32] 



CHAPTER II. THE TISSUES. 



117 



in a circle or in several circles ; the strands thus forming an in- 

 complete hollow cylinder enclosing a central mass of conjunctive 

 tissue, the medulla or pith, whilst the conjunctive tissue between 

 the strands constitutes the medullary rays. 



In all cases the stele (whether one or more) Is at anjearly jtage 

 marked off from the ground tissue, the layer of the grQund : tis8ue 

 which abuts on the stele being specially differentiated as a sheath, 

 the endodermis (p. 115), which forms a continuous covering to the 

 stele or any isolated portion of it. 



All primary stems are typically monostelic to begin with, but 

 as they increase in bulk this type of structure is departed from 

 in various ways ; typical monostely is, however, the rule in the 

 stem of Phanerogams, and is 

 frequent in that of Pteridophyta 

 (in Hymenophyllum, Osmunda, 

 Lycopodium, Isoetes, some 

 species of Selaginella). Some 

 stejns are, however, polystclic. 

 In these the original single stele 

 passes over, as the stem, grows 

 and enlarges, into a varying 

 number of steles which can be 

 traced to the growing-point as 

 distinct plerome-strands. Poly- 

 stely is rare in stems which 

 have a growing-point without 

 an~apical cell, whilst it is com- 

 mon in steins where the grow- 

 ing-point has an apical cell, or 

 a group of common initial cells : 

 hence it is rare in Phanerogams 

 (occurs in Auricula and Gun- 

 nera), and is common in Pteridophyta (especially Leptosporangiate 

 Ferns, and some Selaginellas : see p. 102). 



A common modification of the polystelic structure is that which 

 is termed gamostelic ; in this case the several steles are not dis- 

 tinct for any considerable distance in their longitudinal course ; 

 but some or all of them fuse with each other at more or less 

 frequent intervals ; this is common in Ferns. 



The general morphology of the tissues of the leaf is essentially 

 the same as that of the stem which bears it. When the stem is 



FIG. 96. Part of a transverse section of 

 the stele of a root of Iris florentina : e scle- 

 rotic endodermis, with / a thin-walled pass- 

 age-cell; v bast; wood-vessel; c cortical 

 ground-tissue; p pericycle. (After Stras- 

 bnrger:x240.) 



