Chapter II.
Chapter II.
The lowest organisms, as the slime molds, do not have an enclosing membrane, but consist of a naked mass of protoplasm. With this exception plants have an outer wall or membrane. They may consist of a single cell, as in the Bacteria, or a chain of cells, as in the filamentous Algz, or a mass of cells, as in the majority of plants, and are accordingly designated as unicellular or multicellular. The cell-wall is composed for the most part of cellulose, but may be modified in various ways.
Nomenclature.—The names for describing plants have been derived for the most part from studies of the higher plants, they having exclusively attracted the attention of botanists at first. But with the light which has been thrown on the relationship
PRINCIPAL GROUPS OF PLANTS. 3
of the higher and lower groups of plants by the more recent study of the lower forms the older terminology has been somewhat modified. Thus, for example, we speak of the root and shoot, with its leaves, as the vegetative organs of the higher plants, and in describing the corresponding organs (where they exist) in the lower plants, we either apply these terms directly, or indi- rectly by saying that the latter are root-like, stem-like, etc. On the other hand, we now speak of the sexual organs of the higher plants as antheridia and odgonia (or archegonia) instead of classifying them roughly as stamens and pistils, the latter names being retained but with a different signification. : Plant Organs.—Depending upon the fact that the plant re- quires nourishment for its growth and development and that it has also to carry on the work of reproduction or propagation, —1i.e., the production of new plants,—we distinguish between vegetative or nutritive organs and propagative or reproductive organs. The vegetative organs, such as the root, stem and leaves in higher plants, manufacture the food necessary for the life of the plant, while certain other more or less specialized organs or cells carry on the work of reproduction. In the lower plants, however, the whole structure is much simpler, and in some instances a cell which performs the work of a nutritive cell at one stage may become a reproductive cell at another, or, as in the case of the unicellular Algz, all the various functions of the plant may be carried on by a single cell. Generally speaking, there are two principal ways in which plants are multiplied or reproduced: (1) By CELL DIVISION or cell fission, and (2) by the formation of special cells known as SPoRES. In cell division (Fig. 85) the nucleus and cytoplasm of a cell divide to form two new cells or protoplasts, which become distinct by the formation of a wall or cell-plate between the two halves. All growth in plants is dependent upon this method, and in growing parts the cells are said to be in a state of division. Owing to the plasticity of the plant organism, detached portions will often grow and give rise to new plants, as in the case of cut- tings. Growth here as in the parent plant is accompanied by cell division. In some of the lower Algz (Fig. 10) cell division is the only method of propagation, and as only the ordinary vegetative or
4 A TEXT-BOOK OF BOTANY.
nutritive cells of the plant are involved in the process it is some- times spoken of as vegetative multiplication.
In both lower and higher plants, with the exceptions just noted, reproduction is also carried on by means of spores.
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Fic. 5. Ulothrix sonata. A, young filament with rhizoid cell (r); B, piece of filament showing escape of swarm spores; C, a swarm spore or zodéspore with 4 cilia; D, biciliate gametes escaping from a filament; E, F, G, showing different stages of union of two gametes; H, young zygote or zygospore in which the cilia have been absorbed; J, 1-celled plant developed from zygote; K, young plant organizing zodspores.—After Dodel-Port.
Depending upon their origin two classes of spores are distin- guished, namely, (a) asexual spores, and (b) sexual spores, In the production of asexual spores the contents of a certain cell called a mother cell or sPoRANGIUM break up into a number of new cells sometimes called daughter cells, which escape through the cell-wall. In the lower plants, particularly those growing
PRINCIPAL GROUPS OF PLANTS. 5
in water or in moist places, these cells are provided with short thread-like appendages known as cilia, which enable them to move about in the water. They are known as ZOOSPORES or swarm spores (Fig. 5, B, C), and each individual zodspore is able to produce a new plant.
The number of zoospores formed in a sporangium is usually 2 to 8, as in Ulothrix, but the number may be larger. The method of cell formation which gives rise to zoOspores is sometimes spoken of as INTERNAL DIVISION from the fact that they arise within the old cell and retain no relation to the old wall as is the case in cell fission, The zoospores are at first naked protoplasts, but later, on coming to rest, may form a wall. Sexual spores, on the other hand, are formed by the union of two cells known as GAMETES. When the gametes are similar the resulting spore is known as a ZYGOSPORE or zygote (Fig. 5, E, F, G). When the gametes are unlike, the spore produced by their union is known as an oOsporRE. In the latter case one of the gametes is larger — than the other, is less active, and is spoken of as the female gamete, odsphere, or egg (Figs. 12, 21). The other more active cell is known as the male gamete, antherozoid or sperm (Fig. 51, JIJ). The cell giving rise to the odsphere is known as the odgonium (Figs. 12, 21), while the one in which the anthero- zoid or sperm originates is called the antheridium (Figs. 12, 21, 22, 51).
PLANT GROUPS.
Until a comparatively recent time, botanists divided the plant kingdom into two large groups, as follows:
The flowering plants, or Phanerogams, meaning “ reproductive process evident,” so applied because the reproduction of the plant was readily seen to develop in the flower through the agency of the pistil and stamens.
The non-flowering plants, or Cryptogams, meaning “ repro- ductive process concealed,” so applied to the lower plants like the ferns, mosses, sea-weeds, etc., because in these plants the method of reproduction was not known.
Now, however, after a considerable amount of study, it has been learned that a great many of the lower plants have repro-