The absence of cold weather in tropical countries allows trees to mature without a thick coating of bark or corky material. The trees all have a green and fresh appearance. Monocotyledonous plants prevail. Ferns of all varieties, especially the largest tree ferns, are abundant. PLANT LIFE IN THE TEMPERATE ZONES. In the state of New York conditions are those of a typical temperate flora. Extremes of cold and heat are found, the temperature ranging from 30° Fahrenheit below zero in the winter to 100° or over in the summer. Conditions of moisture show an average rainfall of from 60 to 130 cm. Conditions of moisture in the country cause great differences in the plant covering. In the eastern part of the United States the rainfall is sufficient to give foothold to great forests, which aid in keeping the water in the soil. In the middle West the rainfall is less, the prairies are covered with grasses and other plants which have become adapted to withstand dryness. In the desert region of the Southwest we find true xerophytes, cacti, switch plants, yuccas, and others, all plants which are adapted to withstand almost total absence of moisture. In the temperate zone the water supply is the primary factor which determines the form of plant growth. REFERENCE BOOKS FOR THE PUPIL Andrews, Botany All the Year Round. American Book Company. D. C. Heath and Company. FOR THE TEACHER Bailey, The Survival of the Unlike. The Macmillan Company. Darwin, Animals and Plants under Domestication, Chaps. IX, XII. D. Appleton and Company. Kerner. Natural History of Plants. 4 Vols. Henry Holt and Company. Schimper, Plant Geography. Clarendon Press. Year Book, Department of Agriculture, 1894, 1895, 1898, 1900. XI. FLOWERLESS PLANTS SYSTEMATIC BOTANY. -The plant world is divided into many tribes or groups. Any one who has visited a hothouse or a large garden is likely to notice this fact. 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 may differ slightly; indeed no two individuals are exactly alike It is known that in some cases seeds from plants which have thus varied to a considerable degree may reproduce these variations in the young plants. This fact is made use of by plant breeders to produce new kinds of plants. Species are grouped together in a larger group called a genus. For example many kinds of peas - the garden edible varieties, the wild beach peas, the sweet peas, and many others—are all grouped in one genus (called pisum, Latin for pea) because they have certain structural characteristics in common. NOMENCLATURE. - When we wish to identify a plant, we look it up by means of its generic and specific names in much the same way that we look up a name in a city directory. As in a directory the last name of the person is placed first, as Jones, John, so we find the Latin name Phaseolus given to the beans as a genus. Phaseolus vulgaris is the name of the common bean; Phaseolus lunatus, the pole or lima bean; and Phaseolus multiflorus, the scarlet runner. SYSTEM OF CLASSIFICATION ARTIFICIAL. 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 or Tribe, Order, and Class. Thus the whole plant and animal kingdom is artificially massed in separate divisions, the smallest of which contains a few 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. Phanerogams and Cryptogams. In the widest sense the plant world is divided into two great groups, the flowering plants, or Phanerogams, and the flowerless plants, or Cryptogams. This is an old system of classification, but it shows one very important distinction in the plant kingdom. The flowerless plants are much simpler in structure than the flowering plants. We are apt to entirely overlook them in a casual glance at vegetation in a landscape. Thousands of species exist so small that we cannot see them with the unaided eye. Many kinds hide themselves in the water, while still others may lie flat on the ground or cling to the bark of trees and thus escape observation. Yet one of the cryptogams is over a thousand feet in length, one of the longest plants in the world. Classification of the Plant Kingdom. The entire plant kingdom has been grouped as follows by the later botanists:true flowering plants. 1. Spermatophytes. { dinosperms, the pines and their allies. 2. Pteridophytes. The fern plants and their allies. 3. Bryophytes. Moss plants and their allies. 4. Thallophytes. Plants in which the plant body is a thallus, that is, the body is not divided into root, stem, and leaves. A seaweed and a mushroom are good examples. The Thallophytes form two groups: the Alga and the Fungi. The extent of the plant kingdom can only be hinted at, because each day new species are added to the lists. There are nearly 150,000 species of flowering plants and perhaps half as many flowerless plants. The latter consist of nearly 3500 species of ferns, some 800 species of mosses, over 5600 lichens (plants consisting of a partnership between alga and fungi), approximately 45,000 species of fungi, and 15,000 species of algæ. Flowering plants re Sexual Reproduction in Flowering Plants. produce 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 embryo 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 ger mination of the pollen grain on the stigmatic surface of the flower, enters the ovule in the pollen tube and unites with the egg cell. This process, known as fertilization, is the most important event in the life of the plant, for it is only by means of this process that the ovule is stimulated to become a seed. The fertilized egg grows into the young plant within the seed, known as the embryo (see page 34). This method of reproduction, called sexual reproduction, is found in the spermatophytes, that is, all seed-producing plants. In the flowerless plants a somewhat similar process takes place. Seeds are not formed, however, but structures called spores reproduce the plants. Sexual and Asexual Spores. A spore is usually considered to be a cell which has become dormant, but which will under favorable conditions again germinate to form a new plant. A spore, as we shall see, may be formed in one of several ways. If formed by the union of two cells, as is the fertilized egg, it is then said to be a sexual spore. If, as is frequently the case, the spore is formed by the separation of a bit of protoplasm from the plant to form a resting cell, then it is called an asexual spore. In most of the so-called spore plants" both sexual and asexual spores are formed at different times during the life history of the plant. A spore of bread mold, highly magnified. PTERIDOPHYTES 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. The ferns are among the most beautiful of our plants, and the study of a common form will amply repay the time so spent. The Polypody (Polypodium vulgare)!. The habitat of the polypody is damp woods and rocky glens. The ferns are usually hard to get up 1 Hunter and Valentine's Manual, page 93. entire. The stem is underground. Large compound leaves (called fronds) are given off at intervals along the stem. rootstock. Try to find a reason for calling it a stem rather than a root. Note the arrangement of the leaflets, or pinna, of the fern frond. Some of the pinnæ will 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. Examine a sorus with your hand lens to make out its position on the pinna with reference to the veins. Mount a small bit of the fern leaf which contains a sorus under a very low power of the compound microscope. How are the sporangia arranged in the sorus? If a single sporangium is gently separated from the mass and mounted carefully in alcohol and water on a slide, the following structures may be found: (a) the stalk; (b) a thick wall row of cells, yellow We call the underground stem a Rock fern, polypody. Notice the underground stem giving off roots (R) from its under surface, and leaves (C) from the upper surface. The com-. pound leaf or frond may bear sori (S) on the under side of the leaflets. in most specimens, which form the annulus, or ring; (c) the covering of Section of sorus; 8, sporangia; i, indusium, or covering; b, blade of the leaf.- Wossidlo. thin-walled cells over the remainder of the sporangium. A comparison of several sporangia will bring out the fact that in an opened sporangium the annulus is never broken. It is always the thin-walled cells that are ruptured. The dark-colored spores may be seen in the opened sporangia. If fresh material is obtainable, it will be possible to see how the spores get out of the sporangium. A drop of glycerine run under the cover slip of a slide holding a fresh unopened sporangium soon causes the sporangium to snap open. If the sporangium is dry and on the under surface of the fern leaf, the spores will be scattered for a considerable distance. An explanation for this snapping open of the sporangium is found when we notice that the outer walls of the cells forming the annulus are thinner than the inner walls. This allows water to escape more rapidly on one side, and pressure from without causes the cells to bend outward. A sporangium. |