(C, E, after Boodle; D, after Bommer.)
To Prof. Jeffrey[731] we owe the term protostele which he applied to a type of stele consisting of a central core of xylem surrounded by phloem, pericycle, and endodermis. While admitting that steles of this type may sometimes be the result of the modification of less simple forms, we may confidently regard the protostele as representing the most primitive form of vascular system. The genus Lygodium affords an example of a protostelic fern; a solid column of xylem tracheae and parenchyma is completely encircled by a cylinder of phloem succeeded by a multi-layered pericycle and an endodermis of a single layer of cells. In this genus the stele is characterised by marginal groups of protoxylem; it is exarch. An almost identical type is represented by species of Gleichenia, but here the stele is mesarch, the protoxylem being slightly internal ([fig. 237], C). Trichomanes scandens ([fig. 238]) has an exarch protostele like that of Lygodium; but, as Boodle[732] has suggested, the protostelic form in this case is probably the result of modification of a collateral form of stele such as occurs in Trichomanes reniforme ([fig. 237], E). A second type of stele has been described in species of Lindsaya[733] in which the xylem includes a small group of phloem near the dorsal surface. This Lindsaya type is often passed through in the development of “seedling” ferns and may be regarded as a stage in a series leading to another well-marked type, the solenostele. The solenostele[734], a hollow cylinder of xylem lined within and without by phloem, pericycle, and endodermis, occurs in several genera belonging to different families, e.g. Dipteris, species of Pteris, species of Lindsaya, Polypodium, Jamesonia, Loxsoma, Gleichenia and other genera. In a smaller number of ferns the stele consists of what may be called a medullated protostele similar to the common form of stele in Lepidodendron: this type is found in species of Schizaea and in Platyzoma ([fig. 239]). It is important to notice that in the solenostele and as a rule in the medullated protostele when a leaf-trace passes out from the rhizome stele the vascular cylinder is interrupted by the formation of a foliar gap (Platyzoma[735], [fig. 239], is an exception). This fact has been emphasized by Jeffrey[736] who draws a distinction between the Lycopodiaceous type of stele, which is not broken by the exit of leaf-traces, and the fern stele in which foliar gaps are produced: the former he speaks of as the cladosiphonic type (Lycopsida) and the latter as the phyllosiphonic (Pteropsida).
Fig. 238. Stele of Trichomanes scandens: px, protoxylem; s, endodermis.
From Tansley, after Boodle.
Fig. 239. Platyzoma microphylla. l.t., leaf-trace; i.e., internal endodermis. (After Tansley; modified from Boodle.)
The transition to a hollow cylinder of xylem from a protostele may be described as the result of the replacement of some of the axial conducting tracheae by parenchyma or other non-vascular tissue consequent on an increase in diameter of the whole stele and the concentration of the true conducting elements towards the periphery[737].
The occurrence of the internal cylinder of phloem, pericycle, and endodermis in a solenostele is rendered intelligible by a study of fern seedlings and by a comparative examination of transitional types connecting protosteles and solenosteles through medullated protosteles and steles of the Lindsaya type. A further stage in stelar evolution is illustrated by what is termed the dictyostele, the arrangement of vascular tissue characteristic of Nephrodium Filix-mas, Cyathea ([fig. 240]), Polypodium vulgare and many other common ferns.
Fig. 240. Cyathea Imrayana. (From Tansley after de Bary.) (Sclerenchyma represented by black bands.)