A common fern is the polypody (Polypodium vulgare), the habitat of which is damp woods and rocky glens. These ferns are hard to procure entire, as they have an underground stem, from which at intervals the leaves or fronds arise. The leaflets or pinna at certain seasons show a series of little brown dots on the under surface. These structures, called collectively the sori (singular sorus), are made up of a number of tiny spore cases. These spore cases, or sporangia, hold the asexual spores. These spores under favorable conditions of heat and moisture may germinate to form a tiny thread of cells which soon develops into a flat, heart-shaped body not much bigger than a pinhead, called a prothallus. tion of the prothallus with a compound microscope reveals the fact that scattered among the rhizoids are some tiny rounded elevations; immediately above the rhizoids and between them and the little groove (see Figure) in the prothallus are other structures; both the above structures are too minute to find with the naked eye. Archegonia. The last named are archegonia; they are found to be very tiny flask-shaped organs almost embedded in the surface of the prothallus. Each archegonium contains a single large egg cell. Antheridia. The other structures found among the rhizoids are the antheridia. Each antheridium contains a large number of very minute objects which are able to move about in water by means of lashlike threads of protoplasm. Each of these motile cells is called an antherozoid; they have, in fact, the same function as the sperm cells of A sporangium. the flowering plants. Because this part of the plant holds the egg cells and sperm cells, we recognize it as the sexual generation of the fern. Fertilization. The sperm cells swim to the egg cells in water (rain or dew), being attracted to the mouth of the flask-shaped archegonium by an acid secretion which is poured out by the cells forming the neck of the flask. Fertilization is essentially the same process that has been described for the flowering plants, the sperm cell uniting with the egg cell to form a single cell, the fertilized egg. Sporophyte and Gametophyte. - The direct result of fertilization is the growth of the egg cell by repeated division to form a little fern plant. Later the young plant strikes root, the prothallus dies away, and we have a fern plant which will later in the season produce asexual spores. The leafy fern plant, because it produces asexual spores, is called the sporophyte. The prothallus, which forms the eggs and sperms, both of which are known as gametes, or sex cells, is called the gametophyte. Alternation of Generations. The fern plant like the moss also passes through two entirely different stages, or generations. The spore ger distinguishing marks of the ferns and their allies. Fern plants show a great diversity in form and size. They vary from the great tree ferns of the tropics, some of which are thirty to forty feet in height, to tiny forms of almost microscopic size. The leaves of the ferns are among the most complex in form of any that we know. The Horsetails. — These comprise a small group of plants, recognized by their erect habit of growth, the leaves coming out in whorls on the stem. In most forms the stem contains considerable silica. This gave to the plant its former useful place in the household and its name of the scouring rush. If you burn one of these plants very carefully on a tin plate over a very hot fire, the delicate skeleton of silica may be seen. The horsetails, or Equisetums, were once a very important part of the earth's vegetation. Before the coal fields were formed, the ancestors of these plants flourished as trees. A large amount of the coal of this country is undoubtedly formed from the trunks of the Equisetums of the Carboniferous age. At present they are represented by a very few species, none of which are over four or five feet in height. Club Mosses. Another relative of the fern is the club moss (lycopodium). It is familiar to us as a Christmas decoration under the name of ground pine. It is chiefly of interest now as the representative of another group of plants that flourished during the Carboniferous age. Economic Value of Ferns. - It may be said that the ferns as a group have formed a large part of the enormous deposits of almost pure carbon that we call coal, from which we now derive the energy to run our many engines. Flowering plants Sexual Reproduction in Flowering Plants. reproduce their kind by the formation of seeds. As we know, the flower produces in the ovary structures which are known as ovules. In the interior of the ovule is found a clear protoplasmic area which is called the embyro sac. In this area is a cell (the egg cell) which is destined to form the future plant. In the pollen grain is found another cell, the sperm. This cell, after the germination of the pollen grain on the stigmatic surface of the flower, enters the ovule in the pollen tube and unites with the egg cell. The fertilized egg grows into the young plant within the seed, known as the embryo (see page 37). This method of reproduction, called sexual reproduction, is found in the spermatophytes, that is, all seed-producing plants. Botanists have shown that in the spermatophytes there exists an alternation of generations as in the mosses and ferns. The pollen grain is believed to contain the male gametophyte, while within the embryo sac is found the female gametophyte. Most of the life of the flowering plant is thus seen to be passed in the asexual or sporophyte stage. Thus we see that all plants - and all animals as well — form the cells which compose their bodies by either sexual or asexual growth, and the stage of asexual growth is usually separated from the period of sexual growth. Systematic Botany. - The plant world is divided into many tribes or groups. And not only are plants placed in large groups which have some very conspicuous characters in common, but smaller groupings can be made in which perhaps only a few plants having common characters may be placed. If we plant a number of peas so that they will all germinate under the same conditions of soil, temperature, and sunlight, the seedlings that develop will each differ one from another in a slight degree. But in a general way they will have many characters in common, as the shape of the leaves, the possession of tendrils, form of the flower and fruit. The smallest group of plants or animals having certain characters in common that make them different from all other plants or animals is called a species. Individuals of such species differ slightly; for no two individuals are exactly alike. Species are grouped together in a larger group called a genus. For example, many kinds of peas- the garden peas, the wild beach peas, the sweet peas, and many others are all grouped in one genus (called Lathyrus, or vetchling) because they have certain structural characteristics in common. Plant and animal genera are brought together in still larger groups, the classification based on general likenesses in structure. Such groups are called, as they become successively larger, Family, Order, and Class. Thus the whole plant and animal kingdom is grouped into divisions, the smallest of which contains individuals very much alike; and the largest of which contains very many groups of individuals, the groups having some characters in common. This is called a system of classification. Classification of the Plant Kingdom. The entire plant kingdom has been grouped as follows by botanists: 1. Spermatophytes. Angiosperms, true flowering plants. 2. Pteridophytes. The fern plants and their allies. 3. Bryophytes. Moss plants and their allies. 4. Thallophytes. The Thallophytes form two groups: the Algæ and the Fungi. The extent of the plant kingdom can only be hinted at, because each year new species are added to the lists There are about 110,000 species of flowering plants and nearly as many flowerless plants. The latter consist of over 3500 species of fernlike plants, some 16,500 species of mosses, over 5600 lichens (plants consisting of a partnership between alga and fungi), approximately 55,000 species of fungi, and about 16,000 species of algæ. REFERENCE BOOKS ELEMENTARY Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Book Company. Andrews, Botany All the Year Round, Chap. X. American Book Company. Atkinson, Lessons in Botany, Chaps. III, XIX-XXIX. Ginn and Company. How to Grow Mushrooms, Bul. 53, U.S. Department of Agriculture. Mushroom Poisoning, Cir. 13, U.S. Department of Agriculture. ADVANCED Atkinson, Mushrooms, Edible and Poisonous. Andrews and Church. Bergen and Davis, Foundations of Botany (Cryptogamic portion). Ginn and Com pany. Coulter, Barnes, and Cowles, A Textbook of Botany, Vol. I. American Book Com pany. De Bary, Comparative Anatomy of Phanerogams and Ferns. Clarendon Press. De Bary, Comparative Morphology and Biology of the Fungi, Mycetozoa, and Bacteria. Clarendon Press. Goodale, Physiological Botany. American Book Company. Grout, Mosses with a Hand Lens. A. J. Grout. Leavitt, Outlines of Botany. American Book Company. Marshall, The Mushroom Book. Doubleday, Page, and Company. Sedgwick and Wilson, General Biology. Henry Holt and Company. Strasburger, Noll, Schenck, and Schimper, A Textbook of Botany. The Macmillan Company. Henry Holt and Company. Underwood, Our Native Ferns and their Allies. Henry Holt and Company. |