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Mechanical Tissues. — Plants growing in the air require certain skel-
etal structures to give them the necessary rigidity. These support-
ing tissues are known as mechanical tissues, but are not necessarily
devoted to this purpose only. The strongly distended cells of ordi-
nary parenchyma give firmness, and may to some extent be consid-
ered mechanical tissue, but large aerial plants require something





-vb




E



Fio. 38. — Af cross-section of the stem of a Begonia, showing the circle of vascular
bundles ( x 3) . B, collenchyma, or thick-angleil tissue from the outer part of the
cortex of the same plant, more highly magnified. C\ cross-section of the peduncle
of the inflorescence of Phttniz Canariensis, showing the numerous scattered vascu-
lar bundles (x 2). D, cross-section of an internode of the shoot of Equisetum
Imngatvmt showing the ring of vascular bundles, v6, alternating with large air-
spaces, /. Ef sclerenchyma, or fibrous tissue, from the onter part of the stem
(X 250). The shaded portions of A and C indicate the mechanical tissues.

more, and we find special tissues developed. In the vascular plants
there is generally found below the epidermis a greater or less devel-
oped system of supporting tissues (Hypoderma), which may be in
the form of elongated, thick-walled fibres, with pointed ends (Prosen-
ch3rma, e.g. Wood-fibres), or thick -angled elongated elements (Collen-
chyma, e.g. Begonia), or shorter, very thick-walled stony cells.
(Sclerenchyma, e.g. the rhizomes of most Ferns).

Most important in this connection are the vascular bundles of the
higher plants, which form a very complete skeleton of firm, woody



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68 BOTANY



tissue. The wood of the stem, and the framework, or veins of the
leaves, belong to the vascular system. The mechanical elements of
the vascular bundle are of two kinds, Fibres — either wood or bast
fibres — and tracheary tissue. The latter is also the principal water-
conducting tissue of these plants, and may be composed either of
Tracheids, which are single elongated cells, or Vessels, which are
rows of cells in which the transverse partitions have disappeared.
Both forms of tracheae, when mature, are destitute of living contents,
and their walls are marked by rings, spirals, reticulations, or pits,
due to unequal thickening in the growing wall.



Fio. 39. — A, cross-section of a vascular bnndle from the scape of Iris Florentina
(X 260) ; t, tracheary tissue; pA, phloem. B, longitudinal section of the same;
tt spiral, i\ reticulate vessels; «, a sieve-tube.

In some of the lower plants, like some Seaweeds, firmness is given
to the plant by great thickening of the walls of the superficial cells,
such as occurs in many forms which are exposed to the heavy surf.
Others, like the calcareous Algae, attain the eame end by a heavy
deposit of lime in their outer cells.

Protective Tissues. — All of the superficial cells of plants exposed
to the air are provided with a heavily cutinized membrane, which is
especially developed in plants of dry regions. This thick cuticle
prevents excessive loss of water from the delicate inner tissues.
The layers of cork-cells in the stems of woody plants serve the same
purpose.



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THE PLANT-CELL 50



Conductiye Tissues

Besides the tracheary tissue already referred to, there are other
forms of conducting tissue met with. Most important are the sieve-
tubes (Fig. 40) found in the outer or bast portion
(phloem) of the vascular bundles. The sieve-
tubes closely resemble the tracheae of the woody
part of the bundle, but differ in not having the
walls lignified, and in retaining the living cell-
contents. WTiile the tracheae are mainly con-
cerned with the conduction of water, the
sieve-tubes are the important agents in the
transfer of assimilated food-elements. Very
similar in appearance to the sieve-tubes of the
vascular plants are those found in many of
the lai-ge Alps, or Brown Algae.

Another type of conducting tissue is seen in
the so-called Laticiferous ducts, which occur
in plants with milky juice, like the Poppy, Let-
tuce, Milkweed, Euphorbia, etc. Sometimes pio. 40.— Longitudinal
the latex is red, e.g. Bloodroot (Sanguinaria), section of part of a
yellow (Argemone), or colorless (Eschscholtzia). rhlzTXlX^';

The laticiferous the cytoplasm has
ducts may be *>««'» contracted by
either very long f^^^"" ^' ^'^^'^'^^
and branched

single elements, e.g. Euphorbia, or
the much more common irregularly
branching system formed by the
coalescence of many cells (Fig. 41).
It is somewhat questionable how far
the laticiferous ducts are of impor-
tance in the transfer of plastic mate-
rials. Much of the contents are
apparently excretions, whose func-
tions, if any, are not certainly known.

Special Secretory Cells

Special secretory cells are of wide
FiG.41.— Anastomosinglaticiferoos occurrence. Such are the cells secret-
ing the various aromatic substances
to which plants owe their character-
istic odors. The oil-glands in the Orange and Lemon belong to this
cat^ory, as do the mucilage and oil-cells in many Liverworts.



vessels from the stem of Sonchus
oleraceus.



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60



BOTANY



CELL-FORMATION

New cells may arise by division, or by the union of two (occasion-
ally several) into a single cell.

Fission. — The commonest form of cell-multiplication is the divi-
sion of the cell into two, usually equal, parts. This mode of division,
or Fission (Fig. 42), is the only method by which new cells are formed
in the lowest organisms, such as Bacteria. In the Bacteria, where a
distinct nucleus cannot be certainly demonstrated, the cell-division
consists merely in the constriction of the protoplast, and its division
without the complicated changes in the nucleus which characterize
cell-division in the higher plants. Sometimes there is no evident con-
striction of the protoplast, but a division-wall cuts the cell into two
parts, which may remain connected, and by repeated divisions give rise
to a cell-row. In these lowest forms, all the cells are alike, and there




Fio. 42. — A, ceU of a Bacteriam, ChromaUum WeissiU in process of divisioir
(X 1000). B, a living cell of Cladophora glomerata, in process of division; the
division-wall is not complete. C, the same cell an hoar later (x 200). D, cells of
Yeast, Saccharomyces cerevisia, multiplying by badding (X 700).

is no distinction between vegetative and reproductive cells. In some-
what more specialized forms, certain cells may be somewhat changed,
and become modified into thick-walled resting spores, which are,
however, derived from ordinary vegetative cells.

Where a definite nucleus is present in the cell, as occurs always in
the cells of the typical plants, the division of the protoplast is pre-
ceded by a division of the nucleus. The only exceptions to this are
multinucleate cells, or Coenocytes, in which nuclear division and cell-
division are quite independent. The formation of the division-wall
may begin as an equatorial ring of cellulose, which grows centripetally,
until it cuts the protoplast in two ; or there may be formed simulta-
neously in the protoplast an equatorial cell-plate, which extends com-
pletely across the cell. i V*

Karytiklnesis

The division of the protoplast is preceded by extensive changes in
the nucleus, which finally become divided into two daughter-nucleL
These changes are known as Mitosis, or Karyokinesis.



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THE PLANT-CELL 61



The Resting NucleuB. — The resting nucleus (Fig. 44, A) contains a
complicated network, made up of linin-threads, in which are imbedded
more or less numerous chromatin-granules. One or more nucleoli are
also usually present.

Prophases. — The first ^

sign of approaching divi-
sion is a shortening and
thickening of the linin-
filaments, which sometimes

may be shown to constitute ^...

a single long and very

much tangled thread. This j^

is accompanied by an in- ^

crease in the amount of
chromatin, which forms
a series of disks arranged

along the linin-thread, like P~

beads, separated by short
intervals (Fig. 44, D). The
spaces between the chro-
matin disks may almost
completely disappear as -^(J

the thread shortens, so that «_ >,o r. n # *u * *: * /^ *

' Fio. 43. — Cells from the root-tip of an Onion,

the thread appears almost dividing by mitosis, or karyokinesis (x 625).
homogeneous. There next

follows a longitudinal splitting of the nuclear filament, which thus
forms two threads, lying close together and often hard to distinguish.

OiTomosomes. — Each filament divides transversely into a definite
number of pieces — nuclear segments, or Chromosomes, which are in
pairs, one segment of each pair belonging to each half into which the
original nuclear filament splits longitudinally. The two chromosomes
of each pair sometimes fuse more or less completely together. The
chromosomes appear homogeneous, and stain very strongly with the
usual nuclear stains. Their form varies a good deal, from almost
globular to elongated, straight, or bent rods.

While these changes are taking place in the nuclear filament, the
nucleolus usually shows signs of disorganization, and finally is no
longer visible. Just what ibstance is still doubtful.

Spindle-fibres. — In the diately surrounding the

nuclear cavity, there may l lely fine filaments, which

sometimes form a thick ts it the nucleus, but later

show a more or less distind 4, B, C). These begin to

penetrate into the nuclear ^ , 11 becomes less evident,

and finally quite unrecognizable.

MetafibAS^- — As the nuclear membrane disappears, the chromo-



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62 BOTANY



somes arrange themselves in a more or less distinct plate which
occupies the equator of the dividing cell. The cytoplasmic fibres are
now seen to converge at several points in the cytoplasm, and some of
them are connected with the chromosomes, which may each show a
sheaf of these attached to it, while other fibres remain free. The
several converging points, or poles, in the cytoplasm move toward
each other, and usually form two in the long axis of the cell, and
at equal distances from the equatorial nuclear plate. The free
fibres run from pole to pole, while the bundles connected with the

B



ch




Fiu. 4A. — At pollen mother-cell of Podophyllum peltatmn, bliowiug the resting
nucleus, with the net-work of unclear filaments, and the nucleolus. B, late
prophase of division ; the nuclear segments (chromosomes) are separate, the
spindle-fibres arranged in several groups. C, completed nuclear spindle; the
chromosomes have divided into two. />, part of the nuclear filament of Hellehorus
futidua, showing the chromatin-granules imbedded in the linin-thread. E^ a later

. stage, showing the splitting of the filament. (All figures after Mottier.)

chromosomes are attached to one pole only. The whole mass of
fibres is spindle-shaped, hence the whole figure is known as the
Nuclear spindle, and the filaments as Spindle-fibres.

In the nuclear plate the pairs of chromosomes separate, and begin
to move toward opp ipindle, perhaps due to

the contraction of t \ attached to each. It

has also been conjee s, sometimes found at

the poles, may be c( m of the chromosomes

to the poles. Besid ^ fibres, which run from

pole to pole, and the " mantle-fibres," which are attached to the chro-
mosomes, there have also been detected, at the outside of the spindle,



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THE PLANT-CELL 63



free fibres which are attached at one end at the poles, but end free
in the surrounding cytoplasm.

Anaphases. — As the chromosomes approach the poles of the spin-
dle, they become crowded together, and finally grow together, end to
end, and constitute a single filament, which gradually assumes the
condition found in the resting nucleus. The nucleolus is formed
again, as well as the nuclear membrane, and the nucleus has now all
the characters of the typical resting nucleus.

Cell-plate. — While the two groups of chromosomes are moving
toward the poles, there suddenly becomes evident, in the equator of
the spindle, a disk, formed of small granular bodies, which finally
coalesce into a continuous membrane, — the Cell-plate. The granules
of which the young cell-plate is composed are formed by swellings
in the connecting fibres, whose substance, apparently, is transformed
into the elements of the cell-plate. In case the Cell-plate does not
extend entirely across the cell, new elements are added to its margin
by the peripheral spindle-fibres. The cell-plate finally splits into
two lamellae, and thus the division of the protoplast is completed.
The new cell-wall is then deposited in the space between the proto-
plasts, in the same way that
a cell-wall is formed upon the
surface of a naked protoplast,
such as a zo5spore.

The changes in the nucleus
up to the formation of the
nuclear plate are known as
the Prophases ; the separation
of the chromosomes and their
movements to the poles, the
Metaphases ; the reconstitu- ^ *
tion of the daughter-nuclei,
the Anaphases.

Direct Nuclear Division. —

Sometimes in large cells, like Fio. 45. — Direct (amitotic) nuclear division

the internodes of the Characeae, *° *" .'"^'"f'^^j. ^^" ""! . ^'^"'''* /'""vi^i*

, ^, • XV i. i (X 750): n, dividing nuclei,

and those in the stem of ^

Tradescantia, the nucleus may become constricted, or divided directly.

This is known as direct or amitotic division, but only occurs in old

cells, and is never accompanie^^l^^ division of the protoplast

(Fig. 45).



^nie^^l^^ c



The form of fission known IstS^^^ consists simply in a protru-
sion of the cell-wall, which is then separated from the parent-cell by
fission. This occurs rejjularly in the Yeast-fungi, and is also seen in
the branching of many filamentous Algse.



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BOTANY





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Internal Cell-division

Internal cell-division differs from the ordinary form of fission only
in having the division confined to the protoplast, a new cell-wall

being formed about the new cells,
either while still contained within
the mother-cell or after their escape.
Where the protoplast divides after
each nuclear division, it is hardly
distinguishable from typical fission;
but often there is repeated nuclear
division and a simultaneous division
of the protoplast into as many parts as
there are nuclei. InternaJ division
is especially common in the formation
of the reproductive cells of many
plants, such as the zoospores and
spermatozoids of many Algae, the
pollen-spores of Flowering Plants, etc.

Free Cell-formation

Free cell-formation is a form of
internal cell-division, where a cell-wall
is formed about the nuclei in the proto-
plasm, leaving a certain amount of the
cytoplasm unused. The commonest
example of this is found in the forma-
tion of the so-called "Ascospores" of
many Fungi and Lichens. Free cell-
formation has also been observed in the
development of the embryo in Ephedra
and some other Gymnosperms.



Fid. 46. — A, sporogenoos ceU of
Azollafiliculoides, dividing into
four by internal division
(X800). Bf an older stage,
with the fonr spores completely
divided ; only two of the spores
are completely shown in the
section. C, ascos, or spore-
sac, of a Cup-fungus (Peziza),
containing eight spores formed
by free cell-formation (x 260).



Conjugation

In most plants there arise, at certain times, new cells, formed by
the union of the protoplasm of two independent cells. These unit-
ing cells are the sexual cells, or Gametes, and the cell produced from
their union is a Zyg nplest form the gametes are

entirely similar, eithe ciliated cells, e.g. Pandorina,

or non-motile, as in S] le protoplast of one cell flows

through a tube into a I

In most plants ther 'erence in the character of the

two gametes. One is much larger than the other, and is passive —
this is the female cell (Egg or Ovum). The other, the male or sperm-



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THE PLANT-CELL 66



I



cell, is much smaller and often actively motile, when it is termed a
Spermatozoid. The fusion of the latter with the egg constitutes fer-
tilization, or fecundation, without which the eggy except in rare cases,
is incapable of further development. The greater part of the sper-
matozoid is composed of nuclear matter, which fuses more or less
completely with the
nucleus of the egg- . {

cell before the latter
divides.

The differentiation
of sexual cells has j /-v » / p

taken place quite in- ~ * ^

dependently in sev-
eral widely separated
groups of plants,
where nearly every
gradation between Fio- 47. — -4, single gamete. B, conjugating plano-
Tw»rfpotlv RimilftT ^am gametes of Ectocarpus siliculosus (X 790). C, male

periecwy similar gam gamete. D, female gamete of Cutlena mvltifida

etes and well-marked (x960). E, egg. F, spermatozoid of Fucus vest-
male and female cells cuioms (x300). {A, B, after Bbrthold; C7, />,
may stUl be seen. »^^' ^™^>

Thus in the Brown Algae many forms, including the largest ones,
produce no sexual cells at all, but only zoospores, which germinate
directly. Ectocarpus and various other allied genera produce simi-
lar motile gametes (Planogametes) ; Cutleria produces two kinds of
motile gametes, of very unequal size ; while in Fucus, the common
Eockweed, the non-motile egg-cells are enormously larger than the
active, ciliated spermatozoids (Fig. 47).





BIBLIOGRAPHY

•87. 1. De Bary, A. Comparative Anatomy of the Ferns and Flowering
Plants. Oxford, 1887.

Chamberlain, C. J. Methods in Plant Histology. Chicago, 1001.

Fischer, A. Fixirung, Fftrbung and Bau des Protoplasmas. Jena, 1899.

Haberlandt, G. Physiologische Pflanzenanatomie. Leipzig, 1896.

Henneguy, L. F. Lemons sur la Cellule. Paris, 1898.

Hertwig, O. Die Zelle und die Gewebe. Jena, 1898.

Sachs, J. Text-book of Botany. Oxford, 1882.

Strasburger, E. Histologische Beitrfige, I- VI. Jena, 1890-1900.

Das botanische Practicum. 3d edition. Jena, 1897.

Tachirch, A. Angewandte Pflan^'naii^tomie. Leipzig, 1889.

Van Tieghem, Ph. Traits de jguitrfQufe. Paris, 1898.

Vines, S. H. Students' Text-Wl^oTOi§any. London and New York,
1896. W-l:r/' fi

Wieener, J. Anatomic and Phy^ai^gii^ den Pflanzen. Vienna, 1898.

Wilson, £. B. The Cell in Development and Inheritance. New York.
1900. (This contains an excellent bibliography of the subject. )
'93. 15. Zimmermann, A. Botanical Microtechnique. New York, 1893.



'01.


2.


'99.


3.


'96.


4.


'98.


5.


'98.


6.


'82.


7.


'00.


8.


'97.


9.


'89.


10.


'98.


11.


^96.


12.


'98.


13.


'00.


14.



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CHAPTER IV
CLASSIFICATION; THE SIMPLEST PLANT-FORMS

It is generally assumed that a real genetic relationship exists
among all plants, and the aim of a natural system of classification
is to express the degree of this relationship. An ideal classification
would represent the family tree of the vegetable kingdom, but un-
fortunately such a classification is not to be hoped for, owing to the
complete disappearance of many plant-forms, which has resulted in
the survival of many isolated types that are only distantly related
to other known forms, and to which it is impossible, at present, to
assign a certain position in the system of classification. Among
such isolated groups may be mentioned the Diatoms and Gharacese.

Factors in ClasBification. — In determining the degree of relation-
ship between plants, probably the general structure, or morphology,
is of the first importance ; but as certain parts, especially the repro-
ductive structures, are less subject to change from external con-
ditions, these less variable structures are, of course, especially
important in classification. Where plants are obviously closely
related, as, for instance, two species of the same genus, it is differ-
ences, rather than resemblances, that are considered in assigning
them their places. Where relationships are less obvious, it often
becomes necessary to study all phases of the development of the
plant — its " life-history " — in order to determine its affinities with
other forms. No single point of structure can be safely used alone,
and, so far as possible, all the structures must be considered.

Ontogeny and Phylogeny. — It is assumed that the life-history, or
" Ontogeny," of the individual repeats, to some extent, the evolution
of the race, " Phylogeny," and a study of the developing organism,
is often of the greatest importance in making out its relationship to
other and especially lower forms. All Mosses and Ferns, for exam-
ple, produce minute motile repro(^ctive cells (sperraatozoids), which
closely resemble similar cells ajong the Algae, and indicate that
these land plants have sprung from aquatic ancestors resembling the
existing Green Algae.

The geological record, so far as it goes, is of very great value in
tracing the evolution of the vegetable kingdom; but unfortunately



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CLASSIFICATION 67



the record is very incomplete, especially as regards the very perish-
able structures of the lower plants, and we can never expect to have
much light thrown on the origin of these lower plant-types, from a
study of fossils.

Classification

The vegetable kingdom may be divided into a number of primary
groups, " Subkingdoms," or " Branches," as to whose limits there is
a good deal of difference of opinion. We shall assume here five of
these subkingdoms, viz. Schizophyta, Algae, Fungi, ArchegoniataB,
Spermatophyta. Besides these there are two groups of organisms,
sometimes included among plants, the Myxomycetes (Mycetozoa)
and the Magellata, both of which show unmistakable animal aflfini-
ties as well.

Each subkingdom is divided into classes, these into orders, fami-
lies, genera, and species, which are sometimes still farther subdivided.

THE SIMPLEST ORGANISMS

Many of the lowest organisms known are so simple in structure as
to make it impossible to decide positively whether their affinities are
with plants or animals. They are simply undifferentiated living be-
ings, such as we may reasonably infer existed before there were any
true plants or animals.

Protista. — To these lowest forms of life Haeckel gave the name
" Protista," and assumed that some of them consisted of quite undif-
ferentiated protoplasm. The more perfect methods of investiga-
tion now in use have demonstrated that it is exceedingly doubtful
whether any organisms of such extreme simplicity really exist, and
most of the Protista have been relegated to one or the other of the
two great organic kingdoms. Nevertheless, there are two groups of
organisms, the Flagellata and the Myxomycetes or Mycetozoa, which
seem to lie on the border line between plants and animals.

FLAGELLATA

The Flagellata (Fig. 48) are unicellular organisms, which are provided with
one or two (occasionally more) flagella or cilia, by means of which they are able
to move rapidly in the water. The cell may be quite naked, or there may be a
more or less marked membrane, whj^h very rarely, however, is composed of
celhiloee. The cell contains a single nucleus, and sometimes chromatophores,
which may be either green (Euglena) or brown (Hydrurus). The forms which
possess chromatophores ate able to assimilate carbon-dioxide, like normal plants,
but those which are destitute of these feed upon organic matter. Some of the
more highly organized forms possess a ntouth, so that they can ingest solid food,
which in the lower forms may be taken in at any part of the protoplast.



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68



BOTANY



Reproduction is either by a division (mostly longitudinal) which may occur
while the cell is active, or it may first become encysted, after which the proto-
plast divides into two new individuals.
No sexual reproduction has yet been
certainly demonstrated for any of them.

Affinities of Flagellata. — The
Flagellata show affinities on the
one hand with the Infusoria, and
on the other with the lower
plants. The Volvocaceae, which
are sometimes included with the
Flagellata, are forms which to
a certain extent connect the typi-
cal green plants with the true
Flagellata. The Myxomycetes
or Slime-moulds, the Brown Algae,
and possibly the Bacteria, also
show evidences of relationship
with the Flagellata, which are
Fio. 48.-FlageUata. .4, B Chromu- ^hus seen to be a group almost

lina ovalts. A^ active cell, showing , . j» ^ i_ i.



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