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a localized absorptive region, a well-developed bydrom in the axis of the plant, as well as similar hydroid strands in the leaf-midribs, are constantly met with. In higher forms the conducting strands of the leaves are continued downwards into the stem, and eventually come into connexion with the central hydroid cylinder, forming a complete cylindrical investment apparently distinct from the latter, and exhibiting a differentiation into hydrom, leptom, and amylom which almost completely parallels that found among the true vascular plants. Similar differentiation, differing in some details, takes place independently in the other generation, the sporogonium. The stereom of the moss is found mainly in the outer cortex and in the midrib of the leaf. Vascular Plants. The body of the sporophyte in the great majority of the vascular plants (Pteridophytes and Phanerogams) shows a considerable increase in complexity over that found in the Bryophytes. The principal new feature in the external conformation of the body is the acquirement of “ true ” roots, the nearest approach to which in the lower forms we saw in the “ rhizome ” of Polytrichaceoe. The primary root is a downward prolongation of the primary axis of the plant. From this, as well as from various parts of the shoot system, other roots may originate. The root differs from the shoot in the characters of its surface tissues, in the arrangement of its vascular system and in the mode of growth at the apex, all features which are in direct relation to its normally subterranean life and its fixative and absorptive functions. Within the limits of the sporophyte generation the Pteridophytes and Phanerogams also differ from the Bryophytes in possessing special assimilative and transpiring organs, the leaves, though these organs are also developed in the gametophyte of many liverworts and of all the mosses. The leaves, again, have special histological features adapted to the performance of their special functions. Alike in root, stem, and leaf, we can trace a three-fold division of tissue-systems, a division of which there are indications among the lower plants, and which Tissue- js the expression of the fundamental conditions systems. ^ evolution of a bulky differentiated plantbody. From the primitive uniform mass of undifferentiated assimilating cells, there is, on the one hand, a specialization of a surface layer regulating the immediate relations of the plant with its surroundings. In the typically submerged algae and in submerged plants of every affinity this is the absorptive and the main assimilative layer, and may also by the production of mucilage be of use in the protection of the body in various ways. In the terrestrial plants it differs in the subterranean and subaerial parts, being in the former pre-eminently absorptive, and in the latter protective—provision at the same time being made for the gaseous interchange of oxygen and carbon dioxide necessary for respiration and feeding. This surface layer in the typically subaerial “ shoot ’ of the sporophyte in Pteridophytes and Phanerogams is known as the epidermis, though the name is restricted by some writers, on account of developmental differences, to the surface layer of the shoot of Angiosperms, and by others extended to the surface layer of the whole plant in both these groups. On the other hand, we have an internal differentiation of conducting tissue, the main features of which have already been fully described. The remaining tissue of the plant-body, a tissue that we must regard phylogenetically as the remnant of the undifierentiated tissue of the primitive thallus, but which often undergoes further differentiation of its own, the better to fulfil its characteristically vital functions for the whole plant, is
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known, from its peripheral position in relation to the primitively central conducting tissue, as the cortex. The remaining important function for which provision has to be made in any plant-body of considerable size, especially when raised into the air, is that of support. Special tissues may be developed for this purpose in the cortex, or in immediate connexion with the conducting system, according to the varying needs of the particular type of plant-body. In relation to its characteristic function of protection, the epidermis, which, as above defined, consists of aouter single layer of cells, has typically thickened and cuticularized ^pyderm/s. walls. These serve not only to protect the plant ^ against slight mechanical injury from without, but also and especially to prevent the evaporation of water from within, and to protect the internal tissues from the soaking in of rain, &c. At intervals it is interrupted by pores leading from the air outside to the intercellular space system below. These stomata are each surrounded by a pair of peculiarly modified epidermal cells called guard-cells, which open and close the pore according to the need for transpiration. The structure of the stomata of the sporophyte of vascular plants is fundamentally the same as that of the stomata on the sporogonium of the true mosses and of the liverwort ^4wi!/mceros, a striking fact that lends countenance to the theory which would derive the Pteridophytic sporophyte from the sporogonium of an Anthoceros-ke ancestor. The stomata serve for all gaseous interchange between the plant and the surrounding air. The guard-cells contain chlorophyll, which is absent from typical epidermal cells, the latter acting as a tissue for water storage. Sometimes the epidermis is considerably more developed by tangential division of its cells, forming a manylayered water-tissue. The extremely various modifications of the epidermis, especially in connexion with the occurrence of hairs of the most various kinds, and the numberless differences in the distribution of the stomata, and their relation to the general surface of the epidermis, are concisely treated as to the leading facts in Botany (Encij. Brit. vol. iv. pp. 89-91), and in Histology (vol. xii. p. 17). Mention may be made here of a class of epidermal organ, the Injdathode, the wide distribution and variety of which have been revealed by recent research. These are special organs usually situated on foliage fjyaathodes. leaves for the excretion of water in liquid form ^ when transpiration is diminished so that the pressure in the water-channels of the plant has come to exceed a certain limit. They are widely distributed, but are particularly abundant in the tropics. In one type they may take the form of speciallymodified single epidermal cells or multicellular hairs without any direct connexion with the vascular system. The. cells concerned, like all secreting organs, have abundant protoplasm with larger nuclei, and sometimes, in addition, part of the cellwall is modified as a filter. In a second type they are situated at the ends of tracheal strands and consist of groups of richly protoplasmic cells belonging to the epidermis (leaves of many ferns), or to the subjacent tissue (the commonest type in flowering plants); in this last case the cells in question are known as The epithema is penetrated by a network of fine intercellular spaces, which are normally filled with water and debouch on one or more intercellular cavities below the epidermis. Above each cavity is situated a so-called water-stoma, no doubt derived phylogenetically from an ordinary stoma, and enclosed by guardcells which have nearly or entirely lost the power of movement. The pores of the water-stomata are the outlets of the hydathode. The epithema is frequently surrounded by a sheath of cuticularized cells; in other cases the epithema may be absent altogether, the tracheal strand debouching directly on the lacunas of the mesophyll. This last type of hydathode is usually situated on the edge of the leaf. Some hydathodes are active glands, secreting the water they expel from the leaf. Many other types of glands also exist, either in connexion with the epidermis or not, such as nectaries, digestive glands, oil, resin and mucilage glands, &c. The surface layer of the root, sometimes included under the term epidermis, is fundamentally different from the epidermis of the stem. In correspondence wivh its waterabsorbing function it is not cuticularized, but remains usually thin-walled ; the absorbing surface is increased by its cells being often produced into delicate tubes which curl round and adhere firmly to particles of soil, thus at once fixing the root firmly in the soil, and enabling the hair to absorb readily the thin films of water ordinarily surrounding the particles. The root-hair ends blindly and is simply an outgrowth from a surface cell, having no cross walls. It corresponds in function with the rhizoid of a bryophyte. At the apex of a root, covering and I protecting the delicate tissue of the growing point, is a special
