Elements of Structural and Systematic Botany

D >> Douglas Houghton Campbell >> Elements of Structural and Systematic Botany

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Of food plants belonging to this order, the banana (_Musa_) is much
the most important. Others of more or less value are species of
arrowroot (_Maranta_) and ginger (_Zingiber_).

There are three families: I. _Musaceae_ (banana family);
II. _Zingiberaceae_ (ginger family); and III. _Cannaceae_ (_Canna_,
_Maranta_).

ORDER VI.--_Gynandrae_.

By far the greater number of the plants of this order belong to the
orchis family (_Orchideae_), the second family of the order
(_Apostasieae_), being a small one and unrepresented in the United
States. The orchids are in some respects the most highly specialized
of all flowers, and exhibit wonderful variety in the shape and color
of the flowers, which are often of extraordinary beauty, and show
special contrivances for cross-fertilization that are without parallel
among flowering plants.

[Illustration: FIG. 89.--_Gynandrae_. _A_, inflorescence of the showy
orchis (_Orchis spectabilis_), x 1 (_Orchideae_). _B_, a single flower,
with the upper leaves of the perianth turned back to show the column
(_x_). _sp._ the spur attached to the lower petal or lip. _o_, the
ovary, x 1. _C_, the column seen from in front. _an._ the stamen.
_gy._ the stigmatic surface, x 1. _D_, the two pollen masses attached
to a straw, which was inserted into the flower, by means of the viscid
disc (_d_): i, the masses immediately after their withdrawal; ii, iii,
the same a few minutes later, showing the change in position. _E_,
diagram of the flower; the position of the missing stamens indicated
by small circles.]

The flowers are always more or less bilaterally symmetrical
(zygomorphic). The ovary is inferior, and usually twisted so as to
turn the flower completely around. There are two sets of perigone
leaves, three in each, and these are usually much alike except the
lower (through the twisting of the ovary) of the inner set. This
petal, known as the "lip" or "labellum," is usually larger than the
others, and different in color, as well as being frequently of
peculiar shape. In many of them it is also prolonged backward in a
hollow spur (see Fig. 89, _B_). In all of the orchids except the
lady's-slippers (_Cypripedium_) (Fig. 90, _B_), only one perfect
stamen is developed, and this is united with the three styles to form
a special structure known, as the "column" or "gynostemium" (Fig. 89,
_B_, _C_). The pollen spores are usually aggregated into two or four
waxy masses ("pollinia," sing. pollinium), which usually can only be
removed by the agency of insects upon which all but a very few orchids
are absolutely dependent for the pollination of the flowers.

[Illustration: FIG. 90.--Forms of _Orchideae_. _A_, putty-root
(_Aplectrum_), x 1. _B_, yellow lady's-slipper (_Cypripedium_), x 1/2.
_C_, the column of the same, x 1. _an._ one of the two perfect
stamens. _st._ sterile, petal-like stamen. _gy._. stigma. _D_,
_Arethusa_, x 1/2. _E_, section of the column, x 1: _an._ stamen. _gy._
stigma. _F_, the same, seen from in front. _G_, _Habenaria_, x 1. _H_,
_Calopogon_, x 1. In the last the ovary is not twisted, so that the
lip (_L_) lies on the upper side of the flower.]

In the lady-slippers there are two fertile stamens, and a third
sterile one has the form of a large triangular shield terminating the
column (Fig. 90, _C_, _st._).

The ovules of the orchids are extremely small, and are only partly
developed at the time the flower opens, the pollen tube growing very
slowly and the ovules maturing as it grows down through the tissues of
the column. The ripe seeds are excessively numerous, but so fine as to
look like dust.

The orchids are mostly small or moderate-sized plants, few of them
being more than a metre or so in height. All of our native species,
with the exception of a few from the extreme south, grow from fibrous
roots or tubers, but many tropical orchids, as is well known, are
"epiphytes"; that is, they grow upon the trunks and branches of trees.
One genus, _Vanilla_, is a twining epiphyte; the fruit of this plant
furnishes the vanilla of commerce. Aside from this plant, the
economical value of the orchids is small, although a few of them are
used medicinally, but are not specially valuable.

Of the five thousand species known, the great majority are inhabitants
of the tropics, but nevertheless there are within the United States a
number of very beautiful forms. The largest and showiest are the
lady's-slippers, of which we have six species at the north. The most
beautiful is the showy lady's-slipper (_Cypripedium spectabile_),
whose large, pink and white flowers rival in beauty many of the
choicest tropical orchids. Many of the _Habenarias_, including the
yellow and purple fringed orchids, are strikingly beautiful as are the
_Arethuseae_ (_Arethusa_, _Pogonia_, _Calopogon_). The last of these
(Fig. 90, _H_) differs from all our other native orchids in having the
ovary untwisted so that the labellum lies on the upper side of the
flower.

A number of the orchids are saprophytic, growing in soil rich in
decaying vegetable matter, and these forms are often nearly or quite
destitute of chlorophyll, being brownish or yellowish in color, and
with rudimentary leaves. The coral roots (_Corallorhiza_), of which
there are several species, are examples of these, and another closely
related form, the putty-root (_Aplectrum_) (Fig. 90, _A_), has the
flowering stems like those of _Corallorhiza_, but there is a single,
large, plaited leaf sent up later.

ORDER VII.--_Helobiae_.

The last order of the monocotyledons is composed of marsh or water
plants, some of which recall certain of the dicotyledons. Of the three
families, the first, _Juncagineae_, includes a few inconspicuous plants
with grass-like or rush-like leaves, and small, greenish or yellowish
flowers (_e.g._ arrow-grass, _Triglochin_).

The second family (_Alismaceae_) contains several large and showy
species, inhabitants of marshes. Of these the water-plantain
(_Alisma_), a plant with long-stalked, oval, ribbed leaves, and a
much-branched panicle of small, white flowers, is very common in
marshes and ditches, and the various species of arrowhead
(_Sagittaria_) are among the most characteristic of our marsh plants.
The flowers are unisexual; the female flowers are usually borne at the
base of the inflorescence, and the male flowers above. The gynoecium
(Fig. 91, _B_) consists of numerous, separate carpels attached to a
globular receptacle. The sepals are green and much smaller than the
white petals. The leaves (_F_) are broad, and, besides the thickened,
parallel veins, have numerous smaller ones connecting these.

[Illustration: FIG. 91.--Types of _Helobiae_. _A_, inflorescence of
arrowhead (_Sagittaria_), with a single female flower, x 1/2
(_Alismaceae_). _B_, section through the gynoecium, showing the numerous
single carpels, x 3. _C_, a ripe fruit, x 3. _D_, a male flower, x 1.
_E_, a single stamen, x 3. _F_, a leaf of _Sagittaria variabilis_,
x 1/6. _G_, ditch-moss (_Elodea_), with a female flower (_fl._), x 1/2.
(_Hydrocharideae_). _H_, the flower, x 2. _an._ the rudimentary
stamens. _st._ the stigma. _I_, cross-section of the ovary, x 4. _J_,
male inflorescence of eel-grass (_Vallisneria_), x 1. _K_, a single
expanded male flower, x 12. _st._ the stamen. _L_, a female flower,
x 1. _gy._ the stigma.]

The last family is the _Hydrocharideae_. They are submersed aquatics,
or a few of them with long-stalked, floating leaves. Two forms, the
ditch-moss (_Elodea_) (Fig. 91, _G_, _I_) and eel-grass
(_Vallisneria_) are very common in stagnant or slow-running water. In
both of these the plants are completely submersed, but there is a
special arrangement for bringing the flowers to the surface of the
water. Like the arrowhead, the flowers are unisexual, but borne on
different plants. The female flowers (_H_, _L_) are comparatively
large, especially in _Vallisneria_, and are borne on long stalks, by
means of which they reach the surface of the water, where they expand
and are ready for pollination. The male flowers (Fig. 91, _J_, _K_)
are extremely small and borne, many together, surrounded by a
membranous envelope, the whole inflorescence attached by a short
stalk. When the flowers are ready to open, they break away from their
attachment, and the envelope opens, allowing them to escape, and they
immediately rise to the surface where they expand and collect in great
numbers about the open female flowers. Sometimes these are so abundant
during the flowering period (late in summer) that the surface of the
water looks as if flour had been scattered over it. After pollination
is effected, the stem of the female flower coils up like a spring,
drawing the flower beneath the water where the fruit ripens.

The cells of these plants show very beautifully the circulation of the
protoplasm, the movement being very marked and continuing for a long
time under the microscope. To see this the whole leaf of _Elodea_, or
a section of that of _Vallisneria_, may be used.

CHAPTER XVII.

DICOTYLEDONS.

The second sub-class of the angiosperms, the dicotyledons, receive
their name from the two opposite seed leaves or cotyledons with which
the young plant is furnished. These leaves are usually quite different
in shape from the other leaves, and not infrequently are very thick
and fleshy, filling nearly the whole seed, as may be seen in a bean or
pea. The number of the dicotyledons is very large, and very much the
greater number of living spermaphytes belong to this group. They
exhibit much greater variety in the structure of the flowers than the
monocotyledons, and the leaves, which in the latter are with few
exceptions quite uniform in structure, show here almost infinite
variety. Thus the leaves may be simple (undivided); _e.g._ oak, apple;
or compound, as in clover, locust, rose, columbine, etc. The leaves
may be stalked or sessile (attached directly to the stem), or even
grown around the stem, as in some honeysuckles. The edges of the
leaves may be perfectly smooth ("entire"), or they may be variously
lobed, notched, or wavy in many ways. As many of the dicotyledons are
trees or shrubs that lose their leaves annually, special leaves are
developed for the protection of the young leaves during the winter.
These have the form of thick scales, and often are provided with
glands secreting a gummy substance which helps render them
water-proof. These scales are best studied in trees with large, winter
buds, such as the horsechestnut (Fig. 92), hickory, lilac, etc. On
removing the hard, scale leaves, the delicate, young leaves, and often
the flowers, may be found within the bud. If we examine a young shoot
of lilac or buckeye, just as the leaves are expanding in the spring, a
complete series of forms may be seen from the simple, external scales,
through immediate forms, to the complete foliage leaf. The veins of
the leaves are almost always much-branched, the veins either being
given off from one main vein or midrib (feather-veined or
pinnate-veined), as in an apple leaf, or there may be a number of
large veins radiating from the base of the leaf, as in the scarlet
geranium or mallow. Such leaves are said to be palmately veined.

[Illustration: FIG. 92.--End of a branch of a horsechestnut in winter,
showing the buds covered by the thick, brown scale leaves, x 1.]

Some of them are small herbaceous plants, either upright or prostrate
upon the ground, over which they may creep extensively, becoming
rooted at intervals, as in the white clover, or sending out special
runners, as is seen in the strawberry. Others are woody stemmed
plants, persisting from year to year, and often becoming great trees
that live for hundreds of years. Still others are climbing plants,
either twining their stems about the support, like the morning-glory,
hop, honeysuckle, and many others, or having special organs (tendrils)
by which they fasten themselves to the support. These tendrils
originate in different ways. Sometimes, as in the grape and Virginia
creeper, they are reduced branches, either coiling about the support,
or producing little suckers at their tips by which they cling to walls
or the trunks of trees. Other tendrils, as in the poison ivy and the
true ivy, are short roots that fasten themselves firmly in the
crevices of bark or stones. Still other tendrils, as those of the
sweet-pea and clematis, are parts of the leaf.

The stems may be modified into thorns for protection, as we see in
many trees and shrubs, and parts of leaves may be similarly changed,
as in the thistle. The underground stems often become much changed,
forming bulbs, tubers, root stocks, etc. much as in the
monocotyledons. These structures are especially found in plants which
die down to the ground each year, and contain supplies of nourishment
for the rapid growth of the annual shoots.

[Illustration: FIG. 93.--_A_, base of a plant of shepherd's-purse
(_Capsella bursa-pastoris_), x 1/2. _r_, the main root. _B_, upper part
of the inflorescence, x 1. _C_, two leaves: i, from the upper part;
ii, from the base of the plant, x 1. _D_, a flower, x 3. _E_, the
same, with sepals and petals removed, x 3. _F_, petal. _G_, sepal.
_H_, stamen, x 10. _f_, filament. _an._ anther. _I_, a fruit with one
of the valves removed to show the seeds, x 4. _J_, longitudinal
section of a seed, x 8. _K_, the embryo removed from the seed, x 8.
_l_, the first leaves (cotyledons). _st._ the stem ending in the root.
_L_, cross-section of the stem, x 20. _fb._ fibro-vascular bundle.
_M_, a similar section of the main root, x 15. _N_, diagram of the
flower.]

The structure of the tissues, and the peculiarities of the flower and
fruit, will be better understood by a somewhat careful examination of
a typical dicotyledon, and a comparison with this of examples of the
principal orders and families.

One of the commonest of weeds, and at the same time one of the most
convenient plants for studying the characteristics of the
dicotyledons, is the common shepherd's-purse (_Capsella
bursa-pastoris_) (Figs. 93-95).

The plant grows abundantly in waste places, and is in flower nearly
the year round, sometimes being found in flower in midwinter, after a
week or two of warm weather. It is, however, in best condition for
study in the spring and early summer. The plant may at once be
recognized by the heart-shaped pods and small, white, four-petaled
flowers. The plant begins to flower when very small, but continues to
grow until it forms a much-branching plant, half a metre or more in
height. On pulling up the plant, a large tap-root (Fig. 93, _A_, _r_)
is seen, continuous with the main stem above ground. The first root of
the seedling plant continues here as the main root of the plant, as
was the case with the gymnosperms, but not with the monocotyledons.
From this tap-root other small ones branch off, and these divide
repeatedly, forming a complex root system. The main root is very tough
and hard, owing to the formation of woody tissue in it. A
cross-section slightly magnified (Fig. 93, _M_), shows a round,
opaque, white, central area (_x_), the wood, surrounded by a more
transparent, irregular ring (_ph._), the phloem or bast; and outside
of this is the ground tissue and epidermis.

The lower leaves are crowded into a rosette, and are larger than those
higher up, from which they differ also in having a stalk (petiole),
while the upper leaves are sessile. The outline of the leaves varies
much in different plants and in different parts of the same plant,
being sometimes almost entire, sometimes divided into lobes almost to
the midrib, and between these extremes all gradations are found. The
larger leaves are traversed by a strong midrib projecting strongly on
the lower side of the leaf, and from this the smaller veins branch.
The upper leaves have frequently two smaller veins starting from the
base of the leaf, and nearly parallel with the midrib (_C_ i). The
surface of the leaves is somewhat roughened with hairs, some of which,
if slightly magnified, look like little white stars.

Magnifying slightly a thin cross-section of the stem, it shows a
central, ground tissue (pith), whose cells are large enough to be seen
even when very slightly enlarged. Surrounding this is a ring of
fibro-vascular bundles (_L_, _fb._), appearing white and opaque, and
connected by a more transparent tissue. Outside of the ring of
fibro-vascular bundles is the green ground tissue and epidermis.
Comparing this with the section of the seedling pine stem, a
resemblance is at once evident, and this arrangement was also noticed
in the stem of the horse-tail.

Branches are given off from the main stem, arising at the point where
the leaves join the stem (axils of the leaves), and these may in turn
branch. All the branches terminate finally in an elongated
inflorescence, and the separate flowers are attached to the main axis
of the inflorescence by short stalks. This form of inflorescence is
known technically as a "raceme." Each flower is really a short branch
from which the floral leaves arise in precisely the same way as the
foliage leaves do from the ordinary branches. There are five sets of
floral leaves: I. four outer perigone leaves (sepals) (_F_), small,
green, pointed leaves traversed by three simple veins, and together
forming the calyx; II. four larger, white, inner perigone leaves
(petals) (_G_), broad and slightly notched at the end, and tapering to
the point of attachment. The petals collectively are known as the
"corolla." The veins of the petals fork once; III. and IV. two sets of
stamens (_E_), the outer containing two short, and the inner, four
longer ones arranged in pairs. Each stamen has a slender filament
(_H_, _f_) and a two-lobed anther (_an._). The innermost set consists
of two carpels united into a compound pistil. The ovary is oblong,
slightly flattened so as to be oval in section, and divided into two
chambers. The style is very short and tipped by a round, flattened
stigma.

The raceme continues to grow for a long time, forming new flowers at
the end, so that all stages of flowers and fruit may often be found in
the same inflorescence.

The flowers are probably quite independent of insect aid in
pollination, as the stamens are so placed as to almost infallibly shed
their pollen upon the stigma. This fact, probably, accounts for the
inconspicuous character of the flowers.

After fertilization is effected, and the outer floral leaves fall off,
the ovary rapidly enlarges, and becomes heart-shaped and much
flattened at right angles to the partition. When ripe, each half falls
away, leaving the seeds attached by delicate stalks (funiculi, sing.
funiculus) to the edges of the membranous partition. The seeds are
small, oval bodies with a shining, yellow-brown shell, and with a
little dent at the end where the stalk is attached. Carefully dividing
the seed lengthwise, or crushing it in water so as to remove the
embryo, we find it occupies the whole cavity of the seed, the young
stalk (_st._) being bent down against the back of one of the
cotyledons (_f_).

[Illustration: FIG. 94.--_A_, cross-section of the stem of the
shepherd's-purse, including a fibro-vascular bundle, x 150. _ep._
epidermis. _m_, ground tissue. _sh._ bundle sheath. _ph._ phloem.
_xy._ xylem. _tr._ a vessel. _B_, a young root seen in optical
section, x 150. _r_, root cap. _d_, young epidermis. _pb._ ground.
_pl._ young fibro-vascular bundle. _C_ cross section of a small root,
x 150. _fb._ fibro-vascular bundle. _D_, epidermis from the lower side
of the leaf, x 150. _E_, a star-shaped hair from the surface of the
leaf, x 150. _F_, cross-section of a leaf, x 150. _ep._ epidermis.
_m_, ground tissue. _fb._ section of a vein.]

A microscopic examination of a cross-section of the older root shows
that the central portion is made up of radiating lines of
thick-walled cells (fibres) interspersed with lines of larger, round
openings (vessels). There is a ring of small cambium cells around
this merging into the phloem, which is composed of irregular cells,
with pretty thick, but soft walls. The ground tissue is composed of
large, loose cells, which in the older roots are often ruptured and
partly dried up. The epidermis is usually indistinguishable in the
older roots. To understand the early structure of the roots, the
smallest rootlets obtainable should be selected. The smallest are so
transparent that the tips may be mounted whole in water, and will
show very satisfactorily the arrangement of the young tissues. The
tissues do not here arise from a single, apical cell, as we found in
the pteridophytes, but from a group of cells (the shaded cells in
Fig. 94, _B_). The end of the root, as in the fern, is covered with
a root cap (_r_) composed of successive layers of cells cut off from
the growing point. The rest of the root shows the same division of
the tissues into the primary epidermis (dermatogen) (_d_), young
fibro-vascular cylinder (plerome) (_pl._), and young ground tissue
(periblem) (_pb._). The structure of the older portions of such
a root is not very easy to study, owing to difficulty in making
good cross-sections of so small an object. By using a very
sharp razor, and holding perfectly straight between pieces of pith,
however, satisfactory sections can be made. The cells contain so
much starch as to make them almost opaque, and potash should be used
to clear them. The fibro-vascular bundle is of the radial type,
there being two masses of xylem (_xy._) joined in the middle, and
separating the two phloem masses (_ph._), some of whose cells are
rather thicker walled than the others. The bundle sheath is not so
plain here as in the fern. The ground tissue is composed of
comparatively large cells with thickish, soft walls, that contain
much starch. The epidermis usually dies while the root is still
young. In the larger roots the early formation of the cambium ring,
and the irregular arrangement of the tissues derived from its
growth, soon obliterate all traces of the primitive arrangement of
the tissues. Making a thin cross-section of the stem, and magnifying
strongly, we find bounding the section a single row of epidermal
cells (Fig. 94, _A_, _ep._) whose walls, especially the outer ones,
are strongly thickened. Within these are several rows of thin-walled
ground-tissue cells containing numerous small, round chloroplasts.
The innermost row of these cells (_sh._) are larger and have but
little chlorophyll. This row of cells forms a sheath around the ring
of fibro-vascular bundles very much as is the case in the
horse-tail. The separate bundles are nearly triangular in outline,
the point turned inward, and are connected with each other by masses
of fibrous tissue (_f_), whose thickened walls have a peculiar,
silvery lustre. Just inside of the bundle sheath there is a row of
similar fibres marking the outer limit of the phloem (_ph._). The
rest of the phloem is composed of very small cells. The xylem is
composed of fibrous cells with yellowish walls and numerous large
vessels (_tr._). The central ground tissue (pith) has large,
thin-walled cells with numerous intercellular spaces, as in the stem
of _Erythronium_. Some of these cells contain a few scattered
chloroplasts in the very thin, protoplasmic layer lining their
walls, but the cells are almost completely filled with colorless
cell sap.

A longitudinal section shows that the epidermal cells are much
elongated, the cells of the ground tissue less so, and in both the
partition walls are straight. In the fibrous cells, both of the
fibro-vascular bundle and those lying between, the end walls are
strongly oblique. The tracheary tissue of the xylem is made up of
small, spirally-marked vessels, and larger ones with thickened
rings or with pits in the walls. The small, spirally-marked vessels
are nearest the centre, and are the first to be formed in the young
bundle.

The epidermis of the leaves is composed of irregular cells with wavy
outlines like those of the ferns. Breathing pores, of the same type
as those in the ferns and monocotyledons, are found on both
surfaces, but more abundant and more perfectly developed on the
lower surface of the leaf. Owing to their small size they are not
specially favorable for study. The epidermis is sparingly covered
with unicellular hairs, some of which are curiously branched, being
irregularly star-shaped. The walls of these cells are very thick,
and have little protuberances upon the outer surface (Fig. 93, _E_).

Cross-sections of the leaf may be made between pith as already
directed; or, by folding the leaf carefully several times, the whole
can be easily sectioned. The structure is essentially as in the
adder-tongue, but the epidermal cells appear more irregular, and the
fibro-vascular bundles are better developed. They are like those of
the stem, but somewhat simpler. The xylem lies on the upper side.

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