393. The following simple outlines of the anatomy and physiology of
plants (3) are added to the preceding structural part for the better
preparation of students in descriptive and systematic botany; also to
give to all learners some general idea of the life, growth, intimate
structure, and action of the beings which compose so large a part of
organic nature. Those who would extend and verify the facts and
principles here outlined will use the Physiological Botany of the
"Botanical Text Book," by Professor Goodale, or some similar book.
Sec. 1. ANATOMICAL STRUCTURE AND GROWTH.
394. =Growth= _is the increase of a living thing in size and substance_.
It appears so natural that plants and animals should grow, that one
rarely thinks of it as requiring explanation. It seems enough to say
that a thing is so because it grew so. Growth from the seed, the
germination and development of an embryo into a plantlet, and at length
into a mature plant (as illustrated in Sections II. and III.), can be
followed by ordinary observation. But the embryo is already a miniature
plantlet, sometimes with hardly any visible distinction of parts, but
often one which has already made very considerable growth in the seed.
To investigate the formation and growth of the embryo itself requires
well-trained eyes and hands, and the expert use of a good compound
microscope. So this is beyond the reach of a beginner.
395. Moreover, although observation may show that a seedling, weighing
only two or three grains, may double its bulk and weight every week of
its early growth, and may in time produce a huge amount of vegetable
matter, it is still to be asked what this vegetable matter is, where it
came from, and by what means plants are able to increase and accumulate
it, and build it up into the fabric of herbs and shrubs and lofty trees.
396. =Protoplasm.= All this fabric was built up under life, but only a
small portion of it is at any one time alive. As growth proceeds, life
is passed on from the old to the new parts, much as it has passed on
from parent to offspring, from generation to generation in unbroken
continuity. _Protoplasm_ is the common name of that plant-stuff in which
life essentially resides. All growth depends upon it; for it has the
peculiar power of growing and multiplying and building up a living
structure,--the animal no less than the vegetable structure, for it is
essentially the same in both. Indeed, all the animal protoplasm comes
primarily from the vegetable, which has the prerogative of producing it;
and the protoplasm of plants furnishes all that portion of the food of
animals which forms their flesh and living fabric.
397. The very simplest plants (if such may specifically be called plants
rather than animals, or one may say, the simplest living things) are
mere particles, or pellets, or threads, or even indefinite masses of
protoplasm of vague form, which possess powers of motion or of changing
their shape, of imbibing water, air, and even other matters, and of
assimilating these into plant-stuff for their own growth and
multiplication. Their growth is increase in substance by incorporation
of that which they take in and assimilate. Their multiplication is by
spontaneous division of their substance or body into two or more, each
capable of continuing the process.
398. The embryo of a phanerogamous plant at its beginning (344) is
essentially such a globule of protoplasm, which soon constricts itself
into two and more such globules, which hold together inseparably in a
row; then the last of the row divides without separation in the two
other planes, to form a compound mass, each grain or globule of which
goes on to double itself as it grows; and the definite shaping of this
still increasing mass builds up the embryo into its form.
[Illustration: Fig. 433-436. Figures to illustrate the earlier stages in
the formation of an embryo; a single mass of protoplasm (Fig. 433)
dividing into two, three, and then into more incipient cells, which by
continued multiplication build up an embryo.]
399. =Cell-walls.= While this growth was going on, each grain of the
forming structure formed and clothed itself with a coat, thin and
transparent, of something different from protoplasm,--something which
hardly and only transiently, if at all, partakes of the life and action.
The protoplasm forms the living organism; the coat is a kind of
protective covering or shell. The protoplasm, like the flesh of animals
which it gives rise to, is composed of four chemical elements: Carbon,
Hydrogen, Oxygen, and Nitrogen. The coating is of the nature of wood
(is, indeed, that which makes wood), and has only the three elements,
Carbon, Hydrogen, and Oxygen, in its composition.
[Illustration: Fig. 437. Magnified view of some of a simple fresh water
Alga, the Tetraspora lubrica, each sphere of which may answer to an
individual plant.]
400. Although the forming structure of an embryo in the fertilized ovule
is very minute and difficult to see, there are many simple plants of
lowest grade, abounding in pools of water, which more readily show the
earlier stages or simplest states of plant-growth. One of these, which
is common in early spring, requires only moderate magnifying power to
bring to view what is shown in Fig. 437. In a slimy mass which holds all
loosely together, little spheres of green vegetable matter are seen,
assembled in fours, and these fours themselves in clusters of fours. A
transient inspection shows, what prolonged watching would confirm, that
each sphere divides first in one plane, then in the other, to make four,
soon acquiring the size of the original, and so on, producing successive
groups of fours. These pellets each form on their surface a transparent
wall, like that just described. The delicate wall is for some time
capable of expansive growth, but is from the first much firmer than the
protoplasm within; through it the latter imbibes surrounding moisture,
which becomes a watery sap, occupying vacuities in the protoplasmic mass
which enlarge or run together as the periphery increases and distends.
When full grown the protoplasm may become a mere lining to the wall, or
some of it central, as a nucleus, this usually connected with the
wall-lining by delicate threads of the same substance. So, when full
grown, the wall with its lining--a vesicle, containing liquid or some
solid matters and in age mostly air--naturally came to be named a Cell.
But the name was suggested by, and first used only for, cells in
combination or built up into a fabric, much as a wall is built of
bricks, that is, into a
401. =Cellular Structure or Tissue.= Suppose numerous cells like those
of Fig. 437 to be heaped up like a pile of cannon-balls, and as they
grew, to be compacted together while soft and yielding; they would
flatten where they touched, and each sphere, being touched by twelve
surrounding ones would become twelve-sided. Fig. 438 would represent one
of them. Suppose the contiguous faces to be united into one wall or
partition between adjacent cavities, and a _cellular structure_ would be
formed, like that shown in Fig. 439. Roots, stems, leaves, and the whole
of phanerogamous plants are a fabric of countless numbers of such cells.
No such exact regularity in size and shape is ever actually found; but a
nearly truthful magnified view of a small portion of a slice of the
flower-stalk of a Calla Lily (Fig. 440) shows a fairly corresponding
structure; except that, owing to the great air-spaces of the interior,
the fabric may be likened rather to a stack of chimneys than to a solid
fabric. In young and partly transparent parts one may discern the
cellular structure by looking down directly on the surface, as of a
forming root. (Fig. 82, 441, 442).
[Illustration: Fig. 438. Diagram of a vegetable cell, such as it would
be if when spherical it were equally pressed by similar surrounding
cells in a heap.]
[Illustration: Fig. 439. Ideal construction of cellular tissue so
formed, in section.]
[Illustration: Fig. 440. Magnified view of a portion of a transverse
slice of stem of Calla Lily. The great spaces are tubular air-channels
built up by the cells.]
402. The substance of which cell-walls are mainly composed is called
CELLULOSE. It is essentially the same in the stem of a delicate leaf or
petal and in the wood of an Oak, except that in the latter the walls are
much thickened and the calibre small. The protoplasm of each living
cell appears to be completely shut up and isolated in its shell of
cellulose; but microscopic investigation has brought to view, in many
cases, minute threads of protoplasm which here and there traverse the
cell-wall through minute pores, thus connecting the living portion of
one cell with that of adjacent cells. (See Fig. 447, &c.)
[Illustration: Fig. 441. Much magnified small portion of young root of a
seedling Maple (such as of Fig. 82); and 442, a few cells of same more
magnified. The prolongations from the back of some of the cells are root
hairs.]
403. The hairs of plants are cells formed on the surface; either
elongated single cells (like the root-hairs of Fig. 441, 442), or a row
of shorter cells. Cotton fibres are long and simple cells growing from
the surface of the seed.
404. The size of the cells of which common plants are made up varies
from about the thirtieth to the thousandth of an inch in diameter. An
ordinary size of short or roundish cells is from 1/300 to 1/500 of an
inch; so that there may generally be from 27 to 125 millions of cells in
the compass of a cubic inch!
405. Some parts are built up as a compact structure; in others cells are
arranged so as to build up regular air-channels, as in the stems of
aquatic and other water-loving plants (Fig. 440), or to leave irregular
spaces, as in the lower part of most leaves, where the cells only here
and there come into close contact (Fig. 443).
[Illustration: Fig. 443. Magnified section through the thickness of a
leaf of Florida Star-Anise.]
406. All such soft cellular tissue, like this of leaves, that of pith,
and of the green bark, is called PARENCHYMA, while fibrous and woody
parts are composed of PROSENCHYMA, that is, of peculiarly transformed
407. =Strengthening Cells.= Common cellular tissue, which makes up the
whole structure of all very young plants, and the whole of Mosses and
other vegetables of the lowest grade, even when full grown, is too
tender or too brittle to give needful strength and toughness for plants
which are to rise to any considerable height and support themselves. In
these needful strength is imparted, and the conveyance of sap through
the plant is facilitated, by the change, as they are formed, of some
cells into thicker-walled and tougher tubes, and by the running together
of some of these, or the prolongation of others, into hollow fibres or
tubes of various size. Two sorts of such transformed cells go together,
and essentially form the
408. =Wood.= This is found in all common herbs, as well as in shrubs and
trees, but the former have much less of it in proportion to the softer
cellular tissue. It is formed very early in the growth of the root,
stem, and leaves,--traces of it appearing in large embryos even while
yet in the seed. Those cells that lengthen, and at the same time thicken
their walls form the proper WOODY FIBRE or WOOD-CELLS; those of larger
size and thinner walls, which are thickened only in certain parts so as
to have peculiar markings, and which often are seen to be made up of a
row of cylindrical cells, with the partitions between absorbed or broken
away, are called DUCTS, or sometimes VESSELS. There are all gradations
between wood-cells and ducts, and between both these and common cells.
But in most plants the three kinds are fairly distinct.
[Illustration: Fig. 444. Magnified wood-cells of the bark (bast-cells)
of Basswood, one and part of another. 445. Some wood cells from the wood
(and below part of a duct); and 446, a detached wood-cell of the same;
equally magnified.]
[Illustration: Fig. 447. Some wood cells from Buttonwood, Platanus,
highly magnified, a whole cell and lower end of another on the left; a
cell cut half away lengthwise, and half of another on the right; some
pores or pits (_a_) seen on the left; while _b b_ mark sections through
these on the cut surface. When living and young the protoplasm extends
into these and by minuter perforations connects across them. In age the
pits become open passages, facilitating the passage of sap and air.]
409. The proper cellular tissue, or _parenchyma_, is the ground-work of
root, stem, and leaves; this is traversed, chiefly lengthwise, by the
strengthening and conducting tissue, wood-cells and duct-cells, in the
form of bundles or threads, which, in the stems and stalks of herbs are
fewer and comparatively scattered, but in shrubs and trees so numerous
and crowded that in the stems and all permanent parts they make a solid
mass of wood. They extend into and ramify in the leaves, spreading out
in a horizontal plane, as the framework of ribs and veins, which
supports the softer cellular portion or parenchyma.
410. =Wood-Cells, or Woody Fibres=, consist of tubes, commonly between
one and two thousandths, but in Pine-wood sometimes two or three
hundredths, of an inch in diameter. Those from the tough bark of the
Basswood, shown in Fig. 444, are only the fifteen-hundredth of an inch
wide. Those of Buttonwood (Fig. 447) are larger, and are here highly
magnified besides. The figures show the way wood-cells are commonly put
together, namely, with their tapering ends overlapping each
other,--spliced together, as it were,--thus giving more strength and
toughness. In hard woods, such as Hickory and Oak, the walls of these
tubes are very thick, as well as dense; while in soft woods, such as
White Pine and Basswood, they are thinner.
411. Wood-cells in the bark are generally longer, finer, and tougher
than those of the proper wood, and appear more like fibres. For example,
Fig. 446 represents a cell of the wood of Basswood of average length,
and Fig. 444 one (and part of another) of the fibrous bark, both drawn
to the same scale. As these long cells form the principal part of
fibrous bark, or _bast_, they are named _Bast-cells_ or _Bast-fibres_.
These give the great toughness and flexibility to the inner bark of
Basswood (i. e. Bast-wood) and of Leatherwood; and they furnish the
invaluable fibres of flax and hemp; the proper wood of their stems being
tender, brittle, and destroyed by the processes which separate for use
the tough and slender bast-cells. In Leatherwood (Dirca) the bast-cells
are remarkably slender. A view of one, if magnified on the scale of Fig.
444, would be a foot and a half long.
[Illustration: Fig. 448. Magnified bit of a pine shaving, taken parallel
with the silver grain. 449. Separate whole wood-cell, more magnified.
450. Same, still more magnified; both sections represented: _a_, disks
in section, _b_, in face.]
412. The wood-cells of Pines, and more or less of all other Coniferous
trees, have on two of their sides very peculiar disk-shaped markings
(Fig. 448-450) by which that kind of wood is recognizable.
[Illustration: Fig. 451, 452. A large and a smaller dotted duct from
Grape-Vine.]
413. =Ducts=, also called VESSELS, are mostly larger than wood-cells:
indeed, some of them, as in Red Oak, have calibre large enough to be
discerned on a cross section by the naked eye. They make the visible
porosity of such kinds of wood. This is particularly the case with
_Dotted_ ducts (Fig. 451, 452), the surface of which appears as if
riddled with round or oval pores. Such ducts are commonly made up of a
row of large cells more or less confluent into a tube.
_Scalariform_ ducts (Fig. 458, 459), common in Ferns, and generally
angled by mutual pressure in the bundles, have transversely elongated
thin places, parallel with each other, giving a ladder-like appearance,
whence the name.
_Annular_ ducts (Fig. 457) are marked with cross lines or rings, which
are thickened portions of the cell-wall.
[Illustration: Fig. 453, 454. Spiral ducts which uncoil into a single
thread. 455. Spiral duct which tears up as a band. 456. An annular duct,
with variations above. 457. Loose spiral duct passing into annular. 458.
Scalariform ducts of a Fern; part of a bundle, prismatic by pressure.
459. One torn into a band.]
_Spiral_ ducts or vessels (Fig. 453-455) have thin walls, strengthened
by a spiral fibre adherent within. This is as delicate and as strong as
spider-web: when uncoiled by pulling apart, it tears up and annihilates
the cell-wall. The uncoiled threads are seen by gently pulling apart
many leaves, such as those of Amaryllis, or the stalk of a Strawberry
leaflet.
[Illustration: Fig. 460. Milk Vessels of Dandelion, with cells of the
common cellular tissue. 461. Others from the same older and gorged with
milky juice. All highly magnified.]
_Laticiferous ducts_, _Vessels of the Latex_, or _Milk-vessels_ are
peculiar branching tubes which hold _latex_ or milky juice in certain
plants. It is very difficult to see them, and more so to make out their
nature. They are peculiar in branching and inosculating, so as to make a
net-work of tubes, running in among the cellular tissue; and they are
very small, except when gorged and old (Fig. 460, 461).
Sec. 2. CELL-CONTENTS.
414. The living contents of young and active cells are mainly protoplasm
with water or watery sap which this has imbibed. Old and effete cells
are often empty of solid matter, containing only water with whatever may
be dissolved in it, or air, according to the time and circumstances. All
the various products which plants in general elaborate, or which
particular plants specially elaborate, out of the common food which they
derive from the soil and the air, are contained in the cells, and in the
cells they are produced.
415. =Sap= is a general name for the principal liquid contents,--_Crude
sap_, for that which the plant takes in, _Elaborated sap_ for what it
has digested or assimilated. They must be undistinguishably mixed in the
cells.
416. Among the solid matters into which cells convert some of their
elaborated sap two are general and most important. These are
_Chlorophyll_ and _Starch_.
417. =Chlorophyll= (meaning _leaf-green_) is what gives the green color
to herbage. It consists of soft grains of rather complex nature, partly
wax-like, partly protoplasmic. These abound in the cells of all common
leaves and the green rind of plants, wherever exposed to the light. The
green color is seen through the transparent skin of the leaf and the
walls of the containing cells. Chlorophyll is essential to ordinary
assimilation in plants: by its means, under the influence of sunlight,
the plant converts crude sap into vegetable matter.
418. Far the largest part of all vegetable matter produced is that which
goes to build up the plant's fabric or cellular structure, either
directly or indirectly. There is no one good name for this most
important product of vegetation. In its final state of cell-walls, the
permanent fabric of herb and shrub and tree, it is called _Cellulose_
(408): in its most soluble form it is _Sugar_ of one or another kind; in
a less soluble form it is _Dextrine_, a kind of liquefied starch: in the
form of solid grains stored up in the cells it is _Starch_. By a series
of slight chemical changes (mainly a variation in the water entering
into the composition), one of these forms is converted into another.
419. =Starch= (_Farina_ or _Fecula_) is the form in which this common
plant material is, as it were, laid by for future use. It consists of
solid grains, somewhat different in form in different plants, in size
varying from 1/300 to 1/4000 of an inch, partly translucent when wet,
and of a pearly lustre. From the concentric lines, which commonly appear
under the microscope, the grains seem to be made up of layer over layer.
When loose they are commonly oval, as in potato-starch (Fig. 462): when
much compacted the grains may become angular (Fig. 463).
[Illustration: Fig. 462. Some magnified starch-grains, in two cells of a
potato. 463. Some cells of the albumen or floury part of Indian Corn,
filled with starch-grains.]
420. The starch in a potato was produced in the foliage. In the soluble
form of dextrine, or that of sugar, it was conveyed through the cells of
the herbage and stalks to a subterranean shoot, and there stored up in
the tuber. When the potato sprouts, the starch in the vicinity of
developing buds or eyes is changed back again, first into mucilaginous
dextrine, then into sugar, dissolved in the sap, and in this form it is
made to flow to the growing parts, where it is laid down into cellulose
or cell-wall.
[Illustration: Fig. 464. Four cells from dried Onion-peel, each holding
a crystal of different shape, one of them twinned. 465. Some cells from
stalk of Rhubarb-plant, three containing chlorophyll; two (one torn
across) with rhaphides. 466. Rhaphides in a cell, from Arisaema, with
small cells surrounding. 467. Prismatic crystals from the bark of
Hickory. 468. Glomerate crystal in a cell, from Beet-root. 469. A few
cells of Locust-bark, a crystal in each. 470. A detached cell, with
rhaphides being forced out, as happens when put in water.]
421. Besides these cell-contents which are in obvious and essential
relation to nutrition, there are others the use of which is
problematical. Of such the commonest are
422. =Crystals.= These when slender or needle-shaped are called
RHAPHIDES. They are of inorganic matter, usually of oxalate or phosphate
or sulphate of lime. Some, at least of the latter, may be direct
crystallizations of what is taken in dissolved in the water absorbed,
but others must be the result of some elaboration in the plant. Some
plants have hardly any; others abound in them, especially in the foliage
and bark. In Locust-bark almost every cell holds a crystal; so that in a
square inch not thicker than writing-paper there may be over a million
and a half of them. When needle-shaped (rhaphides), as in stalks of
Calla-Lily, Rhubarb, or Four-o'clock, they are usually packed in
sheaf-like bundles. (Fig. 465, 466.)
Sec. 3. ANATOMY OF ROOTS AND STEMS.
423. This is so nearly the same that an account of the internal
structure of stems may serve for the root also.
424. At the beginning, either in the embryo or in an incipient shoot
from a bud, the whole stem is of tender cellular tissue or parenchyma.
But wood (consisting of wood-cells and ducts or vessels) begins to be
formed in the earliest growth; and is from the first arranged in two
ways, making two general kinds of wood. The difference is obvious even
in herbs, but is more conspicuous in the enduring stems of shrubs and
trees.
425. On one or the other of these two types the stems of all
phanerogamous plants are constructed. In one, the wood is made up of
separate threads, scattered here and there throughout the whole diameter
of the stem. In the other, the wood is all collected to form a layer (in
a slice across the stem appearing as a ring) between a central cellular
part which has none in it, the _Pith_, and an outer cellular part, the
_Bark_.
[Illustration: Fig. 471. Diagram of structure of Palm or Yucca. 472.
Structure of a Corn stalk, in transverse and longitudinal section. 473.
Same of a small Palm stem. The dots on the cross sections represent cut
ends of the woody bundles or threads.]
426. An Asparagus-shoot and a Corn-stalk for herbs, and a rattan for a
woody kind, represent the first kind. To it belong all plants with
monocotyledonous embryo (40). A Bean-stalk and the stem of any common
shrub or tree represent the second; and to it belong all plants with
dicotyledonous or polycotyledonous embryo. The first has been called,
not very properly, _Endogenous_, which means inside-growing; the second,
properly enough, _Exogenous_, or outside-growing.
427. =Endogenous Stems=, those of Monocotyls (40), attain their greatest
size and most characteristic development in Palms and Dragon-trees,
therefore chiefly in warm climates, although the Palmetto and some
Yuccas become trees along the southern borders of the United States. In
such stems the woody bundles are more numerous and crowded toward the
circumference, and so the harder wood is outside; while in an exogenous
stem the oldest and hardest wood is toward the centre. An endogenous
stem has no clear distinction of pith, bark, and wood, concentrically
arranged, no silver grain, no annual layers, no bark that peels off
clean from the wood. Yet old stems of Yuccas and the like, that continue
to increase in diameter, do form a sort of layers and a kind of scaly
bark when old. Yuccas show well the curving of the woody bundles (Fig.
471) which below taper out and are lost at the rind.
[Illustration: Fig. 474. Short piece of stem of Flax, magnified, showing
the bark, wood, and pith in a cross section.]
428. =Exogenous Stems=, those of Dicotyls (37), or of plants coming from
dicotyledonous and also polycotyledonous embryos, have a structure which
is familiar in the wood of our ordinary trees and shrubs. It is the same
in an herbaceous shoot (such as a Flax-stem, Fig. 474) as in a
Maple-stem of the first year's growth, except that the woody layer is
commonly thinner or perhaps reduced to a circle of bundles. It was so in
the tree-stem at the beginning. The wood all forms in a cylinder,--in
cross section a ring--around a central cellular part, dividing the
cellular core within, the pith, from a cellular bark without. As the
wood-bundles increase in number and in size, they press upon each other
and become wedge-shaped in the cross section; and they continue to grow
from the outside, next the bark, so that they become very thin wedges or
plates. Between the plates or wedges are very thin plates (in cross
section lines) of much compressed cellular tissue, which connect the
pith with the bark. The plan of a one-year-old woody stem of this kind
is exhibited in the figures, which are essentially diagrams.
[Illustration: Fig. 475. Diagram of a cross section of a very young
exogenous stem, showing six woody bundles or wedges. 476. Same later,
with wedges increased to twelve. 477. Still later, the wedges filling
the space, separated only by the thin lines, or medullary rays, running
from pith to bark.]
429. When such a stem grows on from year to year, it adds annually a
layer of wood outside the preceding one, between that and the bark.
This is exogenous growth, or outside-growing, as the name denotes.
[Illustration: Fig. 478. Piece of a stem of Soft Maple, of a year old,
cut crosswise and lengthwise.]
[Illustration: Fig. 479. A portion of the same, magnified.]
[Illustration: Fig. 480. A small piece of the same, taken from one side,
reaching from the bark to the pith, and highly magnified: _a_, a small
bit of the pith; _b_, spiral ducts of what is called the _medullary
sheath_; _c_, the wood; _d_, _d_, dotted ducts in the wood; _e_, _e_,
annular ducts; _f_, the liber or inner bark; _g_, the green bark; _h_,
the corky layer; _i_, the skin, or epidermis; _j_, one of the medullary
rays, or plates of silver grain, seen on the cross-section.]
430. Some new bark is formed every year, as well as new wood, the former
inside, as the latter is outside of that of the year preceding. The ring
or zone of tender forming tissue between the bark and the wood has been
called the _Cambium Layer_. _Cambium_ is an old name of the
physiologists for nutritive juice. And this thin layer is so gorged with
rich nutritive sap when spring growth is renewed, that the bark then
seems to be loose from the wood and a layer of viscid sap (or _cambium_)
to be poured out between the two. But there is all the while a
connection of the bark and the wood by delicate cells, rapidly
multiplying and growing.
431. =The Bark= of a year-old stem consists of three parts, more or less
distinct, namely,--beginning next the wood,--
1. The LIBER or FIBROUS BARK, the _Inner Bark_. This contains some
wood-cells, or their equivalent, commonly in the form of bast or
bast-cells (411, Fig. 444), such as those of Basswood or Linden, and
among herbs those of flax and hemp, which are spun and woven or made
into cordage. It also contains cells which are named _sieve_-cells, on
account of numerous slits and pores in their walls, by which the
protoplasm of contiguous cells communicates. In woody stems, whenever a
new layer of wood is formed, some new liber or inner bark is also formed
outside of it.
2. The GREEN BARK or _Middle Bark_. This consists of cellular tissue
only, and contains the same green matter (_chlorophyll_, 417) as the
leaves. In woody stems, before the season's growth is completed, it
becomes covered by
3. The CORKY LAYER or _Outer Bark_, the cells of which contain no
chlorophyll, and are of the nature of _cork_. Common cork is the thick
corky layer of the bark of the Cork-Oak of Spain. It is this which gives
to the stems or twigs of shrubs and trees the aspect and the color
peculiar to each,--light gray in the Ash, purple in the Red Maple, red
in several Dogwoods, etc.
4. The EPIDERMIS, or skin of the plant, consisting of a layer of
thick-sided empty cells, which may be considered to be the outermost
layer, or in most herbaceous stems the only layer, of cork-cells.
[Illustration: Fig. 481. Magnified view of surface of a bit of young
Maple wood from which the bark has been torn away, showing the
wood-cells and the bark-ends of medullary rays.]
[Illustration: Fig. 482. Section in the opposite direction, from bark
(on the left) to beginning of pith (on the right), and a medullary ray
extending from one to the other.]
432. The green layer of bark seldom grows much after the first season.
Sometimes the corky layer grows and forms new layers, inside of the old,
for years, as in the Cork-Oak, the Sweet Gum-tree, and the White and the
Paper Birch. But it all dies after a while; and the continual
enlargement of the wood within finally stretches it more than it can
bear, and sooner or later cracks and rends it, while the weather acts
powerfully upon its surface; so the older bark perishes and falls away
piecemeal year by year.
433. So on old trunks only the inner bark remains. This is renewed every
year from within and so kept alive, while the older and outer layers
die, are fissured and rent by the distending trunk, weathered and worn,
and thrown off in fragments,--in some trees slowly, so that the bark of
old trunks may acquire great thickness; in others, more rapidly. In
Honeysuckles and Grape-Vines, the layers of liber loosen and die when
only a year or two old. The annual layers of liber are sometimes as
distinct as those of the wood, but often not so.
434. =The Wood= of an exogenous trunk, having the old growths covered
by the new, remains nearly unchanged in age, except from decay. Wherever
there is an annual suspension and renewal of growth, as in temperate
climates, the annual growths are more or less distinctly marked, in the
form of concentric rings on the cross section, so that the age of the
tree may be known by counting them. Over twelve hundred layers have been
counted on the stumps of Sequoias in California, and it is probable that
some trees now living antedate the Christian era.
435. The reason why the annual growths are distinguishable is, that the
wood formed at the beginning of the season is more or less different in
the size or character of the cells from that of the close. In Oak,
Chestnut, etc., the first wood of the season abounds in dotted ducts,
the calibre of which is many times greater than that of the proper
wood-cells.
436. =Sap-wood, or Alburnum.= This is the newer wood, living or recently
alive, and taking part in the conveyance of sap. Sooner or later, each
layer, as it becomes more and more deeply covered by the newer ones and
farther from the region of growth, is converted into
437. =Heart-wood, or Duramen.= This is drier, harder, more solid, and
much more durable as timber, than sap-wood. It is generally of a
different color, and it exhibits in different species the hue peculiar
to each, such as reddish in Red-Cedar, brown in Black-Walnut, black in
Ebony, etc. The change of sap-wood into heart-wood results from the
thickening of the walls of the wood-cells by the deposition of hard
matter, lining the tubes and diminishing their calibre; and by the
deposition of a vegetable coloring-matter peculiar to each species. The
heart-wood, being no longer a living part, may decay, and often does so,
without the least injury to the tree, except by diminishing the strength
of the trunk, and so rendering it more liable to be overthrown.
438. =The Living Parts of a Tree=, of the exogenous kind, are only
these: first, the rootlets at one extremity; second, the buds and leaves
of the season at the other; and third, a zone consisting of the newest
wood and the newest bark, connecting the rootlets with the buds or
leaves, however widely separated these may be,--in the tallest trees
from two to four hundred feet apart. And these parts of the tree are all
renewed every year. No wonder, therefore, that trees may live so long,
since they annually reproduce everything that is essential to their life
and growth, and since only a very small part of their bulk is alive at
once. The tree survives, but nothing now living has been so long. In it,
as elsewhere, life is a transitory thing, ever abandoning the _old_, and
renewed in the _young_.
Sec. 4. ANATOMY OF LEAVES.
439. The wood in leaves is the framework of ribs, veins, and veinlets
(125), serving not only to strengthen them, but also to bring in the
sap, and to distribute it throughout every part. The cellular portion is
the green pulp, and is nearly the same as the green layer of the bark.
So that the leaf may properly enough be regarded as a sort of expansion
of the fibrous and green layers of the bark. It has no proper corky
layer; but the whole is covered by a transparent skin or _epidermis_,
resembling that of the stem.
440. The cells of the leaf are of various forms, rarely so compact as to
form a close cellular tissue, usually loosely arranged, at least in the
lower part, so as to give copious intervening spaces or air passages,
communicating throughout the whole interior (Fig. 443, 483). The green
color is given by the chlorophyll (417), seen through the very
transparent walls of the cells and through the translucent epidermis of
the leaf.
[Illustration: Fig. 483. Magnified section of a leaf of White Lily, to
exhibit the cellular structure, both of upper and lower stratum, the
air-passages of the lower, and the epidermis or skin, in section, also a
little of that of the lower face, with some of its stomates.]
441. In ordinary leaves, having an upper and under surface, the green
cells form two distinct strata, of different arrangement. Those of the
upper stratum are oblong or cylindrical, and stand endwise to the
surface of the leaf, usually close together, leaving hardly any vacant
spaces; those of the lower are commonly irregular in shape, most of them
with their longer diameter parallel to the face of the leaf, and are
very loosely arranged, leaving many and wide air-chambers. The green
color of the lower is therefore diluted, and paler than that of the
upper face of the leaf. The upper part of the leaf is so constructed as
to bear the direct action of the sunshine; the lower so as to afford
freer circulation of air, and to facilitate transpiration. It
communicates more directly than the upper with the external air by means
of _Stomates_.
442. =The Epidermis= or skin of leaves and all young shoots is best seen
in the foliage. It may readily be stripped off from the surface of a
Lily-leaf, and still more so from more fleshy and soft leaves, such as
those of Houseleek. The epidermis is usually composed of a single
layer, occasionally of two or three layers, of empty cells, mostly of
irregular outline. The sinuous lines which traverse it, and may be
discerned under low powers of the microscope (Fig. 487), are the
boundaries of the epidermal cells.
[Illustration: Fig. 484. Small portion of epidermis of the lower face of
a White-Lily leaf, with stomata.]
[Illustration: Fig. 485. One of these, more magnified, in the closed
state. 486. Another stoma, open.]
[Illustration: Fig. 487. Small portion of epidermis of the Garden
Balsam, highly magnified, showing very sinuous-walled cells, and three
stomata.]
443. =Breathing-pores, or Stomates, Stomata= (singular, a
_Stoma_,--literally, a mouth) are openings through the epidermis into
the air-chambers or intercellular passages, always between and guarded
by a pair of thin-walled guardian cells. Although most abundant in
leaves, especially on their lower face (that which is screened from
direct sunlight), they are found on most other green parts. They
establish a direct communication between the external air and that in
the loose interior of the leaf. Their guardian cells or lips, which are
soft and delicate, like those of the green pulp within, by their greater
or less turgidity open or close the orifice as the moisture or dryness
varies.
444. In the White Lily the stomata are so remarkably large that they may
be seen by a simple microscope of moderate power, and may be discerned
even by a good hand lens. There are about 60,000 of them to the square
inch of the epidermis of the lower face of this Lily-leaf, and only
about 3000 to the same space on the upper face. It is computed that an
average leaf of an Apple-tree has on its lower face about 100,000 of
these mouths.
Sec. 5. PLANT FOOD AND ASSIMILATION.
445. Only plants are capable of originating organizable matter, or the
materials which compose the structure of vegetables and animals. The
essential and peculiar work of plants is to take up portions of earth
and air (water belonging to both) upon which animals cannot live at all,
and to convert them into something organizable; that is, into something
that, under life, may be built up into vegetable and animal structures.
All the food of animals is produced by plants. Animals live upon
vegetables, directly or at second hand, the carnivorous upon the
herbivorous; and vegetables live upon earth and air, immediately or at
second hand.
446. =The Food of plants=, then, primarily, is earth and air. This is
evident enough from the way in which they live. Many plants will
flourish in pure sand or powdered chalk, or on the bare face of a rock
or wall, watered merely with rain. And almost any plant may be made to
grow from the seed in moist sand, and increase its weight many times,
even if it will not come to perfection. Many naturally live suspended
from the branches of trees high in the air, and nourished by it alone,
never having any connection with the soil; and some which naturally grow
on the ground, like the Live-forever of the gardens, when pulled up by
the roots and hung in the air will often flourish the whole summer long.
447. It is true that fast-growing plants, or those which produce much
vegetable matter in one season (especially in such concentrated form as
to be useful as food for man or the higher animals) will come to
maturity only in an enriched soil. But what is a rich soil? One which
contains decomposing vegetable matter, or some decomposing animal
matter; that is, in either case, some decomposing organic matter
formerly produced by plants. Aided by this, grain-bearing and other
important vegetables will grow more rapidly and vigorously, and make a
greater amount of nourishing matter, than they could if left to do the
whole work at once from the beginning. So that in these cases also all
the organic or organizable matter was made by plants, and made out of
earth and air. Far the larger and most essential part was air and water.
448. Two kinds of material are taken in and used by plants; of which the
first, although more or less essential to perfect plant-growth, are in a
certain sense subsidiary, if not accidental, viz.:--
_Earthy constituents_, those which are left in the form of ashes when a
leaf or a stick of wood is burned in the open air. These consist of some
_potash_ (or _soda_ in a marine plant), some _silex_ (the same as
flint), and a little _lime_, _alumine_, or _magnesia_, _iron_ or
_manganese_, _sulphur_, _phosphorus_, etc.,--some or all of these in
variable and usually minute proportions. They are such materials as
happen to be dissolved, in small quantity, in the water taken up by the
roots; and when that is consumed by the plant, or flies off pure (as it
largely does) by exhalation, the earthy matter is left behind in the
cells,--just as it is left incrusting the sides of a teakettle in which
much hard water has been boiled. Naturally, therefore, there is more
earthy matter (i. e. more ashes) in the leaves than in any other part
(sometimes as much as seven per cent, when the wood contains only two
per cent); because it is through the leaves that most of the water
escapes from the plant. Some of this earthy matter incrusts the
cell-walls, some goes to form crystals or rhaphides, which abound in
many plants (422), some enters into certain special vegetable products,
and some appears to be necessary to the well-being of the higher orders
of plants, although forming no necessary part of the proper vegetable
structure.
_The essential constituents_ of the organic fabric are those which are
dissipated into air and vapor in complete burning. They make up from 88
to 99 per cent of the leaf or stem, and essentially the whole both of
the cellulose of the walls and the protoplasm of the contents. Burning
gives these materials of the plant's structure back to the air, mainly
in the same condition in which the plant took them, the same condition
which is reached more slowly in natural decay. The chemical elements of
the cell-walls (or cellulose, 402), as also of starch, sugar, and all
that class of organizable cell-material, are carbon, hydrogen, and
oxygen (399). The same, with nitrogen, are the constituents of
protoplasm, or the truly vital part of vegetation.
449. These chemical elements out of which organic matters are composed
are supplied to the plant by water, carbonic acid, and some combinations
of nitrogen.
_Water_, far more largely than anything else, is imbibed by the roots;
also more or less by the foliage in the form of vapor. Water consists of
oxygen and hydrogen; and cellulose or plant-wall, starch, sugar, etc.,
however different in their qualities, agree in containing these two
elements in the same relative proportions as in water.
_Carbonic acid_ gas (Carbon dioxide) is one of the components of the
atmosphere,--a small one, ordinarily only about 1/2500 of its
bulk,--sufficient for the supply of vegetation, but not enough to be
injurious to animals, as it would be if accumulated. Every current or
breeze of air brings to the leaves expanded in it a succession of fresh
atoms of carbonic acid, which it absorbs through its multitudinous
breathing-pores. This gas is also taken up by water. So it is brought to
the ground by rain, and is absorbed by the roots of plants, either as
dissolved in the water they imbibe, or in the form of gas in the
interstices of the soil. Manured ground, that is, soil containing
decomposing vegetable or animal matters, is constantly giving out this
gas into the interstices of the soil, whence the roots of the growing
crop absorb it. Carbonic acid thus supplied, primarily from the air, is
the source of the carbon which forms much the largest part of the
substance of every plant. The proportion of carbon may be roughly
estimated by charring some wood or foliage; that is, by heating it out
of contact with the air, so as to decompose and drive off all the other
constituents of the fabric, leaving the large bulk of charcoal or carbon
behind.
_Nitrogen_, the remaining plant-element, is a gas which makes up more
than two thirds of the atmosphere, is brought into the foliage and also
to the roots (being moderately soluble in water) in the same ways as is
carbonic acid. The nitrogen which, mixed with oxygen, a little carbonic
acid, and vapor of water, constitutes the air we breathe, is the source
of this fourth plant-element. But it is very doubtful if ordinary plants
can use any nitrogen gas directly as food; that is, if they can directly
cause it to combine with the other elements so as to form protoplasm.
But when combined with hydrogen (forming ammonia), or when combined with
oxygen (nitric acid and nitrates) plants appropriate it with avidity.
And several natural processes are going on in which nitrogen of the air
is so combined and supplied to the soil in forms directly available to
the plant. The most efficient is _nitrification_, the formation of nitre
(nitrate of potash) in the soil, especially in all fertile soils,
through the action of a bacterial ferment.
450. =Assimilation= in plants is the conversion of these inorganic
substances--essentially, water, carbonic acid, and some form of combined
or combinable nitrogen--into vegetable matter. This most dilute food the
living plant concentrates and assimilates to itself. Only plants are
capable of converting these mineral into organizable matters; and this
all-important work is done by them (so far as all ordinary vegetation is
concerned) only
451. _Under the light of the sun, acting upon green parts or foliage_,
that is, upon the chlorophyll, or upon what answers to chlorophyll,
which these parts contain. The sun in some way supplies a power which
enables the living plant to originate these peculiar chemical
combinations,--to organize matter into forms which are alone capable of
being endowed with life. The proof of this proposition is simple; and it
shows at the same time, in the simplest way, what a plant does with the
water and carbonic acid it consumes. Namely, 1st, it is only in sunshine
or bright daylight that the green parts of plants give out oxygen
gas,--then they regularly do so; and 2d, the giving out of this oxygen
gas is required to render the chemical composition of water and carbonic
acid the same as that of _cellulose_, that is, of the plant's permanent
fabric. This shows why plants spread out so large a surface of foliage.
Leaves are so many workshops, full of machinery worked by sun-power. The
emission of oxygen gas from any sun-lit foliage is seen by placing some
of this under water, or by using an aquatic plant, by collecting the air
bubbles which rise, and by noting that a taper burns brighter in this
air. Or a leafy plant in a glass globe may be supplied with a certain
small percentage of carbonic acid gas, and after proper exposure to
sunshine, the air on being tested will be found to contain less carbonic
acid and just so much the more oxygen gas.
452. Now if the plant is making cellulose or any equivalent
substance,--that is, is making the very materials of its fabric and
growth, as must generally be the case,--all this oxygen gas given off by
the leaves comes from the decomposition of carbonic acid taken in by the
plant. For cellulose, and also starch, dextrine, sugar, and the like are
composed of carbon along with oxygen and hydrogen in just the
proportions to form water. And the carbonic acid and water taken in,
less the oxygen which the carbon brought with it as carbonic acid, and
which is given off from the foliage in sunshine, just represents the
manufactured article, cellulose.
453. It comes to the same if the first product of assimilation is sugar,
or dextrine which is a sort of soluble starch, or starch itself. And in
the plant all these forms are readily changed into one another. In the
tiny seedling, as fast as this assimilated matter is formed it is used
in growth, that is, in the formation of cell-walls. After a time some or
much of the product may be accumulated in store for future growth, as
in the root of the turnip, or the tuber of the potato, or the seed of
corn or pulse. This store is mainly in the form of starch. When growth
begins anew, this starch is turned into dextrine or into sugar, in
liquid form, and used to nourish and build up the germinating embryo or
the new shoot, where it is at length converted into cellulose and used
to build up plant-structure.
454. But that which builds plant-fabric is not the cellular structure
itself; the work is done by the living protoplasm which dwells within
the walls. This also has to take and to assimilate its proper food, for
its own maintenance and growth. Protoplasm assimilates, along with the
other three elements, the nitrogen of the plant's food. This comes
primarily from the vast stock in the atmosphere, but mainly through the
earth, where it is accumulated through various processes in a fertile
soil,--mainly, so far as concerns crops, from the decomposition of
former vegetables and animals. This protoplasm, which is formed at the
same time as the simpler cellulose, is essentially the same as the flesh
of animals, and the source of it. It is the common basis of vegetable
and of animal life.
455. _So plant-assimilation produces all the food and fabric of
animals._ Starch, sugar, the oils (which are, as it were, these
farinaceous matters more deoxidated), chlorophyll, and the like, and
even cellulose itself, form the food of herbivorous animals and much of
the food of man. When digested they enter into the blood, undergo
various transformations, and are at length decomposed into carbonic acid
and water, and exhaled from the lungs in respiration,--in other words,
are given back to the air by the animal as the very same materials which
the plant took from the air as its food,--are given back to the air in
the same form that they would have taken if the vegetable matter had
been left to decay where it grew, or if it had been set on fire and
burned; and with the same result, too, as to the heat,--the heat in this
case producing and maintaining the proper temperature of the animal.
456. The protoplasm and other products containing nitrogen (gluten,
legumine, etc.), and which are most accumulated in grains and seeds (for
the nourishment of their embryos when they germinate), compose the most
nutritious vegetable food consumed by animals; they form their proper
flesh and sinews, while the earthy constituents of the plant form the
earthy matter of the bones, etc. At length decomposed, in the secretions
and excretions, these nitrogenous constituents are through successive
changes finally resolved into mineral matter, into carbonic acid, water,
and ammonia or some nitrates,--into exactly or essentially the same
materials which the plants took up and assimilated. Animals depend upon
vegetables absolutely and directly for their subsistence; also
indirectly, because
457. _Plants purify the air for animals._ In the very process by which
they create food they take from the air carbonic acid gas, injurious to
animal respiration, which is continually poured into it by the breathing
of all animals, by all decay, by the burning of fuel and all other
ordinary combustion; and they restore an equal bulk of life-sustaining
oxygen needful for the respiration of animals,--needful, also, in a
certain measure, for plants in any work they do. For in plants, as well
as in animals, work is done at a certain cost.
Sec. 6. PLANT WORK AND MOVEMENT.
458. As the organic basis and truly living material of plants is
identical with that of animals, so is the life at bottom essentially the
same; but in animals something is added at every rise from the lowest to
highest organisms. Action and work in living beings require movement.
459. Living things move; those not living are only moved. Plants move as
truly as do animals. The latter, nourished as they are upon organized
food, which has been prepared for them by plants, and is found only here
and there, must needs have the power of going after it, of collecting
it, or at least of taking it in; which requires them to make spontaneous
movements. But ordinary plants, with their wide-spread surface, always
in contact with the earth and air on which they feed,--the latter
everywhere the same, and the former very much so,--might be thought to
have no need of movement. Ordinary plants, indeed, have no locomotion;
some float, but most are rooted to the spot where they grew. Yet
probably all of them execute various movements which must be as truly
self-caused as are those of the lower grades of animals,--movements
which are overlooked only because too slow to be directly observed.
Nevertheless, the motion of the hour-hand and of the minute-hand of a
watch is not less real than that of the second-hand.
[Illustration: Fig. 488. Two individuals of an Oscillaria, magnified.]
460. =Locomotion.= Moreover, many microscopic plants living in water are
seen to move freely, if not briskly, under the microscope; and so
likewise do more conspicuous aquatic plants in their embryo-like or
seedling state. Even at maturity, species of Oscillaria (such as in Fig.
488, minute worm-shaped plants of fresh waters, taking this name from
their oscillating motions) freely execute three different kinds of
movement, the very delicate investing coat of cellulose not impeding the
action of the living protoplasm within. Even when this coat is firmer
and hardened with a siliceous deposit, such crescent-shaped or
boat-shaped one-celled plants as _Closterium_ or _Naricula_ are able in
some way to move along from place to place in the water.
[Illustration: Fig. 489. A few cells of a leaf of Naias flexilis, highly
magnified: the arrows indicate the courses of the circulating currents.]
461. =Movements in Cells=, =or Cell-circulation=, sometimes called
_Cyclosis_, has been detected in so many plants, especially in
comparatively transparent aquatic plants and in hairs on the surface of
land plants (where it is easiest to observe), that it may be inferred to
take place in all cells during the most active part of their life. This
motion is commonly a streaming movement of threads of protoplasm,
carrying along solid granules by which the action may be observed and
the rate measured, or in some cases it is a rotation of the whole
protoplasmic contents of the cell. A comparatively low magnifying power
will show it in the cells of Nitella and Chara (which are cryptogamous
plants); and under a moderate power it is well seen in the Tape Grass of
fresh water, Vallisneria, and in Naias flexilis (Fig. 489). Minute
particles and larger greenish globules are seen to be carried along, as
if in a current, around the cell, passing up one side, across the end,
down the other and across the bottom, completing the circuit sometimes
within a minute or less when well warmed. To see it well in the cell,
which like a string of beads form the hairs on the stamens of
Spiderwort, a high magnifying power is needed.
462. =Transference of Liquid from Cell to Cell=, and so from place to
place in the plant, the absorption of water by the rootlets, and the
exhalation of the greater part of it from the foliage,--these and
similar operations are governed by the physical laws which regulate the
diffusion of fluids, but are controlled by the action of living
protoplasm. Equally under vital control are the various chemical
transformations which attend assimilation and growth, and which involve
not only molecular movements but conveyance. Growth itself, which is the
formation and shaping of new parts, implies the direction of internal
activities to definite ends.
463. =Movements of Organs.= The living protoplasm, in all but the lowest
grade of plants, is enclosed and to common appearance isolated in
separate cells, the walls of which can only in their earliest state be
said to be alive. Still plants are able to cause the protoplasm of
adjacent cells to act in concert, and by their combined action to effect
movements in roots, stems, or leaves, some of them very slow and
gradual, some manifest and striking. Such movements are brought about
through individually minute changes in the form or tension in the
protoplasm of the innumerable cells which make up the structure of the
organ. Some of the slower movements are effected during growth, and may
be explained by inequality of growth on the two sides of the bending
organ. But the more rapid changes of position, and some of the slow
ones, cannot be so explained.
464. =Root-movements.= In its growth a root turns or bends away from
the light and toward the centre of the earth, so that in lengthening it
buries itself in the soil where it is to live and act. Every one must
have observed this in the germination of seeds. Careful observations
have shown that the tip of a growing root also makes little sweeps or
short movements from side to side. By this means it more readily
insinuates itself into yielding portions of the soil. The root-tips will
also turn toward moisture, and so secure the most favorable positions in
the soil.
465. =Stem-movements.= The root end of the caulicle or first joint of
stem (that below the cotyledons) acts like the root, in turning downward
in germination (making a complete bend to do so if it happens to point
upward as the seed lies in the ground), while the other end turns or
points skyward. These opposite positions are taken in complete darkness
as readily as in the light, in dryness as much as in moisture:
therefore, so far as these movements are physical, the two portions of
the same internode appear to be oppositely affected by gravitation or
other influences.
466. Rising into the air, the stem and green shoots generally, while
young and pliable, bend or direct themselves toward the light, or toward
the stronger light when unequally illuminated; while roots turn toward
the darkness.
467. Many growing stems have also a movement of _Nutation_, that is, of
nodding successively in different directions. This is brought about by a
temporary increase of turgidity of the cells along one side, thus bowing
the stem over to the opposite side; and this line of turgescence travels
round the shoot continually, from right to left or from left to right
according to the species: thus the shoot bends to all points of the
compass in succession. Commonly this nutation is slight or hardly
observable. It is most marked in
468. =Twining Stems= (Fig. 90). The growing upper end of such stems, as
is familiar in the Hop, Pole Beans, and Morning-Glory, turns over in an
inclined or horizontal direction, thus stretching out to reach a
neighboring support, and by the continual change in the direction of the
nodding, sweeps the whole circle, the sweeps being the longer as the
stem lengthens. When it strikes against a support, such as a stem or
branch of a neighboring plant, the motion is arrested at the contact,
but continues at the growing apex beyond, and this apex is thus made to
wind spirally around the supporting body.
469. =Leaf-movements= are all but universal. The presentation by most
leaves of their upper surface to the light, from whatever direction that
may come, is an instance; for when turned upside down they twist or bend
round on the stalk to recover this normal position. Leaves, and the
leaflets of compound leaves, change this position at nightfall, or when
the light is withdrawn; they then take what is called their sleeping
posture, resuming the diurnal position when daylight returns. This is
very striking in Locust-trees, in the Sensitive Plant (Fig. 490), and
in Woodsorrel. Young seedlings droop or close their leaves at night in
plants which are not thus affected in the adult foliage. All this is
thought to be a protection against the cold by nocturnal radiation.
470. Various plants climb by a coiling movement of their leaves or their
leaf-stalks. Familiar examples are seen in Clematis, Maurandia,
Tropaeolum, and in a Solanum which is much cultivated in greenhouses
(Fig. 172). In the latter, and in other woody plants which climb in this
way, the petioles thicken and harden after they have grasped their
support, thus securing a very firm hold.
471. =Tendril movements.= Tendrils are either leaves or stems (98, 168),
specially developed for climbing purposes. Cobaea is a good example of
partial transformation; some of the leaflets are normal, some of the
same leaf are little tendrils, and some intermediate in character. The
Passion-flowers give good examples of simple stem-tendrils (Fig. 92);
Grape-Vines, of branched ones. Most tendrils make revolving sweeps, like
those of twining stems. Those of some Passion-flowers, in sultry
weather, are apt to move fast enough for the movement actually to be
seen for a part of the circuit, as plainly as that of the second-hand of
a watch. Two herbaceous species, Passiflora gracilis and P. sicyoides
(the first an annual, the second a strong-rooted perennial of the
easiest cultivation), are admirable for illustration both of revolving
movements and of sensitive coiling.
[Illustration: Fig. 490. Piece of stem of Sensitive Plant (Mimosa
pudica), with two leaves, the lower open, the upper in the closed
state.]
472. =Movements under Irritation.= The most familiar case is that of the
Sensitive Plant (Fig. 490). The leaves suddenly take their nocturnal
position when roughly touched or when shocked by a jar. The leaflets
close in pairs, the four outspread partial petioles come closer
together, and the common petiole is depressed. The seat of the movements
is at the base of the leaf-stalk and stalklets. Schrankia, a near
relative of the Sensitive Plant, acts in the same way, but is slower.
These are not anomalous actions, but only extreme manifestations of a
faculty more or less common in foliage. In Locust and Honey-Locusts for
example, repeated jars will slowly produce similar effects.
473. Leaf-stalks and tendrils are adapted to their uses in climbing by
a similar sensitiveness. The coiling of the leaf-stalk is in response to
a kind of irritation produced by contact with the supporting body. This
may be shown by gentle rubbing or prolonged pressure upon the upper face
of the leaf-stalk, which is soon followed by a curvature. Tendrils are
still more sensitive to contact or light friction. This causes the free
end of the tendril to coil round the support, and the sensitiveness,
propagated downward along the tendril, causes that side of it to become
less turgescent or the opposite side more so, thus throwing the tendril
into coils. This shortening draws the plant up to the support. Tendrils
which have not laid hold will at length commonly coil spontaneously, in
a simple coil, from the free apex downward. In Sicyos, Echinocystis, and
the above mentioned Passion-flowers (471), the tendril is so sensitive,
under a high summer temperature, that it will curve and coil promptly
after one or two light strokes by the hand.
[Illustration: Fig. 491. Portion of stem and leaves of Telegraph-plant
(Desmodium gyrans), almost of natural size.]
474. Among spontaneous movements the most singular are those of
Desmodium gyrans of India, sometimes called Telegraph-plant, which is
cultivated on account of this action. Of its three leaflets, the larger
(terminal) one moves only by drooping at nightfall and rising with the
dawn. But its two small lateral leaflets, when in a congenial high
temperature, by day and by night move upward and downward in a
succession of jerks, stopping occasionally, as if to recover from
exhaustion. In most plant-movements some obviously useful purpose is
subserved: this of Desmodium gyrans is a riddle.
475. =Movements in Flowers= are very various. The most remarkable are in
some way connected with fertilization (Sect. XIII.). Some occur under
irritation: the stamens of Barberry start forward when touched at the
base inside: those of many polyandrous flowers (of Sparmannia very
strikingly) spread outwardly when lightly brushed: the two lips or lobes
of the stigma in Mimulus close after a touch. Some are automatic and
are connected with dichogamy (339): the style of Sabbatia and of
large-flowered species of Epilobium bends over strongly to one side or
turns downward when the blossom opens, but slowly erects itself a day or
two later.
476. =Extraordinary Movements connected with Capture of Insects.= The
most striking cases are those of Drosera and Dionaea; for an account of
which see "How Plants Behave," and Goodale's "Physiological Botany."
477. The upper face of the leaves of the common species of Drosera, or
Sundew, is beset with stout bristles, having a glandular tip. This tip
secretes a drop of a clear but very viscid liquid, which glistens like a
dew-drop in the sun; whence the popular name. When a fly or other small
insect, attracted by the liquid, alights upon the leaf, the viscid drops
are so tenacious that they hold it fast. In struggling it only becomes
more completely entangled. Now the neighboring bristles, which have not
been touched, slowly bend inward from all sides toward the captured
insect, and bring their sticky apex against its body, thus increasing
the number of bonds. Moreover, the blade of the leaf commonly aids in
the capture by becoming concave, its sides or edges turning inward,
which brings still more of the gland-tipped bristles into contact with
the captive's body. The insect perishes; the clear liquid disappears,
apparently by absorption into the tissue of the leaf. It is thought that
the absorbed secretion takes with it some of the juices of the insect or
the products of its decomposition.
[Illustration: Fig. 492. Plant of Dionaea muscipula, or Venus's Fly-trap,
reduced in size.]
478. Dionaea muscipula, the most remarkable vegetable fly-trap (Fig. 176,
492), is related to the Sundews, and has a more special and active
apparatus for fly-catching, formed of the summit of the leaf. The two
halves of this rounded body move as if they were hinged upon the midrib;
their edges are fringed with spiny but not glandular bristles, which
interlock when the organ closes. Upon the face are two or three short
and delicate bristles, which are sensitive. They do not themselves move
when touched, but they propagate the sensitiveness to the organ itself,
causing it to close with a quick movement. In a fresh and vigorous
leaf, under a high summer temperature, and when the trap lies widely
open, a touch of any one of the minute bristles on the face, by the
finger or any extraneous body, springs the trap (so to say), and it
closes suddenly; but after an hour or so it opens again. When a fly or
other small insect alights on the trap, it closes in the same manner,
and so quickly that the intercrossing marginal bristles obstruct the
egress of the insect, unless it be a small one and not worth taking.
Afterwards and more slowly it completely closes, and presses down upon
the prey; then some hidden glands pour out a glairy liquid, which
dissolves out the juices of the insect's body; next all is re-absorbed
into the plant, and the trap opens to repeat the operation. But the same
leaf perhaps never captures more than two or three insects. It ages
instead, becomes more rigid and motionless, or decays away.
479. That some few plants should thus take animal food will appear less
surprising when it is considered that hosts of plants of the lower
grade, known as Fungi, moulds, rusts, ferments, Bacteria, etc., live
upon animal or other organized matter, either decaying or living. That
plants should execute movements in order to accomplish the ends of their
existence is less surprising now when it is known that the living
substance of plants and animals is essentially the same; that the beings
of both kingdoms partake of a common life, to which, as they rise in the
scale, other and higher endowments are successively superadded.
480. =Work uses up material and energy= in plants as well as in animals.
The latter live and work by the consumption and decomposition of that
which plants have assimilated into organizable matter through an energy
derived from the sun, and which is, so to say, stored up in the
assimilated products. In every internal action, as well as in every
movement and exertion, some portion of this assimilated matter is
transformed and of its stored energy expended. The steam-engine is an
organism for converting the sun's radiant energy, stored up by plants in
the fuel, into mechanical work. An animal is an engine fed by vegetable
fuel in the same or other forms, from the same source, by the
decomposition of which it also does mechanical work. The plant is the
producer of food and accumulator of solar energy or force. But the
plant, like the animal, is a consumer whenever and by so much as it does
any work except its great work of assimilation. Every internal change
and movement, every transformation, such as that of starch into sugar
and of sugar into cell-walls, as well as every movement of parts which
becomes externally visible, is done at the expense of a certain amount
of its assimilated matter and of its stored energy; that is, by the
decomposition or combustion of sugar or some such product into carbonic
acid and water, which is given back to the air, just as in the animal it
is given back to the air in respiration. So the respiration of plants is
as real and as essential as that of animals. But what plants consume or
decompose in their life and action is of insignificant amount in
comparison with what they compose.