Outlines of Lessons in Botany Part 12

(2) If breath from the lungs be pa.s.sed by means of a slender gla.s.s 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 a.s.similation 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 a.s.similation 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 a.s.similation, 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 pota.s.sium, 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 a.s.similate 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.]

5. _Respiration_.--Try the following experiment in germination.

Place some seeds on a sponge under an air-tight gla.s.s. Will they grow?

What causes them to mould?

Seeds will not germinate without free access of air. They must have free oxygen to breathe, as must every living thing. We know that an animal breathes in oxygen, that the oxygen unites with particles of carbon within the body and that the resulting carbonic acid gas is exhaled.[1] The same process goes on in plants, but it was until recently entirely unknown, because it was completely masked during the daytime by the process of a.s.similation, which causes carbonic acid to be inhaled and decomposed, and oxygen to be exhaled.[2] In the night time the plants are not a.s.similating and the process of breathing is not covered up. It has, therefore, long been known that carbonic acid gas is given off at night. The amount, however, is so small that it could not injure the air of the room, as is popularly supposed. Respiration takes place princ.i.p.ally through the stomata of the leaves.[3] We often see plants killed by the wayside dust, and we all know that on this account it is very difficult to make a hedge grow well by a dusty road. The dust chokes up the breathing pores of the leaves, interfering with the action of the plant. It is suffocated.

The oxygen absorbed decomposes starch, or some other food product of the plant, and carbonic acid gas and water are formed. It is a process of slow combustion.[4] The energy set free is expended in growth, that is, in the formation of new cells, and the increase in size of the old ones, and in the various movements of the plant.

[Footnote 1: See page 13.]

[Footnote 2: This table ill.u.s.trates the differences between the processes.

a.s.sIMILATION PROPER. RESPIRATION.

Takes place only in cells Takes place in all active cells.

containing chlorophyll.

Requires light. Can proceed in darkness.

Carbonic acid absorbed, Oxygen absorbed, carbonic oxygen set free. acid set free.

Carbohydrates formed. Carbohydrates consumed.

Energy of motion becomes Energy of position becomes energy of position. energy of motion.

The plant gains in dry The plant loses dry weight.

weight.

Physiological Botany, page 356.]

[Transcriber's Note: Two footnote marks [3] and [4] above in original text, but no footnote text was found in the book]

This process of growth can take place only when living _protoplasm_ is present in the cells of the plant. The substance we call protoplasm is an alb.u.minoid, like the white of an egg, and it forms the flesh of both plants and animals. A living plant can a.s.similate its own protoplasm, an animal must take it ready-made from plants. But a plant can a.s.similate its food and grow only under the mysterious influence we call life. Life alone brings forth life, and we are as far as ever from understanding its nature. Around our little island of knowledge, built up through the centuries by the labor of countless workers, stretches the infinite ocean of the unknown.

_Gray's First Lessons_. Sect. VII, XVI, --2, --4, --5, --6, 476-480.

_How Plants Grow_. Chap. I, 119-153, Chap. III, 261-280.