color on tree trunks, stone houses, etc., is due to millions of these little

plants.

Diatoms.

These plants are found in vast numbers living on the mud or stones at the bottom of small streams. The plant body is inclosed in a cell wall composed largely of silica. Many of the diatoms are free-swimming. They compose a large percentage of the living organisms found near the ocean's surface.

Diatoms are found as fossils, and make up a large proportion of many rocks. The siliceous skeletons in such rocks are of commercial importance, the rock forming a basis for polishing powders.

The

Fungi, Parasites, and Saprophytes. thallus plants may be grouped in two great divisions: the Alga, water-loving thallophytes containing chlorophyll, and the Fungi, thallus plants which do not contain chlorophyll. As a direct result of the lack of chlorophyll in the cells, the fungi are unable to make their own. food. They must obtain food from other plants or animals. Some take up their abode upon living plants or animals (in which case they are called from some dead organic matter.

r

S

Bread mold: r, rhizoids; s, sporangium.

Various forms of Diatoms.

parasites); others obtain their food The latter are called saprophytes. The above facts make the group of the fungi of immense economic importance to man.

Mold (Rhizopus nigricans).One of the most common of all our fungi is the black mold which appears growing upon bread, cake, and other organic substances under certain conditions of temperature and moisture.

The tangled mass of threads which cover the bread is called the mycelium, each thread being called a hypha. Many of the hyphæ are prolonged into tiny upright threads, bearing at the top a little ball. With the low power of the microscope each of these structures is seen to contain many tiny bodies called spores. These

spores have been formed by the division of the protoplasm making up the ball or sporangium into many separate bodies.

This method of the production of spores is evidently asexual. These spores, if grown under favorable conditions, will produce more mycelia, which in turn bear sporangia. It has been found, however, that at some time during the life of the mold another method of reproduction is likely to occur.

Formation of Zygospores. Two hypha which are close-lying put out threads which communicate. The end of each of the

B

threads cuts off a cell, and the two cells, each from a different hypha, flow together and mingle. In this condition they remain as a single resting cell. This cell, which puts a heavy D wall around itself, is a zygospore.

Here again we have a process of conjugation similar to that we observed in the pond scum. The ultimate result of the conjugation of the two cells is that a new plant grows from the zygospore after a period of rest. During the resting stage the spore may undergo very unfavorable conditions, even to extreme dryness, heat, or cold. Conjugation of black mold: A, The use of the zygospore to the plant B, C, D, successive stages in is evidently to continue the species the formation of the zygospore. during an unfavorable time in the life history of the plant. The process of conjugation is probably a sexual process, as we have called it in pond scum.

A

Physiology of the Growth of Mold. Mold, in order to grow rapidly, evidently needs oxygen, moisture, and heat. It obtains its food from the material on which it lives. This it is able to do by means of digestive ferments which are given out by the rhizoids or rootlike parts of the hypha, by means of which the mold clings to the bread. These ferments change the starch of

1 It seems to have been proved recently that zygospores are formed by the union of two cells, from different filaments, one of which has male, the other female, characters.

the bread to sugar and the proteid to a soluble form which will pass by osmosis into the hypha. Thus the plant is enabled to absorb the material. This food is then used to supply energy and make protoplasm. This seems to be the usual method by which saprophytes assimilate the materials on which they live.

Other Saprophytic Fungi. The mushroom resembles a tiny umbrella. The upper part is known to botanists as the cap; the cap is held up by a stalk or stipe. The under surface of the cap discloses a number of structures which radiate out from

the central stipe to the edge of

the cap.
These are the gills.
If you place the cap of a mush-
room gills downward on the
surface of a piece of white
paper, being careful not to dis-
turb for at least twelve hours,
it will be found that when the
cap is removed a print of the
shape and size of the gills re-
mains on the paper. This is a
spore print. It has been caused
by the spores of the plant, which
have fallen from the place where
they were formed between the
gills to the surface of the paper.

Photographed

Mycelium. - The mushroom is, then, the spore-bearing part of the plant. Where is the plant body? This question is answered if we dig up a little of the earth surrounding a mushroom. In the rich black soil is seen a mass of little whitish threads. These threads form the mycelium of the fungus. The hyphæ of this part of the plant body take food from the organic matter in the soil and digest it in the same manner as did the hyphæ of black mold. The mushroom is a saprophyte. No sexual stage has yet been discovered. Food Value of Mushrooms. The food value of the edible mushroom has been much overestimated. Recent experiments seem to show that, although they have a slight food value, they are far from taking the place of nitrogenous foods, as was formerly believed by scientists.

Other Fungi. Many other plants, both useful and harmful to man, belong in this group; among them are the yeasts, the various parasitic

rusts and smuts, causing plant diseases, and, most important of all, the bacteria. We shall consider several of these plants later in their direct relation to the human race.

Mosses

Mosses are mostly shade-loving and moisture-loving plants. They form velvety carpets in many of our forests, but they often show their preference for moist localities by covering the wooded shores of lakes and swamps.

Pigeon-wheat Moss. One of the mosses frequently seen and easily recognized is the so-called pigeon-wheat moss (Polytrichum commune).

G

G

Unlike some mosses, it often inhabits dry localities. It may be found on some dry hillock close to the edge of the woods, where it forms a reddish brown carpet. This red color is due largely to the presence of a great number of little upright stalks, bearing at the summit tiny capsules, which seem to grow up from the leafy moss plant. The resemblance of a large number of these stalks and capsules to a mimic field of grain has given the name pigeon-wheat moss to this form.

Forms of Plants. Three kinds of moss plants appear to be present: leafy plants, others bearing a stalk and capsule, and still others which terminate at the end in a little rosette of leaves, inclosing what appears to be a tiny flower.

Leafy Moss Plant. A leafy moss plant has rhizoids or hairlike roots, an upright stem, and green leaves. In the plants which have a stalk and capsule, the stalk grows directly from the end of the leafy plant. This capsule is provided with an outer cap which seems to have somewhat the structure of a thatched roof. Under the cap is found a lid, or cover, to the capsule. If this cover is removed and the capsule turned upside down, the dust that escapes will be found to be made up of a great number of spores. Sporophyte. The capsule is the spore-producing part (sporangium) of the moss plant. The stalk and capsule together form the sporophyte or spore-producing generation of the moss.

Two moss plants, showing the gametophyte (G) and the sporophyte (S).

If we were to plant the spores of the moss in damp sand, taking care to keep the sand moist and warm, we might get them to grow. The spore germinates into a threadlike structure, very tiny, and not at all like the adult moss plant. This thread is called a protonema.

Adult Moss Plants. The protonema soon develops rhizoids; tiny buds appear which in time form the adult moss plant. These adult plants may grow only leaves, and become what are known as sterile plants; or they may develop into a plant that bears at the summit the little rosette of leaves previously referred to. Within the rosette lie a number of tiny organs which hold large numbers of sperm cells. Other moss plants not so tall as the sperm-producing plants bear at the summit of the stem a tuft of leaves which hide a number of small flask-shaped structures, each of which contains a single egg cell. These plants form the sexual generation of the moss. This stage of the plant is called the gametophyte, because it produces the gametes or sexual cells, eggs and sperms. After a sperm cell has been transferred (usually by means of a drop of dew) to the egg cell, a fusion of the two cells takes place. This, we remember, is the process of fertilization. In the mosses the fertilization of the egg cell results in the growth of that part of the plant which forms and bears the asexual

spores.

Alternation of Generations. In the mosses we have what is known as an alternation of generations. The leafy moss, bearing among its leaves the organs producing sperms and eggs, antheridia and archegonia, gives place to a stalk and capsule bearing the asexual spores. This spore-bearing portion of the plant does not appear until after fertilization; then it grows directly out of that part of the plant which produces the egg cell. In fact, if we make a microscopic examination of the egg-producing structure (the archegonium) directly after fertilization, we find that the sporophyte is a direct outgrowth from the fertilized egg cell. Thus the sexual stage alternates with the asexual stage in the life of the plant.

Sporophyte a Parasite. One interesting fact comes out in connection with this growth of the sporophyte. It has no green leaves and must therefore obtain all its nourishment from the leafy moss plant, or gametophyte. The spore-bearing part of the plant is thus actually a parasite upon the gametophyte.

Ferns and their Allies

The Ferns and their Allies. The fern plants include the true ferns, the horsetails or scouring rushes, and the club mosses. The true ferns are moisture-loving and shade-loving plants; they play an important part in the vegetation of the tropical forests. Many forms are found in the temperate regions; we even have some common ferns that remain green all winter. Fossil ferns have been found in Greenland, thus showing that at one time the climate at the north was milder than it now is.