The cortex of the leaf-stalk is transitional in structure to that of the stem and leaf-blade. The leaf-stalk is traversed by one or more vascular strands connected below to the central con ductive system of the stem, and leading forward in the leaf-blade to a complex system of minor strands which ramify the meso phyll and run their course in the plane of junction of the spongy and palisade tissue. The layer of mesophyll immediately investing each bundle is phloeotherma and usually takes the form of a special parenchymatous sheath of elongated cells, distinguished otherwise from the rest of the mesophyll only by their poverty in chloroplasts. In a few rare cases the phloeothermal cells are rich in chlorophyll. These bundle-sheaths are considered important in the conduction of carbohydrates from the assimilative cells to the conductive strands and in the supply of water from the latter for transpiration. Their function is thus considered in some measure comparable with that of the phloeotherma of the stem.
The cortex of the roots of Pteridophytes is commonly exten sively sclerotic at maturity, but in flowering plants the root cortex generally remains a living parenchymatous food-storing tissue. Its innermost layer is invariably an endodermis.
Among the most striking characters of Pteridophytes and Phanerogams is the possession of a double (hydrom-leptom) conducting system, such as has been noted in the higher mosses, but with more sharply defined and specialized features. This is the vascular system, and associated with it are other tissues con sisting of parenchyma and special stereom. The whole tissue system is known as the stelar system (from the way in which in its simpler forms it runs through the whole axis of the plant as a column). When it is remembered that the moss plant is a gametophyte while the vascular plant is a sporophyte it will be realized that the vascular system of the latter is not the result of elaboration of the conductive system of the former, but that these vascular systems are most readily interpreted as somewhat similar products of differentiation in plants fitted for life on land.
Tissue Elements.—The hydroid of a Pteridophyte (e.g., a fern) is initially a living undifferentiated parenchymatous cell. As it dies it usually elongates and its wall is irregularly thickened and becomes woody (lignified). Its contents at maturity are watery solutions and air. The thickenings are on the inner wall surface, are insufficient to line the wall entirely, and take the form of a spiral or complex network. The mature hydroid is
called a tracheid. Its end walls persist and are commonly oblique. When the walls are mainly thickened their unthickened portions are called pits, and according to the varied distribution of the thickenings, the size, form, number and distribution of the pits depend. Thus in spiral tracheids a broad spiral band of un thickened wall persists, while when the thickenings on a wall-face are more or less transverse bars (often joined by short vertical bars) there is a resemblance to the rungs of a ladder, and we have a scalariform tracheid. All manner of transitions from spiral to scalariform tracheids may be found in a single plant. Spiral thickening is, however, characteristic of tracheids in which lig nification begins and is completed while elongation of the cell is still in progress. Scalariform thickening is typical of tracheids which mature late or slowly, so that thickening may be continued after elongation is completed. The pits of scalariform tracheids are typically bordered pits, as the bars of thickening last deposited are widest. The pits on tracheid-walls which are in contact with each other are at similar levels so that at regular intervals the cavities of contiguous tracheids are only separated by a common thin area of wall (the pit-membrane). Sooner or later the pit membrane disappears so that the mature tracheids come to con sist of a framework of lignified bars with numerous small open communications between the cell cavities. Collectively with asso ciated parenchyma the tracheids constitute the xylem or wood.
The leptoid of a Pteridophyte is also an elongated cell. It, however, remains alive, its wall is unlignified, it has a thin lining of protoplasm, but is destitute of a nucleus. It is always in communication with its neighbouring cells in a leptom strand by exceedingly minute wall perforations through which proto plasmic continuity is maintained. It is generally supposed that through these perforations organic substances are passed from cell to cell as through a sieve. The cells are accordingly called sieve-tubes, and collectively with associated parenchyma consti tute the tissue called the phloem, or bast. The phloem is typi cally distributed at the periphery of the xylem when the latter is massed centrally, but may occur in other positions (see below). Between the phloem and the phloeotherma is a mantle of one or more layers of parenchyma forming the pericycle.