THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



627 



but in the stem thev are normally dormant. Each root has a wide cortex, 

 with an outer exodermis (Fig. 634). Inside there is a well-defined endo- 

 dermis, and a small stele consisting of either three or four protoxylem groups, 

 with phloem alternating with them, and a single, central metaxylem vessel. 

 The chief interest in the root of Equisetiim is the fact that the endodermis 

 consists of two layers, a large-celled outer layer, and a smaller-celled inner 

 layer which replaces the pericycle and from which lateral roots originate. 

 There is a three-sided apical cell, which produces a root cap as well as the 

 root tissues. 



There is no main root in the mature plant oi Equisetiim, for this structure, 

 which appears in the embryo, only lasts for a short time and is soon superseded 

 by adventitious roots. 



PELTATE 

 SCALE 



Development of the Strobilus 



The cone of Eqiiisetum is a terminal structure, arising either on the main 

 fertile shoot or, rarely, on a branch. It possesses a fairly stout axis, giving 

 rise to crowded, alternating whorls of T-shaped peltate scales, termed 

 sporangiophores, on which the sporangia are developed (Fig. 635). Each 

 sporangiophore has a short cylindrical stalk and expands at its distal end 

 into a flat plate, on the under-surface of which from five to ten sporangia are 

 attached (Fig. 636). The peltate heads of the sporangiophores arise in such 

 close contact that, as a result of mutual pres- 

 sure, they become hexagonal in outline. The 

 sporangia develop towards the axis and fill up all 

 the space that is left between the peltate scales. 

 At the base of the cone there is a ring-like out- 

 growth, the annulus, which is sometimes toothed 

 and which appears to be a reduced leaf sheath. 



The cone axis is hollow, and although the 

 sporangiophores are in whorls the vascular 

 anatomy is irregular and shows no nodes or 

 internodes. Gaps in the stele are present, but 

 they are not directly related to the sporangi- 

 ophore traces. The metaxylem is much more 

 developed than in the stem, forming a discon- 

 tinuous ring, and is in close contact with the protoxylems. The latter 

 form the traces to the sporangiophores, passing up the stalk of the scale 

 and dividing radially at the top, one branch going to each sporangium. 



SPORANGIUM 

 PEDICEL 



Fig. 636. — Equisetum arvense. 

 A single sporangiophore 

 showing the peltate head 

 with ring of attached 

 sporangia. 



Development of the Sporangium 



Each sporangium arises from a single superficial cell on the under side of 

 the expanded peltate scale. This cell divides periclinally, and the inner 

 segment gives rise to the archesporium. The outer segment forms part 

 of the wall, the rest being formed from the sporangiophore. This wall is 

 at first several cells thick, but the inner layers, termed the tapetum, 



