XIV] SELAGINELLA 53 



surrounded by parenchymatous elements, is enclosed by phloem 

 with external protophloem elements. The characteristic features 

 of the stele are shown in the diagrammatic drawing of a section 

 of another species — S. Willdenowii — represented in fig. 131, A. 

 A pericycle composed of one or two layers of chlorophyll- 

 containing cells encircles the whole stele which is suspended 

 in a lacuna by trabeculae (fig. 131, A, B, t) connecting the 

 pericycle with the inner edge of the broad cortex. The 

 trabeculae consist in part of endodermal cells characterised by 

 cuticular bands. The cortex is usually differentiated into three 

 fairly distinct regions. Mechanical tissue of thick-walled fibres 

 constitutes the outer region (a) ; the middle cortex consists of 

 thinner-walled parenchyma, the elements of which become 

 smaller and rather more compactly arranged in the inner zone. 

 The middle cortex is frequently characterised by the presence 

 of spaces and by the hyphal or trabecular structure of the 

 tissue, a feature which, as Bower^ pointed out, is common to 

 many recent and fossil members of the Lycopodiales. In some 

 cases, e.g. S. erythropus, from tropical America, the cortex 

 of the creeping stem consists entirely of thick-walled cells. 

 Selaginella grandis (fig. 130) has "a short decumbent stem 

 rooted at close intervals 2," from which thick erect aerial shoots 

 rise to a height of one foot or more. In the apical region 

 these erect axes give oflf repeatedly forked foliage shoots 

 on which the spiral phyllotaxis of the homophyllous axis 

 is gradually replaced by four rows of two kinds of leaves 

 (fig. 130, 2). The anatomy of this species agrees with that 

 of S. Martensii. The trailing or semi-erect and homophyllous 

 shoots of Selaginella spinosa^ present a distinct type of vascular 

 anatomy. The upper part of the ascending stem has an axial 

 strand of xylem with seven peripheral groups of spiral proto- 

 xylem tracheae (fig. 131, B); in the trailing portion of the 

 shoot the protoxylem elements occur as one central group in 

 the solid rod of metaxylem through which the leaf-traces pass 

 on their way to the axial protoxylem. This type is important 



1 Bower (93). " Harvey-Gibson (94) p. 152. 



3 ibid. (94) p. 194; Scott (96) p. 9. 



