In other cases the leaf-gaps are very broad and long, the meristeles
separating them being reduced to comparatively slender strands,
while there is present in each gap a network of fine vascular threads,
some of which run out to the leaf, while others form cross-connexions
between these “leaf-trace” strands and also with the main cauline
meristeles. Finally the cauline meristeles themselves may be
resolved into a number of fine threads. Such a structure may be
spoken of as a dissected dictyostele.
In some solenostelic ferns, and in in any dictyostelic ones additional vascular strands are present which do not form part of the primary Polycycly. vascular tube. They usually run freely in the pith and join the primary tube in the neighbourhood of the leaf-gaps. Sometimes a complete internal vascular cylinder, having the same structure as the primary one, and concentric with it, occurs in the pith, and others may appear, internal to the first (Matonia, Saccoloma). Junctions of the first internal cylinder are made with the primary (external) cylinder at the leaf-gaps, and of the second internal cylinder with the first in the same neighbourhood (fig. 8). In dictyostelic ferns similar internal (dictyostelic) cylinders are found in some forms, and occasionally a large series of such concentric cylinders is developed (Marattiaceae) (fig. 9). In such cases the vascular system is said to be polycyclic in contrast with the ordinary monocyclic condition. These internal strands or cylinders are to be regarded as peculiar types of elaboration of the stele, and probably act as reservoirs for water-storage which can be drawn upon when the water supply from the root is deficient.
The vascular supply of the leaf (leaf-trace) consists of a single strand only in the haplostelic and some of the more primitive Leaf-trace and Petlolar Strands. siphonostelic forms. In the “microphyllous” groups of Pteridophytes (Lycopodiales and Equisetalis) in which the leaves are small relatively to the stem, the single bundle destined for each leaf is a small strand whose departure causes no disturbance in the cauline stele. In the “megaphyllous” forms, on the other hand, (Ferns) whose leaves are large relatively to the stem, the departure of the correspondingly large trace causes a gap (leaf-gap) in the vascular cylinder, as already described. In the haplostelic ferns the leaf-trace appears as a single strand with a tendency to assume the shape of a horseshoe on cross-section, and this type is also found in the more primitive solenostelic types. In the more highly developed forms, as already indicated, the leaf-trace is split up into a number of strands which leave the base and sides of the leaf-gap independently. In the petiole these strands may increase in number by branching, and though usually reducible to the outline of the primitive “horseshoe,” more or less elaborated, they may in some of the complex polycylic dictyostelic types (Marattiaceae) be arranged in several concentric circles, thus imitating the arrangement of strands formed in the stem. The evolution of the vascular structure of the petiole in the higher ferns is strikingly parallel with that of the stem, except in some few special cases.
There is good reason to believe that the haplostele is primitive in the evolution of the vascular system. It is found in most of Parallel of Ontogeny with Phylogeny. those Pteridophytes which we have other reasons for considering as primitive types, and essentially the same type is found, as we have seen, in the independently developed primitive conducting system of the moss-stem. This type of stem is therefore often spoken of as protostelic. In the Ferns there is clear evidence that the amphiphloic haplostele or protostele succeeded the simple (ectophloic) protostele in evolution, and that this in its turn gave rise to the solenostele, which was again succeeded by the dictyostele. Polycycly was derived independently from monocycly in solenostelic and in dictyostelic forms. In the formation of the stem of any fern characterized in the adult condition by one of the more advanced types of vascular structure all stages of increase in complexity from the haplostele of the first-formed stem to the particular condition characteristic of the adult stem are gradually passed through by a series of changes exactly parallel with those which we are led to suppose, from the evidence obtained by a comparison of the adult forms, must have taken place in the evolution of the race. There is no more striking case in the plant kingdom of the parallel between ontogeny (development of the individual) and phylogeny (development of the race) so well known in many groups of animals.
The stele of most Lycopods is a more or less modified protostele, but in the genus Lycopodium a peculiar arrangement of the xylem Aberrant Stelar Systems of Pteridophytes. and phloem is found, in which the latter, instead of being stelar confined to a peripheral mantle of tissue, forms bands running across the stele and alternating with similar bands of xylem (fig. 12). In Selaginella the stelar system shows profounder modifications. In some forms we find a simple protostele, exarch-polyarch in one species (S. spinosa), exarch-diarch in several (fig. 10). In other species, however, a peculiar type of polystely is met with, in which the original diarch stele gives rise to so-called dorsal and ventral stelar “cords” which at first lie on the surface of the primary stele, but eventually, at a higher level separate from it and form distinct “secondary” steles resembling the primary one. Similar cords may be formed on, and may separate from, these secondary steles, thus giving rise to a series of steles arranged in a single file (fig. 11). In the creeping stem of one species (S. Lyallii) a polycyclic solenostele is found exactly parallel with that of the rhizome of ferns. The gaps in the outer tubular stele, however, are formed by the departure of aerial branch-traces, instead of leaf-traces as in the ferns. The first formed portion of the stem in all species of Selaginella which have been investigated possesses an exarch haplostele. The stele of Equisetum is of a very peculiar type whose relations are not completely clear. It consists of a ring of endarch collateral bundles, surrounding a hollow pith. The protoxylem of each is a leaf-trace, while the metaxylem consisting of a right and a left portion forms a quite distinct cauline system. All the metaxylems join at the nodes into a complete ring of xylem. The whole stele may be surrounded by a common external endodermis; sometimes there is an internal endodermis in addition, separating the bundles from the pith; while in other cases each bundle possesses a separate endodermis surrounding it. At the nodes the relation of the endodermis to the bundles undergoes rather complex but definite changes. It is probable that this type of stele is a modification of a primitive protostele, in which the main mass of stelar xylem has become much reduced and incidentally separated from the leaf-traces.
During recent years a number of fossil (Carboniferous and Permian) plants have been very thoroughly investigated in the light of modern Stelar System of Cycadofilices. anatomical knowledge, and as a result it has become clear that in those times a large series of plants existed intermediate in structure between the modern ferns and the modern Gymnosperms (especially Cycads), and to these the general name “Cycadofilices” has been applied. We now know that many at least of the Cycadofilices bore seeds, of a type much more complex than that of most modern seed plants, and in some cases approximating to the seeds of existing Cycads. Among the Cycadofilices a series of stages is found leading from the primitive fern-protostele to the type of siphonostele characteristic of the Cycads which agrees in essentials in all the Spermophytes. The main events in this transition appear to have been (1) disappearance of the central xylem of the protostele and replacement by pith, leading to the survival of a number of (mesarch) collateral bundles (see below) at the periphery of the stele; (2) passage from mesarchy to endarchy of these bundles correlated with a great increase in secondary thickening of the stele. The leaves of the more primitive members of this series were entirely fern-like and possessed a fern-like vascular strand; while in the later members, including the modern Cycads, the leaf bundles, remaining unaffected by secondary thickening, are mesarch, while those of the stem-stele have become endarch. Besides the types forming this series, there are a number of others (Medulloseae and allied forms) which show numerous, often very complex, types of stelar structure, in some cases polystelic, whose origin and relationship with the simpler and better known types is frequently obscure. Among the existing Cycads, though the type of vascular system conforms on the whole with that of the other existing seed-plants, peculiar structures are often found (e.g. indications of polystely, frequent occurrence of extra-stelar concentric bundles, “anomalous” secondary thickening) which recall these complex types of stelar structure in the fossil Cycadofilices.
The typical structure of the vascular cylinder of the adult primary stem in the Gymnosperms and Dicotyledons is, like Structure of the Stele in Seed-plants. that of the higher ferns, a hollow cylinder of vascular tissue enclosing a central parenchymatous pith. But, unlike the ferns, there is in the seed-plants no internal phloem (except as a special development in certain families) and no internal endodermis. The xylem and phloem also, rarely form perfectly continuous layers as they do in a solenostelic fern. The vascular tissue is typically separable into distinct collateral bundles (figs. 13, 23), the xylem of which is usually wedge-shaped in cross-section with the protoxylem elements at the inner extremity, while the phloem forms a band on the outer side of the xylem, and separated from it by a band of conjunctive tissue (mesodesm). These collateral bundles are separated from one another by bands of conjunctive tissues called primary medullary rays, which may be quite narrow or of considerable width. When the pith is large celled, the xylems of the bundles are separated from it by a distinct layer of conjunctive tissue called the endocycle, and a similar layer, the pericycle, separates the phloem from the cortex. The inner layer of the cortex (phloeoterma) may form a well-marked endodermis, or differ in other ways from the rest of the cortex. The pericycle, medullary rays, endocycle and mesoderm all form parts of one tissue system, the external conjunctive, and are only topographically separable. The external conjunctive is usually a living comparatively small-celled tissue, whose cells are considerably elongated in the direction of the stem-axis and frequently contain abundant starch. Certain regions of it, particularly the whole or part of the pericycle, but sometimes also the endocycle, are typically converted into thick-walled hard (sclerenchymatous) tissue usually of the prosenchymatous (fibrous) type, which is important in strengthening the stem, particularly in enabling it to resist bending strains. The relatively peripheral position in the stem of the pericycle is important in this connexion. Various secondary meristems (see p. 741) also arise in the external conjunctive.
