. The general arrangement

will be seen to differ from that of most other herbaceous plants.

Below the epidermis a thick walled band of sclerenchyma with lignified

walls will be seen distinct from the vascular bundles, which readily

take a stain. The vascular bundles are separate and distinct, and the

structure of the bundle is abnormal, there being in each a separate

central mass of xylem, with the phloem masses lying, the one central,

the other in the peripheral side. Between the xylem and the phloem

masses is the cambium layer. The structure being the same in both will

serve for the study of the punctate sieve-plates; these are readily

stained with eosin, as shown in Sach’s text-book.

_Laticiferous Tissues_ (Fig. 318).--In cutting sections of latex care

must be taken to at once transfer them to alcohol so as to prevent the

flow of the latex from the cells, otherwise the laticiferous vessels

will be much less easily traced. The better method is to plunge the

root of the dandelion (_Leontodon taraxacum_), after cleaning, into

alcohol, and there let it remain until it has become hardened; then cut

thin tangential sections from the phloem, and longitudinal sections

through the cambium, and mount them in potash and glycerine. The

laticiferous vessels appear circular in the transverse sections with

brown contents; these are distributed in groups round the central

xylem. Observe in such sections the presence of sphere crystals of

inulin. These are formed quite irrespective of the cell-walls.

Laticiferous cells are readily seen in the cortex of _Euphorbia

splendens_, cut just outside the vascular ring. Long tubes will be seen

to run through the cortical parenchyma, with thick cellulose walls and

granular contents. These are the laticiferous cells, the branching of

which distinguishes them from the preceding structure. Included in the

granular contents are starch grains of a peculiar dumb-bell form.

_Leaf or Petiole._--The general morphology of leaf tissue is

essentially the same as that of the stem from which it proceeds. In the

typical monostotic stem of Phanerogamæ each leaf receives a portion

of the stele or central cylinder of the stem. Such portion is termed

a meristele, and may be either entire or split up into a number of

schizosteles.

The microscopical structure of leaves should be studied in the whole

organ, and by the aid of isolating elements. The whole or portion of a

leaf should be soaked in chloral hydrate solution; this will render it

transparent, whereby the internal structure can be studied as a whole.

Sections should be prepared from fresh leaves, or dried ones softened

by soaking in water. Cut them transversely, both in the direction of

the mid-rib and at right angles to it. This is best effected by placing

the material between two pieces of elder pith or fresh carrot. Sections

of the whole are made and transferred to a dish of water. Leaf sections

are easily made for examination by macerating the leaves in solution

of caustic potash varying in strength from one to five per cent. The

epidermis on both sides may be detached, and the elements of the

mesophyll and vascular bundles isolated for separate examination.

Potassium permanganate proves to be a useful reagent. A weak solution

causes the protoplasmic structures to swell up, thus assisting in the

observation of the structure of the chromatophores. This solution may

also be employed as a macerating fluid. Beautiful preparations are

obtained in this way of the sieve-tubes of Vitis.

Special structural peculiarities are to be observed in the leaves of

various plants in which the epidermis consists of more than a single

layer of cells (_e.g._, the leaves of Ficus, Peperaceæ, Begoniaceæ,

&c.), cystoleths in the cells of the epidermis of Urtica; glandular

structure in Ruta, Psorales; the coriaceous leaves of the Cherry

Laurel, and the cylindrical leaves of Stonecrop (_Sedum acre_).

_Reproductive Organs._--The development of the rudiments of flowers is

of an extremely interesting nature, and the complete flower should be

carefully studied. Median sections are best suited for the purpose.

In the large majority of plants the calyx is developed first, then

the corolla, and next the stamens. Preparations should be made from

materials hardened in alcohol, or first fixed with a strong solution of

picric acid and then hardened in alcohol.

_Pollen-grains._--Microspores are found lying free in sections made

of the reproductive organs; these may be transferred to a glycerine

fluid and examined under a high power. They are mostly spherical, with

granular protoplasmic contents, in which with much difficulty two

nuclei can be made out. Mount and examine, as types of the various

forms of granules, the pollen of Helianthus, Althœa, Cucurbita,

Ænothera, Orchis, Mimosa, Tulipa, &c. Mount any of these pollen-grains

in a weak solution of cane-sugar (about five per cent.), examine with

a high power, and note the configuration of their walls with a medium

power under polarised light. If transverse sections be made from very

young buds, the development of the anther and the pollen may be traced.

The material should be preserved in strong alcohol, and the sections

treated with equal parts of alcohol and glycerine, and exposed in a

watch-glass that the alcohol may evaporate. By this method sections may

be prepared for illustrating the formation of the _tapetum_, special

mother-cells, and division of the nucleus.

[Illustration: Fig. 319.--Pollen Grains.

A. Pollen-grain of Clove-pink; B. Poppy; C. Passion-flower (_Passiflora

cœrulea_); D. _Cobœa scandens_.]

_Starch Granules._--One of the most universally distributed materials

found in plants is starch composed of two substances, _granulose_,

which constitutes by far the largest part, and a skeleton of

_farinose_. It is only the former of these that stains blue with

iodine solutions; the latter partially assumes a brownish colour. The

structure of starch granules is not of equal density throughout; the

hilum or nuclear portion is most conspicuous, around which the rest

of the material is deposited in layers, indicative of stratification.

The several layers next to the hilum are less dense than those

farthest from it. The position of the hilum determines the form of the

grain, a few being rounded, others oval or elongated. The grain also

contains different proportions of water; this conveys the appearance

of concentric lines or curves about the nucleus. The latter is more

conspicuous in the potato starches, as seen in Plate XIII., Nos.

6-15. Starch grains, in nearly all cases, are formed by the agency

of proteid bodies, either chloroplasts or amyloplasts, and under the

action of sunlight are gradually broken up and employed in the process

of growth. There are some plants, however, notably the Compositeæ, in

which another carbohydrate, _inulin_, takes the place of starch from

the first, and is used as a reserve food material. For this reason we

look in vain for starch in the cells of Inula, Taraxacum, &c. From the

whole group of fungi starch is absent; this seems to explain the fact

that chlorophyll, or colouring matter, is rarely met with in the fungi,

hence their inability to utilize, like green plants, carbon-dioxide as

food.

[Illustration: Fig. 320.--Swollen Potato Starch, after the application

of potassium hydrate. (Magnified 210 diameters.)]

The tissues which most commonly contain starch, or which contain it

in largest quantity, are those of the parenchymatous series, though

it sometimes occurs in the latex of laticiferous tissues, and even in

ducts and tracheids. In the stems of Dicotyledons it occurs chiefly

in the parenchyma of the middle and inner bark, in the medullary ray

cells, and in the cells of the pith. In the roots of these plants it

has a similar distribution, being for the most part confined to the

middle or inner bark and the medullary rays, pith not being present in

these organs. In succulent stems and roots, of course, it also commonly

occurs in the xylem tissues of the fibro-vascular bundles.

A study of the various kinds of starches is important, since

this material is very largely used as an adulterant. Other than

microscopical means of detecting frauds are practically useless;

assaying is tedious and expensive, while the microscope is always

available and at hand. The limits of variation should be studied in

starches from the same species of plants; the variations are not very

wide, but in most cases characteristic, so that the discrimination is

at all times an easy task. The reagents required are simply iodine and

dilute potassium hydrate, aided by polarised light.

[Illustration: Fig. 321.

_a a a._ Granules and cells of cocoa; _b b b._ Arrowroot,

_Tous-les-mois_; _c c c._ Tapioca starch. (Magnified 300 diameters.)]

The starch grains of the potato are the best to study in the first

instance on account of their large size (Fig. 320).

In arrowroot starch (Fig. 321) the stratification is almost as distinct

as in that of the potato; the grains much resemble each other. Although

somewhat smaller, the grains of arrowroot are more uniform in size. The

starches are much used as an adulterant of drugs and various articles

sold as cocoas.

Wheat-starch (Fig. 322) consists of circular flattened grains varying

much in size, the central nucleus and stratification of which are very

difficult to distinguish.

In the smaller starches the hilum becomes more indistinct, and without

stratification, as in rice-starch, the latter being angular in shape.

The hilum in other leguminous plants forms a longitudinal cleft; white

rye-starch exhibits distinct cracks. Compound grains are occasionally

met with, as in the oat. In Plate XIII. will be found small groups of

starches taken under the same medium power for the sake of comparison.

In the microscopical examination of starches first use a 2/3-inch or a

1/2-inch and then a 1/6-inch objective.

[Illustration: Fig. 322.

_a._ Husks of Wheat-starch, swollen by

reagents and heat; _b._ A portion of cellulose; _c._ Rice-starch,

magnified 420 diameters.]

The bran of the husk of wheat when broken by grinding is seen to

be composed of two coats of hexagonal cells, the outer of which

is detached by the roasting process. The hexagonal cell layer is,

however, so little altered as to be perfectly distinguishable under the

microscope. Thus even a small admixture of roasted corn with coffee

or chicory can be detected without much difficulty. As to whether

starch granules should be regarded as crystalline or colloid bodies,

a difference of opinion still prevails. There are, however, reasons

for believing that the polarisation effects produced by starch grains

are not due to crystalline structure but to stress or strain, of the

same nature as the polarisation of glass when it is subject to strain.

The polarising phenomena are precisely such as would be induced in

any transparent solid composed of layers, the inner of which being

kept in a state of stress by the compression exerted by the outer

layers. Moreover, when by use of a swelling reagent, such as caustic

potash solution, the outer wall of the starch is made to expand by the

imbibition of water, the polarisation effects immediately disappear.

Were the solid particles of crystal thus forced apart by water each

particle would still exhibit polarisation phenomena.

Want of space will not permit me to further enlarge upon other

micro-chemical substances that enter into the composition of plants;

as, for example, the oil secreting glands. These when present take the

place of starch. There is, however, one product among the cell contents

of plants of some interest to the microscopist--those extremely

fine crystals known as _raphides_, composed of calcium-phosphate

and oxalate. Mr. Gulliver insisted upon the value of raphides as

characteristic of several families of plants. Schleiden states that

“needle-formed crystals, in bundles of from twenty to thirty in a cell,

are present in almost all plants,” and that so really practical is the

presence or absence of raphides, that by studying them he has been able

to pick out pots of seedling Onagraceæ, which had been accidentally

mixed with pots of other seedlings of the same age, and at that period

of growth when no other botanical character would have been so readily

sufficient.

If we examine a portion of the layers of an onion (Plate XIV., No. 3),

or a thin section of the stem or root of the garden rhubarb (No. 4), we

shall find many cells in which either bundles of needle-shaped crystals

or masses of a stellate form occur, not strictly raphides.

Raphides were first noticed by Malpighi in Opuntia, and subsequently

described by Jurine and Raspail. According to the latter observer,

the needle-shaped or acicular are composed of phosphate, and the

stellate of oxalate of lime. There are others having lime as a basis,

in combination with tartaric, malic, and citric acids, all of which

are destroyed by acetic acid; others are soluble in many of the fluids

employed in mounting. These crystals vary in size from the 1/40th

of an inch, while others are as small as the 1/1000th. They occur in

all parts of the plant; in the stem, bark, leaf, petals, fruit, root,

and even in the pollen, and occasionally in the interior of cells. In

certain species of aloe, as _Aloe verrucosa_, we are able to discern

small silky filaments; these are bundles of the acicular form of

raphides, and probably, as in sponges, act as a skeleton support to the

internal soft pulpy mass.

[Illustration: PLATE XIV.

STELLATE AND CRYSTALLINE TISSUE OF PLANTS.]

In portions of the cuticle of the medicinal squill (_Scilla maritima_)

large cells are found full of needle-shaped crystals. These cells,

however, do not lie in the same plane as the smaller cells of the

cuticle. In the cuticle of an onion every cell is occupied either by an

octahedral or a prismatic crystal of calcium oxalate. In some specimens

the octahedral form predominates; in others, even from the same plant,

the crystals are prismatic and arranged in a stellate form, as in that

of the grass (_Pharus cristatus_). (Plate XIV., No. 6.)

Raphides of peculiar figure are found in the bark of certain trees. In

the hickory (_Carya alba_) may be observed masses of flattened prisms

having both extremities pointed. In vertical sections from the stem of

_Elæagnus angustifolia_, numerous raphides of large size are embedded

in the pith, and also found in the bark of the apple-tree, and in elm

seeds, every cell containing two or more minute crystals.

In the Graminaceæ, especially the canes; in the _Equisetum hyemale_,

or Dutch rush; in the husk of rice, wheat, and other grains, silica in

some form or other is abundant. Some have beautifully-arranged masses

of silica with raphides. The leaves of _Deutzia scabia_, No. 7, are

remarkable for their stellate hairs, developed from the cuticle of both

their upper and under surfaces; forming most interesting and attractive

objects examined under polarised light. (Plate VIII., No. 173.)

Silica is found in the structure of Rubiaceæ both in the stem and

leaves, and, if present in sufficient thickness, depolarises light.

This is especially the case in the glandular hairs on the margins of

the leaves. One of the order Compositæ, a plant popularly known as the

“sneezewort” (_Archillæ ptarmica_), has a large amount of silica in the

hairs found about the serratures of its leaves.

All plants are provided with hairs; some few with hairs of a defensive

character. Those in the _Urtica dioica_, commonly called the

_Stinging-nettle_, are glandular hairs, developed from the cuticle, and

contain an irritating fluid; in other hairs a circulation is visible:

examined under a power of 100 diameters, they present the appearance

seen at Plate XIII., No. 19.

[Illustration: Fig. 323.

A. Cotton; B. Fibres of Flax; C. Filaments of Silk; D. Wool of Sheep.]

The fibrous tissue of plants is of great value in many manufactures.

It supplies material for our linens, cordage, paper, and other

industries. This tissue is remarkable for toughness of fibre, and

exhibits an approach to indestructibility, in the use it is put to in

connection with the electric light. It is of importance, then, to be

able to distinguish it from other fibres with which it is often mixed

in various manufactures. Here the use of the microscope is found of

considerable importance. In flax and hemp, in which the fibres are

of great length, there are traces of transverse markings at short

intervals. In the rough condition in which flax is imported into

this country, the fibres have been separated, to a certain extent,

by a process termed _hackling_, and further subjected to hackling,

maceration, and bleaching, before it can be reduced to the white silky

condition required by the spinner and weaver, and finally assumes the

appearance of structureless tubes, Fig. 323 B. China-grass, New Zealand

flax, and some other plants produce a similar material, but are not so

strong, in consequence of the outer membrane containing more _lignine_.

It is important to the manufacturer that he should be able to determine

the true character of some of the textures employed in articles of

clothing; this he may do by the aid of the microscope. In linen we find

each component thread made up of the longitudinal, unmarked fibres of

flax; but if cotton has been mixed, we recognise a flattened, more or

less rounded band, as in Fig. 323 A, having a very striking resemblance

to hair, which, in reality, it is; since, in the condition of elongated

cells, it lines the inner surface of the pod. These, again, should

he contrasted with the filaments of silk, Fig. 323 C, and also of

wool, Fig. 323 D. The latter may be at once recognised by the zigzag

transverse markings on its fibres. The surface of wool is covered with

furrowed and twisted fine cross lines, of which there are from 2,000

to 4,000 in an inch. On this structure depends its _felting_ property,

in judging of fleeces, attention should be paid to the fineness and

elasticity of the fibre--the furrowed and scaly surface, as shown by

the microscope, the quantity of fibre in a given surface, the purity

of the fleece, upon which depend the success of the scouring and

subsequent operations.

In the mummy-cloths of the Egyptians flax only was used, whereas the

Peruvians used cotton alone. By the many improvements introduced into

manufacturing processes, flax has been reduced to the fineness and

texture of silk, and even made to resemble other materials.

[Illustration: Fig. 324.

1. Woody Fibre from the root of the Elder, exhibiting small pores; 2.

Woody fibre of fossil wood, showing large pores; 3. Woody fibre of

fossil wood, bordered with pores and spiral fibres; 4. Fossil wood from

coal.]

_Fossil Plants._--It is well known that the primordial forests furnish

a number of families of plants familiar to the modern algæologist. The

cord-like plant, _Chorda filium_, known as “dead men’s ropes,” from

its proving fatal at times to the too adventurous swimmer who gets

entangled in its thick wreaths, had a Lower Silurian representative,

known to palæontologists as _Palæochorda_, or ancient chorda, which

existed, apparently, in two species,--a larger and a smaller. The

still better known _Chondrus crispus_, the Irish moss, or Carrageen

moss, has likewise its apparent, though more distant representative,

in chondritis, a Lower Silurian algal, of which there seems to exist

at least three species. The fucoids, or kelpweeds, appear to have also

their representatives in such plants as _Fucoides gracilis_, of the

Lower Silurians of the Malverns; in short, the Thallogens of the first

ages of vegetable life seem to have resembled in the group, and in at

least their more prominent features, the algæ of the existing time.

And with the first indications of land we pass from the thallogens to

the acrogens--from the seaweeds to the fern-allies. The Lycopodiaceæ,

or club-mosses, bear in the axils of their leaves minute circular

cases, which form the receptacles of their spore-like seeds. And when

high in the Upper Silurian system, and just when preparing to quit it

for the Lower Old Red Sandstone, we detect our earliest terrestrial

organisms, we find that they are composed exclusively of those little

spore-receptacles.

The existing plants whence we derive our analogies in dealing with the

vegetation of this early period contribute but little, if at all, to

the support of animal life. The ferns and their allies remain untouched

by the grazing animals. Our native club-mosses, though once used in

medicine, are positively deleterious; horsetails (_Equisetaceæ_),

though harmless, so abound in silex, which wrap them round with a

cuticle of stone, that they are rarely cropped by cattle; while the

thickets of fern which cover our hill and dell, and seem so temptingly

rich and green in their season, scarce support the existence of a

single creature, and remain untouched, in stem and leaf, from their

first appearance in spring until they droop and wither under the frosts

of early winter.

The flora of the coal measures was the richest and most luxuriant,

in at least individual productions, with which the fossil botanist

has formed an acquaintance. Never before or since did our planet bear

so rank a vegetation as that of which the numerous coal seams and

inflammable shales of the carboniferous period form but a portion of

the remains--the portion spared, in the first instance, by dissipation

and decay, and in the second by denuding agencies. Nevertheless almost

all our coal--the stored-up fuel of a world--is not, as it is often

said to be, the product of destroyed forests of conifers and flora

of the profuse vegetation of the earliest periods in the history of

our globe. Later investigations show that our coal measures are the

compressed accumulations of peat-bogs which, layer by layer, have

sunken down under the superimposed weight of the next. The vertical

stems of coniferous trees became imbedded by a natural process of

decay, and were subsequently overwhelmed in the erect position in

which they are found. The true grasses scarcely appear in the fossil

state at all. For the first time, amid the remains of a flora that

seems to have had but few flowers--the Oolitic ages--do we detect, in

a few broken fragments of the wings of butterflies, decided traces

of the flower-sucking insects. Not, however, until we enter into the

great Tertiary division do these become numerous. The first bee makes

its appearance in the amber of the Eocene, locked up hermetically

in its gem-like tomb--an embalmed corpse in a crystal coffin--along

with fragments of flower-bearing herbs and trees. Her tomb remains

to testify to the gradual fitting up of our earth as a place of

habitation for creatures destined to seek delight for the mind and

eye, as certainly as for the proper senses, and in especial marks

the introduction of the stately forest trees, and the arrival of the

charmingly beautiful flowers that now deck the earth.[62]