Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/102

92 seem to have a protective function. The secretions of glands are very various, oily, waxy, resinous, gummy, saccharine, acid, &c.

ORGANS OF PLANTS.

Having now considered the elementary structures and tissues found in the Vegetable Kingdom, we proceed to view them in combination to form the plant. The simplest plant is found amongst Alga3, where, as in the Red-snow plant (Protococcus nivalis, fig. 51) the whole organism con sists of a single isolated cell. Other Algre and all Fungi and Musci are composed of a number of cells united in various ways ; whilst in Ferns and their allies and all riowering plants vessels are formed in addition to the cells. The plants in which the tissues are entirely cellu- lar are termed cellular plants ; those in which vessels are also found are vascular plants. Fig. 51. Cells of the Red-snow plant different stages of growth and development, a, cell in the young state ; b, cell fully formed, with cellules in its interior ready to be discharged, and to form independent plants; c, cell after its contents have been dis charged. That the portions of a plant may be properly maintained two functions have to be performed, namely, nutri tion, on the proper performance of which the life of the individual plant depends, and reproduction, by which the perpetuation of the type is provided for. In such a simple form as the Red-snow plant (fig. 51) those functions are performed by the single cell. In the plants composed of numerous cells a differentiation takes place by which special cells are set apart for particular functions, and thus certain organs are formed in the plant. In the higher plants those organs become more complicated from the introduction of the vascular element. The nutritive organs of plants are generally known as the root, the stem, and the leaves. In all vascular plants and the higher cellular plants an axis or stem having roots and bearing leaves is distinguishable, and such plants have been designated Cormophytes or Phyllophytes. In the lower class of cellular plants, as Fungi and Algae, no such distinction is possible, and there is merely a flattened leafy expansion with dependent filiform processes ; this structure has been termed a thallus, and such plants are Thalloyens or Tkallophytes. [ missing text ] Amongst the higher plants the reproductive organs are, in ordinary language, comprehended under the term flower ; and, as they are conspicuous, such plants have been denominated Flowering, Phanerogamous, or Pliwnogamous. Amongst all cellular plants and in some vascular plants, as Ferns and Equisetum, there are no flowers, and the repro ductive organs are inconspicuous, hence they have been termed Flowerless or Cryptoyamous. In all cases the young plant, or embryo, is completely cellular. But as growth proceeds, that differentiation takes place which dis tinguishes the several classes of plants one from the other. In Phanerogams the first leaves produced upon the embryo plant arc termed primary, seed-lobes, or cotyledons. In some cases these are two in number, and are opposite one another. Plants in which this occurs are Dicotyledonous (fig. 52), as our ordinary forest trees. In other plants the lobes alternate and only one cotyledon is formed ; such are Monocotyledonous (fig. 53), as Grasses, Lilies. In Crypto gams, on the other hand, no such seed-lobes or cotyledons are produced, and they are Acotylalouous (fig. 54). Fig. 54. out a conical root-like pro cess r, while the other ex tremity, containing a nu cleus and granules, forms a cellular frond; b, the same spore, with the root- like process r, dividing. No cotyledons are pro duced. In all plants the original cell tissue which gives origin to its parts is of a uniform nature, and is termed the primary tissue. When all the cells of this tissue are capable of multipli cation and division the tissue is a meristem or generating tissue. If the cells are not so capable, then it is a permanent tissue. The primary tissue al tlie growing point Ot any Shoot Or root is essentially a meristem, and it has been designated the primary meristem to distinguish it from the se condary meristem, which is applied to a tissue in the older parts of a stem or root which remains or be comes capable of division. The growing point of the apex has been termed the punctum vegetationis, and it not unfrequently forms a conical projection, and is then the vegetative cone. By growth at this punctum vegetationis the shoot or root increases in length, and the mode of addition is in many cases of a definite character. Two chief types of growth are recognized. In one of these a single large cell is always present at the apex, termed the apical cell, which may be regarded as the mother-cell, whence by bipartition in a definite manner the whole meristem below it has arisen, as is well seen in vascular Cryptogams and cellular plants. The other type is seen in Phanerogams, and here no such apical cell is visible, but a number of cells are found at the apex by whose multiplica tion the subsequent tissues are formed. But in whatever way formed, a primary meristem is the result of all the processes of growth, and by differentiation of its cells the various parts of the shoot or root are formed. The outer layer of the primary meristem, which extends completely over the punctual vegetationis, is termed the dermatogen ; it is the primordial epidermis, being continuous with the epidermis of the shoot and afterwards becoming epidermis. Underneath the dermatogen several layers of cells are distinguished, continuous with the cortical portion of the shoot or root ; this is the primordial cortex, and constitutes the periblem. Enclosed by this is a central cellular mass, out of which the fibro-vascular bundles and the structures of the central part of the shoot or root are formed; this has been termed the plerome. If the growing axis be a young root there is in addition developed, usually from the dermatogen, a mass of cells at the extremity, constituting a root-cap, or protective covering, to the delicate meris- matic cells beneath ; no such structure is formed in a stem. Thus a stem is structurally distinct from a root in having no root-cap.

In the plerome the fibro-vascular bundles are formed. Certain cells become elongated and prosenchymatous and united in bundles leaving no intercellular spaces ; 