Foliar- or leaf-gaps are formed in essentially the same way as in the ferns, but as the distribution of the bundles of a single trace may be wide and long, there may be a number of gaps in the cylinder associated with the insertion of the bundles of a single leaf-trace. 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 constitu tion of the stele of a flowering plant entirely from bundles as above described, which are either themselves leaf-traces or will form leaf-traces after junction with similar bundles, is the great characteristic of the stem-stele of flowering plants. These col lateral bundles are obviously highly individualized. The external conjunctive tissue is often arranged in relation to each bundle separately, so that, for example, there may be pericyclic fibres immediately outside the phloem-masses alone. In some cases the 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. The bundles sometimes keep their arrangement in a ring corre sponding with the stele, though there may be no continuous cylinder. This condition is known as astely.
The Monocotyledons, one of the primary divisions of Angio sperms, typically possess large leaves with broad sheathing bases, containing numerous bundles. This results in the number of bundles present at any given level of the stem being high. 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 restricted to a very narrow layer, or scarcely recognizable as a definite zone. The mass of conjunctive tissue is developed as a large-celled ground-tissue, and around each bundle there is a fibrous invest ment. In the stems of many water plants various stages of reduc tion 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 the pith. The phloem is seldom so reduced, and there is nor mally a well-marked endodermis. In other cases the reduction goes much further till the endodermis comes to surround nothing but an intercellular channel, formed in place of the stelar tissue.
To the vascular tissue of the typical leaf and root of the Phanerogams reference is made below. But there are peculiarities of organization of the hydroids and leptoids of the bundles of flowering plants which here call for special mention. Generally the end walls of superimposed developing hydroids break down, so that the final product of differentiation is a continuous tube with lignified pitted walls. Such tubes are called vessels, and are
characteristic of the vast majority of Phanerogams, though tracheids still dominate the Gymnosperms. There is every degree of transition between spirally thickened and reticulately thickened vessels. The sieve-tubes of Phanerogams in their most advanced forms which dominate the Angiosperms have the end walls or portions of the end walls specially perforated. Such walls are accordingly called sieve-plates. It is assumed that by such end sieve-plates the longitudinal movement of slimy foods is acceler ated. Associated with the sieve-tubes are, in addition, cells of the same length as the sieve-tubes, generally narrow, thin walled and enucleate, and which are thought to co-operate in some intimate way with the sieve-tubes. They are known as companion cells, and a sieve-tube and its companion cell arise during tissue differentiation from the same mother-cell. On the whole the lep toids and hydroids of the Phanerogams are considerably more specialized than those of the Pteridophytes.
Stelar Tissue of Leaf and Root.—In the leaf of the average Phanerogam the vascular tissue takes the form of a number of branching or approximately parallel, and usually anastomosing, strands, above and below which the mesophyll is raised so as to produce the so-called veins. The vein-system is typically very elaborate, and the bundle-system is concentrated in the leaf-stalk (petiole) as the tributaries of a river are massed in the main stream. The leaf-bundles are always collateral (the phloem being turned downwards and the xylem upwards), and the whole bundle may be protected above and below by fibrous cells. As the bundles are followed towards their blind endings in the mesophyll the fibres first disappear, the sieve-tubes are replaced by narrow elongated parenchyma which soon dies out, and the bundles usually end in short or long spiral or reticulate tracheids covered by the phloeothermic sheath.
• The stele of the primary root of a Gymnosperm or Angiosperm is of radial structure. There are usually two or four xylem strands radially alternating with phloem strands (the stele being thus diarch or tetrarch), and the first formed xylem groups (pro toxylem) are peripheral. A polyarch state is, however, common in adventitious roots (which arise on other stems or roots in 'chance" positions), and the protoxylem groups are then numer ous. The centre of the stele is seldom solid xylem, but is generally pithed, and sclerotic cells are by no means uncommon in the conjunctive tissue. The roots of some palms and orchids show a peculiar "polystelic" structure. Thus it will be seen that while the vascular systems of the stems of Phanerogams are highly individualized, those of the leaves and primary roots are of simpler organization, but are of varied structure.