XI. THE VARIOUS FORMS OF PLANTS AND HOW THEY REPRODUCE THEMSELVES

Problem XX. Some forms of plant life. (Optional.) (Laboratory Manual, Prob. XX.)

(a) An alga.

(b) A fungus.

(c) A moss.

(d) A fern.

Simplest Plant Body a Thallus. It has been found by botanists that the plants which are the simplest in body structure are those which live in the water. Sometimes such simple plants are found upon rocks or on the bark of trees. In such plants we can distin

of plant body are grouped under the general name of thallus. The simplest forms of plants have a thalluslike body.

Adaptation to Environment. Plants, as well as animals, are greatly affected by what immediately surrounds them, their environment. We have shown in our experiments that the environment (conditions of temperature, moisture, soil, etc.) is capable of changing or modifying the structure of plants very greatly. The changes which a plant or animal has undergone, that fit it for conditions in which it lives, are called adaptations to environment.

The principal factors which act on plants and which make up their environment are soil, water, temperature, and light.

The first plants were probably water-loving forms. It seems likely that, as more land appeared on the earth's surface, plants became adapted to changed conditions of life on dry land. With this change in habit came a need of taking in water, of storing it, of conducting it to various parts of the organism. So it does not seem unlikely that plants came to have roots, stems, and leaves, and thus became adapted to their environment on dry land. We find in nature that those plants or animals which are best adapted or fitted to live under certain conditions are the ones which survive or drive other competitors out from their immediate neighborhood. Nature selected those which were best fitted to live on dry land, and those plants eventually covered the earth with their progeny. Eventually, the forms of life grew more and more complex until at last very complicated organisms such as the flowering plants came to live upon the earth. Between the flowering plant and the simplest of all plants are several great plant groups which act as steps in complexity of structure between the most lowly and the most highly specialized plants.

The simplest of all these forms

are the algæ.

Algæ. The algae are a diverse collection of plants, containing some of the smallest and simplest as well as some of the largest plants in the world. The tiny one-celled plant which lives on the bark of trees is an example of the former; the giant kelp of the Pacific Ocean, which attains a length of over one thousand feet, of the latter. The body of the algæ is a thallus, which may be platelike, circular, ribbon-formed, threadlike, or filamentous. It may even be composed of a single cell. A large number of the algae inhabit the water, fresh and salt.

A red seaweed, showing a finely divided thallus body.

In color they

vary from green through the shades of blue-green to yellow, brown, and red. The latter colors are best seen in the seaweeds, all of which, how

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ever, contain chlorophyll. In the red and brown seaweeds the chlorophyll is concealed by other coloring material in the plant body. In the olive-brown fucus (the common rockweed) it is easy to prove the presence of chlorophyll by cutting open the bladders which are found in the plant body. The red seaweeds are among the most beautiful and delicate of all plants. They may be mounted under water upon cardboard and then studied after drying.

Rockweed, a brown alga, showing the distribution on rocks below high-water mark.

Green Algæ. The plants known as the green algae are of more interest to us because of their distribution in fresh water, and also because of their economic importance as a supply of oxygen for fish and other animals in the waters of our inland lakes and rivers. Our attention is called to them in an unpleasant way at times, when, after multiplying very rapidly during the hot summer, they die rapidly in the early fall and leave their remains in our water supply. Much of the unpleasant taste and odor of drinking water comes from this cause.

Pond Scum (Spirogyra). - This alga is well known to every boy or girl who has ever seen a small pond or sluggish stream. It grows as a slimy mass of green threads or filaments. Frequently it is so plentiful as almost to cover the surface of the water, buoyed

up by little bubbles of a gas which seems to arise from the body of the plant. If we collect some of this gas, we can easily prove that it is oxygen. The person who sees a pond with a covering of slimy

A, jar of water containing pond scum; B, same jar after an hour in the sunlight; the pond scum has risen to the top of the jar, buoyed up by the oxygen formed within it.

pond scum, knowing this fact, should no longer feel that the pond is a menace to health, unless it is a place where mosquitoes live and breed.

Under the low power of the microscope, the body of a pond scum

n

Spirogyra: n, nucleus; s, chlorophyll bands.

Ꭶ

L

is seen to be a thread made up of elongated cylindrical cells, each of which contains a spirally wound band of chlorophyll within it.

Careful study shows the presence of strands held in the body of the cell by strands of protoplasm, the remainder of the space within the cell being occupied by the cell sap.

scum.

Pond scum may grow by a simple division of the cells in a filament. This method of asexual reproduction is the way growth takes place in the cells of the root, stem, or leaf of a flowering plant, but another method of reproduction is also seen in pond The cells of two adjoining filaments may push out tubes which meet, thus connecting the cells with each other. Meantime the protoplasm of the cells thus joined condenses into two tiny spheres; the bands of chlorophyll are broken down, and ultimately the contents of one of the cells passes over the tube and mingles with the cell of the neighboring filament, with which it was previously connected by the tube formed from the cell walls. The result of this process of fusion is a thickwalled resting cell which we call a zygospore.

f

f

Conjugation. The process in which two cells of equal size unite to form a single cell is called conjugation. It is believed to be a sexual process which corresponds in a way to the fertilization in the higher plants.1 This cell thus formed can withstand considerable extremes of heat and cold, and may be dried to such an extent that it is fusion in progress. found in dust or in the air. Under favorable conditions, this spore will germinate and

Conjugation of Spirogyra; 28, zygospore; f,

produce a filament.

Pleurococcus. Many other forms of algæ are well known to us. One of the simplest is

A

B

pleurococcus. This little plant consists of a Pleurococcus. A, single cell; single tiny cell, which by division may give rise to two, three, four, or even more cells which cling together in a mass.

The green

B, colony of four cells formed from the original cell A.

1 Material which shows conjugation is not always easy to obtain. usually takes place most freely in the fall of the year. it may be preserved in a 4 per cent solution of formol. cent solution of chromic acid and then preserved in 70 cent formol shows the details of cellular structure.

Conjugation

When material is obtained, Material killed in a 5 per per cent alcohol or 4 per