Remarks. Veins prominent and very straight.
[Footnote 1: The specimen described is a leaf of Copper Beech.]
In describing shapes, etc., the pupils can find the terms in the book as
they need them. It is desirable at first to give leaves that are easily
matched with the terms, keeping those which need compound words, such as
lance-ovate, etc., to come later. The pupils are more interested if they
are allowed to press and keep the specimens they have described. It is not
well to put the pressed leaves in their note books, as it is difficult to
write in the books without spoiling the specimens. It is better to mount
the specimens on white paper, keeping these sheets in brown paper covers.
The pupils can make illustrations for themselves by sorting leaves
according to the shapes, outlines, etc., and mounting them.
3. _Transpiration_.--This term is used to denote the evaporation of water
from a plant. The evaporation takes place principally through breathing
pores, which are scattered all over the surface of leaves and young stems.
The _breathing pores_, or _stomata_, of the leaves, are small openings
in the epidermis through which the air can pass into the interior of the
plant. Each of these openings is called a _stoma_. "They are formed by a
transformation of some of the cells of the epidermis; and consist usually
of a pair of cells (called guardian cells), with an opening between
them, which communicates with an air-chamber within, and thence with the
irregular intercellular spaces which permeate the interior of the leaf.
Through the stomata, when open, free interchange may take place between
the external air and that within the leaf, and thus transpiration be
much facilitated. When closed, this interchange will be interrupted or
impeded."[1]
[Footnote 1: Gray's Structural Botany, page 89. For a description of the
mechanism of the stomata, see Physiological Botany, p. 269.]
In these lessons, however, it is not desirable to enter upon subjects
involving the use of the compound microscope. Dr. Goodale says: "Whether
it is best to try to explain to the pupils the structure of these valves,
or stomata, must be left to each teacher. It would seem advisable to
pass by the subject untouched, unless the teacher has become reasonably
familiar with it by practical microscopical study of leaves. For a teacher
to endeavor to explain the complex structure of the leaf, without having
seen it for himself, is open to the same objection which could be urged
against the attempted explanation of complicated machinery by one who has
never seen it, but has heard about it. What is here said with regard to
stomata applies to all the more recondite matters connected with plant
structure."[1]
[Footnote 1: Concerning a few Common Plants, p. 29.]
There are many simple experiments which can be used to illustrate the
subject.
(1) Pass the stem of a cutting through a cork, fitting tightly into the
neck of a bottle of water. Make the cork perfectly air-tight by coating it
with beeswax or paraffine. The level of the liquid in the bottle will be
lowered by the escape of water through the stem and leaves of the cutting
into the atmosphere.
(2) Cut two shoots of any plant, leave one on the table and place the
other in a glass of water.[1] The first will soon wilt, while the other
will remain fresh. If the latter shoot be a cutting from some plant that
will root in water, such as Ivy, it will not fade at all. Also, leave one
of the plants in the schoolroom unwatered for a day or two, till it begins
to wilt. If the plant be now thoroughly watered, it will recover and the
leaves will resume their normal appearance.
[Footnote 1: Lessons in Elementary Botany, by Daniel Oliver, London.
Macmillan & Co., 1864, pp. 14-15.]
Evaporation is thus constantly taking place from the leaves, and if there
is no moisture to supply the place of what is lost, the cells collapse and
the leaf, as we say, wilts. When water is again supplied the cells swell
and the leaf becomes fresh.
(3) Place two seedlings in water, one with its top, the other with its
roots in the jar. The latter will remain fresh while the first wilts and
dies.
Absorption takes place through the roots. The water absorbed is drawn up
through the woody tissues of the stem (4), and the veins of the leaves
(5), whence it escapes into the air (6).
(4) Plunge a cut branch immediately into a colored solution, such as
aniline red, and after a time make sections in the stem above the liquid
to see what tissues have been stained.[1]
[Footnote 1: The Essentials of Botany, by Charles E. Bessey. New York,
Henry Holt & Co., 1884. Page 74. See also Physiological Botany, pp.
259-260.]
(5) "That water finds its way by preference through the fibro-vascular
bundles even in the more delicate parts, is shown by placing the cut
peduncle of a white tulip, or other large white flower, in a harmless dye,
and then again cutting off its end in order to bring a fresh surface in
contact with the solution,[1] when after a short time the dye will mount
through the flower-stalk and tinge the parts of the perianth according to
the course of the bundles."[2]
[Footnote 1: If the stems of flowers are cut under water they will last a
wonderfully long time. "One of the most interesting characteristics of the
woody tissues in relation to the transfer of water is the immediate change
which the cut surface of a stem undergoes upon exposure to the air,
unfitting it for its full conductive work. De Vries has shown that when a
shoot of a vigorous plant, for instance a Helianthus, is bent down under
water, care being taken not to break it even in the slightest degree,
a clean, sharp cut will give a surface which will retain the power of
absorbing water for a long time; while a similar shoot cut in the open
air, even if the end is instantly plunged under water, will wither much
sooner than the first."--Physiological Botany, p. 263.]
[Footnote 2: Physiological Botany, p. 260.]
(6) Let the leaves of a growing plant rest against the window-pane.
Moisture will be condensed on the cold surface of the glass, wherever the
leaf is in contact with it. This is especially well seen in Nasturtium
(Tropæolum) leaves, which grow directly against a window, and leave the
marks even of their veining on the glass, because the moisture is only
given out from the green tissue, and where the ribs are pressed against
the glass it is left dry.
Sometimes the water is drawn up into the cells of the leaves faster than
it can escape into the atmosphere.[1] This is prettily shown if we place
some of our Nasturtium seedlings under a ward-case. The air in the case is
saturated with moisture, so that evaporation cannot take place, but the
water is, nevertheless, drawn up from the roots and through the branches,
and appears as little drops on the margins of the leaves. That this is
owing to the absorbing power of the roots, may be shown by breaking off
the seedling, and putting the slip in water. No drops now appear on the
leaves, but as soon as the cutting has formed new roots, the drops again
appear.
[Footnote 1: See Lectures on the Physiology of Plants. By Sidney Howard
Vines, Cambridge, England. University Press, 1886. Page 92.]
This constant escape of water from the leaves causes a current to flow
from the roots through the stem into the cells of the leaves. The dilute
mineral solutions absorbed by the roots[1] are thus brought where they
are in contact with the external air, concentrated by the evaporation of
water, and converted in these cells into food materials, such as starch.
The presence of certain mineral matters, as potassium, iron, etc., are
necessary to this assimilating process, but the reason of their necessity
is imperfectly understood, as they do not enter in the products formed.
[Footnote 1: See page 48.]
The amount of water exhaled is often very great. Certain plants are used
for this reason for the drainage of wet and marshy places. The most
important of these is the Eucalyptus tree.[1]
[Footnote 1: Reader in Botany. XII. Transpiration.]
"The amount of water taken from the soil by the trees of a forest and
passed into the air by transpiration is not so large as that accumulated
in the soil by the diminished evaporation under the branches. Hence, there
is an accumulation of water in the shade of forests which is released
slowly by drainage.[1] But if the trees are so scattered as not materially
to reduce evaporation from the ground, the effect of transpiration in
diminishing the moisture of the soil is readily shown. It is noted,
especially in case of large plants having a great extent of exhaling
surface, such, for instance, as the common sunflower. Among the plants
which have been successfully employed in the drainage of marshy soil by
transpiration probably the species of Eucalyptus (notably _E_. _globulus_)
are most efficient."[2]
[Footnote 1: Reader in Botany. XIII. Uses of the Forests.]
[Footnote 2: Physiological Botany, page 283.]
4. _Assimilation_.--It is not easy to find practical experiments on
assimilation. Those which follow are taken from "Physiological Botany" (p.
305).
Fill a five-inch test tube, provided with a foot, with fresh drinking
water. In this place a sprig of one of the following water
plants,--_Elodea Canadensis, Myriophyllum spicatum, M.
verticillatum_, or any leafy _Myriophyllum_ (in fact, any small-
leaved water plant with rather crowded foliage). This sprig should be
prepared as follows: Cut the stem squarely off, four inches or so
from the tip, dry the cut surface quickly with blotting paper, then
cover the end of the stein with a quickly drying varnish, for
instance, asphalt-varnish, and let it dry perfectly, keeping the rest
of the stem, if possible, moist by means of a wet cloth. When the
varnish is dry, puncture it with a needle, and immerse the stem in
the water in the test tube, keeping the varnished larger end
uppermost. If the submerged plant be now exposed to the strong rays
of the sun, bubbles of oxygen gas will begin to pass off at a rapid
and even rate, but not too fast to be easily counted. If the simple
apparatus has begun to give off a regular succession of small
bubbles, the following experiments can be at once conducted:
(1) Substitute for the fresh water some which has been boiled a few
minutes before, and then allowed to completely cool: by the boiling,
all the carbonic acid has been expelled. If the plant is immersed in
this water and exposed to the sun's rays, no bubbles will be evolved;
there is no carbonic acid within reach of the plant for the
assimilative process. But,
(2) If breath from the lungs be passed by means of a slender glass
tube through the water, a part of the carbonic acid exhaled from the
lungs will be dissolved in it, and with this supply of the gas the
plant begins the work of assimilation immediately.
(3) If the light be shut off, the evolution of bubbles will presently
cease, being resumed soon after light again has access to the plant.
(5) Place round the base of the test tube a few fragments of ice, in
order to appreciably lower the temperature of the water. At a certain
point it will be observed that no bubbles are given off, and their
evolution does not begin again until the water becomes warm.
The evolution of bubbles shows that the process of making food is going
on. The materials for this process are carbonic acid gas and water. The
carbonic acid dissolved in the surrounding water is absorbed, the carbon
unites with the elements of water in the cells of the leaves, forming
starch, etc., and most of the oxygen is set free, making the stream of
bubbles. When the water is boiled, the dissolved gas is driven off and
assimilation cannot go on; but as soon as more carbonic acid gas is
supplied, the process again begins. We have seen by these experiments
that sunlight and sufficient heat are necessary to assimilation, and that
carbonic acid gas and water must be present. The presence of the green
coloring matter of the leaves (chlorophyll) is also essential, and some
salts, such as potassium, iron, etc., are needful, though they may not
enter into the compounds formed.
The food products are stored in various parts of the plant for future use,
or are expended immediately in the growth and movements of the plant. In
order that they shall be used for growth, free oxygen is required, and
this is supplied by the respiration of the plant.
Some plants steal their food ready-made. Such a one is the Dodder, which
sends its roots directly into the plant on which it feeds. This is a
_parasite_.[1] It has no need of leaves to carry on the process of making
food. Some parasites with green leaves, like the mistletoe, take the crude
sap from the host-plant and assimilate it in their own green leaves.
Plants that are nourished by decaying matter in the soil are called
_saprophytes_. Indian Pipe and Beech-Drops are examples of this. They need
no green leaves as do plants that are obliged to support themselves.
[Footnote 1: Reader in Botany. XIV. Parasitic Plants.]
Some plants are so made that they can use animal matter for food. This
subject of insectivorous plants is always of great interest to pupils. If
some Sundew (_Drosera_) can be obtained and kept in the schoolroom, it
will supply material for many interesting experiments.[1] That plants
should possess the power of catching insects by specialized movements and
afterwards should digest them by means of a gastric juice like that of
animals, is one of the most interesting of the discoveries that have been
worked out during the last thirty years.[2]
[Footnote 1: See Insectivorous Plants, by Charles Darwin. New York: D.
Appleton and Co., 1875.
How Plants Behave, Chap. III.
A bibliography of the most important works on the subject will be found in
Physiological Botany, page 351, note.]
[Footnote 2: Reader in Botany. XV. Insectivorous Plants.]