. Botany of the living plant. Botany. 330 BOTANY OF THE LIVING PLANT Ferns, and generally in young sporelings there is a simple stele of a type called a " protostele," having a soHd xylem-core, and phloem surrounding it. This is beheved to have been the primitive structure for them all. It is well shown in Botryopteris (Fig. 268). Occasionally this state may be retained through life (Hymenophyllum, Lygodium). But in the vast majority of Ferns the stele expands as the plant grows stronger, and the leaves larger; and in various ways it becomes segre- gated into a number of vascular str
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. Botany of the living plant. Botany. 330 BOTANY OF THE LIVING PLANT Ferns, and generally in young sporelings there is a simple stele of a type called a " protostele, " having a soHd xylem-core, and phloem surrounding it. This is beheved to have been the primitive structure for them all. It is well shown in Botryopteris (Fig. 268). Occasionally this state may be retained through life (Hymenophyllum, Lygodium). But in the vast majority of Ferns the stele expands as the plant grows stronger, and the leaves larger; and in various ways it becomes segre- gated into a number of vascular strands {meristeles), arranged in a cyhndrical network (Fig. 267 E, F). Each mesh in Nephrodium. Fig. 269. Transverse section of rhizome of Bracken, , omitting the external band of scleren- chyma, and showing the outer and inner series of meristeles, and the irregular bands of dark sclerenchyma between them. ( x lo.) corresponds to the insertion of a leaf-base, and is called a foliar mesh, or gap. The vascular strands that run out into the leaf, called col- lectively the leaf-trace, arise from the margin of it, and the cortex and pith are in direct communication through the foliar gaps. Nephrodium gives a comparatively simple example of this subdivision of the stele, which is characteristic of most Ferns, and is to be regarded as derived from the primitively undivided protostele. The leaf-trace in different kinds of Ferns may be a single strand, or many. In either case it sends out branches into the pinnae and pinnules, and they end either in an open venation, as m Nephrodiwn or Pleridium (Fig. 266); or in a network of veins, as in the Adder's tongue. Thus the H. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work.. Bower, F. O. (Frederick Orpen), 1855-1948. London, Macmillan