The innermost layer of the cortex, abutting on the central
cylinder of the stem or on the bundles of the leaves, is called the
phloeoterma, and is often differentiated. In the
leaf-blade it takes the form of special parenchymatous
sheaths to the bundles. The cells of these sheaths are
Phloeoterma.
often distinguished from the rest of the mesophyll by containing
little or no chlorophyll. Occasionally, however, they are particularly
rich in chloroplasts. These bundle sheaths are important
in the conduction of carbohydrates away from the assimilating
cells to other parts of the plant. Rarely in the leaf, frequently
in the stem (particularly in Pteridophytes), and universally
in the root, the phloeoterma is developed as an endodermis (see
below). In other cases it does not differ histologically from the
parenchyma of the rest of the cortex, though it is often distinguished
by containing particularly abundant starch, in which
case it is known as a starch sheath.
One of the most striking characters common to the two highest groups of plants, the Pteridophytes and Phanerogams, is the possession of a double (hydrom-leptom) conducting system, such as we saw among the highest mosses, but with sharply characterized and peculiar features, Vascular System. probably indicating common descent throughout both these groups. It is confined to the sporophyte, which forms the leafy plant in these groups, and is known as the vascular system. Associated with it are other tissues, consisting of parenchyma, mainly starchy, and in the Phanerogams particularly, of special stereom. The whole tissue system is known as the stelar system (from the way in which in primitive forms it runs through the whole axis of the plant in the form of a column). The stelar system of Vascular Plants has no direct phylogenetic connexion with that of the mosses. The origin of the Pteridophyta (q.v.) is very obscure, but it may be regarded as certain that it is not to be sought among the mosses, which are an extremely specialized and peculiarly differentiated group. Furthermore, both the hydrom and leptom of Pteridophytes have marked peculiarities to which no parallel is to be found among the Bryophytes. Hence we must conclude that the conducting system of the Pteridophytes has had an entirely separate evolution. All the surviving forms, however, have a completely established double system with the specific characters alluded to, and since there is every reason to believe that the conditions of evolution of the primitive Pteridophyte must have been essentially similar to those of the Bryophytes, the various stages in the evolution of the conducting system of the latter (p. 732) are very useful to compare with the arrangements met with in the former.
The hydroid of a Pteridophyte or of a Phanerogam is characteristically a dead, usually elongated, cell containing air and water, and either thin-walled with lignified (woody) spiral (fig. 1, P) or annular thickenings, or with thick lignified walls, incompletely perforate by pits (fig. 1, Q.) (usually bordered Tissue Elements. pits) of various shapes, e.g. the pits may be separated by a network of thickenings when the tracheid is reticulate or the may be transversely elongated and separated by bars of thickening like the rungs of a ladder (scalariform thickening). When, in place of a number of such cells called tracheids, we have a continuous tube with the same kind of wall thickening, but composed of a number of cells whose cross walls have disappeared, the resulting structure is called a vessel. Vessels are common in the Angiospermous group of Flowering Plants. The scalariform hydroids of Ferns (fig. 1, N) have been quite recently shown to possess a peculiar structure. The whole of the middle lamella or originally formed cell-wall separating one from another disappears before the adult state is reached, so that the walls of the hydroids consist of a framework of lignified bars with open communication between the cell cavities. The tracheids or vessels, indifferently called tracheal elements, together with the immediately associated cells (usually amylom in Pteridophytes) constitute the xylem of the plant. This is a morphological term given to the particular type of hydrom found in both Pteridophytes and Phanerogams, together with the parenchyma or stereom, or both, included within the boundaries of the hydrom tissue strand. The leptoid of a Pteridophyte (fig. 1, O) is also an elongated cell, with a thin lining of protoplasm, but destitute of a nucleus, and always in communication with the next cell of the leptom strand by perforations (in Pteridophytes often not easily demonstrable), through which originally pass strings of protoplasm which are bored out by a ferment and converted into relatively coarse “slime strings,” along which pass, we must suppose, the organic substances which it is the special function of the leptoids to conduct from one part of the plant to another. The peculiar substance called callose, chemically allied to cellulose, is frequently formed over the surface of the perforated end-walls. The structure formed by a number of such cells placed end to end is called a sieve-tube (obviously comparable with a xylem-vessel), and the end-wall or area of end-wall occupied by a group of perforations, a sieve-plate. When the sieve-tube has ceased to Function and the protoplasm, slime strings, and callose have disappeared, the perforations through which the slime strings passed are left as relatively large holes, easily visible in some cases with low powers of the microscope, piercing the sieve-plate. The sieve-tubes, with their accompanying parenchyma or stereom, constitute the tissue called phloem. This is the term for a morphologically defined tissue system, i.e the leptom found in Pteridophytes and Phanerogams with its associated cells, and is entirely parallel with the xylem. The sieve-tubes differ, however, from the tracheids in being immediately associated, apparently constantly, not with starchy parenchyma, but with parenchymatous cells, containing particularly abundant proteid contents, which seem to have a function intimately connected with the conducting function of the sieve-tubes, and which we may call proteid-cells. In the Angiosperms there are always sister-cells of sieve-tube segments and are called companion-cells (fig. 1, R.).
The xylem and phloem are nearly always found in close association
in strands of various shapes in all the three main organs
of the sporophyte—root, stem and leaf—and form a connected
tissue-system running through the whole body. In the primary
axis of the plant among Pteridophytes and many Phanerogams,
at any rate in its first formed part, the xylem and phloem are
associated in the form of a cylinder (stele), with xylem occupying
the centre, and the phloem (in the upward-growing part or primary
Arrangement in Strands: the
Central Cylinder.
stem) forming a mantle at the periphery (fig. 4). In
the downward growing part of the axis (primary root),
however, the peripheral mantle of phloem is interrupted,
the xylem coming to the surface of the cylinder
along (usually) two or (sometimes) more vertical lines.
Such an arrangement of vascular tissue is called radial,
and is characteristic of all roots (figs. 3 and 10). The cylinder is surrounded
by a mantle of one or more layers of parenchymatous cells,
the pericycle, and the xylem is generally separated from the phloem in
the stem by a similar layer, the mesocycle (corresponding with the
amylom sheath in mosses). The pericycle and mesocycle together
form the conjunctive tissue of the stele in these simplest types.
When the diameter of the stele is greater, parenchymatous conjunctive
tissue often occupies its centre and is frequently called the pith.
In the root the mesocycle, like the phloem, is interrupted, and
runs into the pericycle where the xylem touches the latter (fig. 3).
The whole cylinder is enclosed by the peculiarly differentiated
innermost cell-layer of the cortex, known as the endodermis. This
layer has its cells closely united and sealed to one another, so to
speak, by the conversion of the radial and transverse walls (which
separate each cell from the other cells of the layer), or of a band
running in the centre of these, into corky substance (fig. 1, V.), so that
the endodermal cells cannot be split apart to admit of the formation
of inter cellular spaces, and an air-tight sheath is formed round the
cylinder. Such a vascular cylinder is called a haplostele, and the
axis containing it is said to be haplostelic. In the stele of the root
the strands of tracheids along the lines where the xylem touches
the pericycle are spiral or annular, and are the xylem elements
first formed when the cylinder is developing. Each strand of
spiral or annular first-formed tracheids is called a protoxylem
strand, as distinct from the metaxylem or rest of the xylem, which
consists of thick-walled tracheids, the pits of which are often scalariform.
The thin-walled spiral or annular tracheae of the protoxylem
allow of longitudinal stretching brought about by the active growth
in length of the neighbouring living parenchymatous cells of a growing
organ. During the process the thin walls are stretched and the
turns of the spiral become pulled apart without rupturing the wall
of the tracheid or vessel. If the pitted type of tracheal element
were similarly stretched its continuously thickened walls would
resist the stretching and eventually break. Hence such tracheae
are only laid down in organs whose growth in length has ceased.
The stele is called monarch, diarch, . . . polyarch according as it
contains one, two, . . . or many protoxylems. When the protoxylem
strands are situated at the periphery of the stele, abutting
on the pericycle, as in all roots, and many of the more primitive
Pteridophyte stems, the stele is said to be exarch. When there
is a single protoxylem strand in the centre of the stele, or when, as
is more commonly the case, there are several protoxylem strands
situated at the internal limit of the xylem, the centre of the stem
being occupied by parenchyma, the stele is endarch. This is the
case in the stems of most Phanerogams and of some Pteridophytes.
When the protoxylems have an intermediate position the stele is
mesarch (many Pteridophytes and some of the more primitive
Phanerogams). In many cases external protophloem, usually consisting
of narrow sieve-tubes often wit swollen walls, can be
distinguished from metaphloem.
As the primitive stele of a Pteridophyte is traced upwards from the primary root into the stem, the phloem becomes continuous round the xylem. At the same time the stele becomes more bulky, all its elements increasing in number (fig. 4). Soon a bundle goes off to Evolution of the Stele in Pteridophytes. the first leaf. This consists of a few xylem elements, a segment of phloem, pericycle, and usually an arc of endodermis, which closes round the bundle as it detaches itself from the stele. As the stele is traced farther upwards it becomes bulkier, as do the successive leaf-bundles which leave it. In many Pteridophytes the solid haplostele is maintained throughout the axis. In others a central parenchyma or primitive pith—a new region of the primitive stelar conjunctive—appears in the centre of the xylem. In most ferns internal phloem appears instead of a parenchymatous pith (fig. 5). Sometimes this condition,
