Page:EB1911 - Volume 21.djvu/769

Rh Most of the collateral bundles of this spermophytic type of siphonostele are leaf-trace bundles, i.e. they can be traced upwards from any given point till they are found to pass out of the cylinder, travel through the cortex of the stem and enter a leaf. The remaining bundles (compensation bundles) which go to make up the cylinder are such as have branched off from the leaf-traces, and will, after joining with others similarly given off, themselves form the traces of leaves situated at a higher level on the stem. Purely cauline vascular strands (i.e. confined to the stem) such as are found in the dictyosteles of ferns are rare in the flowering plants. The leaf trace of any given leaf rarely consists of a single bundle only (unifascicular); the number of bundles of any given trace is always odd; they may either be situated all together before they leave the stele or they may be distributed at intervals round the stele. The median bundles of the trace are typically the largest, and at any given level of the stem the bundles destined for the next leaf above are as a whole larger than the others which are destined to supply higher leaves. Leaf-gaps are formed in essentially the same way as in the ferns, but when in the case of a plurifascicular trace the bundles are distributed at intervals round the cylinder it is obvious that several gaps must be formed as the different bundles leave the stele. The gaps, are, however, often filled as they are formed by the development of external conjunctive tissue immediately above the points at which the bundles begin to bend out of the stele, so that sharply defined open gaps such as occur in fern-steles are but rarely met with in flowering plants. The constitution of the stele of a flowering plant entirely from endarch collateral bundles, which are either themselves leaf-traces or will form leaf-traces after junction with other similar bundles, is the great characteristic of the stem-stele of flowering plants. These collateral bundles are obviously highly individualized. The external conjunctive tissue is often arranged in relation to each bundle separately, the pericyclic fibres for instance, already referred to, being often confined to the bands of pericyclic tissue abutting on the phloem of each bundle, while the cortex and pith frequently form rays in the intervals between the adjacent bundles.

In some cases this individualization is carried further, the cortex and pith becoming continuous between the bundles which appear

as isolated strands embedded in a general ground-tissue. Each bundle has its own investment of tissue corresponding with external conjunctive, and now called peridesm. The bundles sometimes keep their arrangement in a ring corresponding with the stele, though the continuous cylinder no longer exists (species of Ranunculus). This condition is known as astely. In some astelic stems (Nymphaeaceae) the number of bundles is greatly increased and they are scattered throughout the ground tissue. A “polystelic” condition arises in some members of this order by the association of collateral bundles round common centres. A similar phenomenon is seen in two widely separated genera of flowering plants: Primula § Auricula and Gunnera (Halorageae).

The monocotyledons, one of the primary divisions of angiosperms,

typically possess large leaves with broad sheathing bases containing a very great number of bundles. This results in the number of bundles present at any given level of the stem being enormously increased. These bundles are scattered in a definite though not superficially obvious order through the conjunctive tissue of the stele, which occupies nearly the whole diameter of the stem, the cortex being reduced to a very narrow layer, or disappearing altogether (fig. 3). The mass of conjunctive tissue is developed as a large-celled “ground-tissue,” and round each bundle there is a “peridesm” which is often fibrous (fig. 16). It is possible to suppose that this condition is derived from the astelic condition already referred to, but the evidence on the whole leads to the conclusion that it has arisen by an increase in the number of the bundles within the stele, the individuality of the bundle asserting itself after its escape from the original bundle-ring of the primitive cylinder.

In the stems of many water-plants various stages of reduction of the vascular system, especially of the xylem, are met with, and

very often this reduction leads to the formation of a compact stele in which the individuality of the separate bundles may be suppressed, so that a closed cylinder of xylem surrounds a pith. The phloem is generally unreduced, and there is normally a well marked endodermis (fig. 17). In other cases the reduction goes much further, till the endodermis eventually comes to surround nothing but an intercellular channel formed in place of the stelar tissue.

In the blade of a typical leaf of a vascular plant—essentially a thin plate of assimilating tissue—the vascular system takes the

form of a number of separate, usually branching and anastomosing strands. These, with their associated stereom, form a kind of framework which is of great importance in supporting the mesophyll; but also, and chiefly, they provide a number of channels, penetrating every part of the leaf, along which water and dissolved salts are conveyed to, and elaborated food-substances from, the mesophyll cells. The bundle-system is of course continuous with that of the petiole and stem. The leaf-bundles are always collateral (the phloem being turned downwards and the xylem upwards), even in Ferns, where the petiolar strands are concentric, and they have the ordinary mesodesm and peridesm of the collateral bundle. The latter is often sclerized, especially opposite the phloem, and to a less extent opposite the xylem, as in the stem. As a bundle is traced towards its blind termination in the mesophyll the peridesmic stereom first disappears, the sieve-tubes of the phloem are replaced by narrow elongated parenchyma cells, which soon die out, and the bundle ends with a strand of tracheids covered by the phloeotermic sheath. 17.—Transverse section of the stele of the stem of a water-plant (Naias); l. intercellular channel representing xylem; ph. phloem; e. endodermis. The structure of the stele of the primary root as it is found in most Pteridophytes and many Phanerogams has been already described. The radial structure is characteristic of all root-steles, which have in essential points a remarkably uniform structure throughout the vascular plants, a fact no doubt largely dependent on the very uniform conditions under which they live. While the stele of the primary root in both Gymnosperms and Angiosperms is usually diarch or tetrarch, the large primary root-steles of many adventitious roots are frequently polyarch, sometimes with a very large number of protoxylems. Such a stele seldom has the centre filled up with xylem, this being replaced by a large-celled pith, so that a siphonostelic structure is acquired (fig. 15). Sometimes, however, the centre of a bulky root stele has strands of metaxylem (to which may be added strands of metaphloem) scattered through it, the interstices being filled with conjunctive. The conjunctive of a root-stele possessing a pith is often sclerized between the pith and the pericycle. Sometimes all the parenchyma within the stele undergoes this change. In the roots of some palms and orchids a “polystelic” structure obtains.

In certain families of Angiosperms a peculiar tissue, called laticiferous tissue is met with. This takes the form of long usually