152 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 30. 



whilst the conjunctive tissue between the strands constitutes the 

 medullary rays. 



In all cases the stele (whether one or more) is at an early stage 

 marked off from the extra-stelar ground-tissue, the layer of the 

 ground-tissue which abuts on the stele being specially differentiated 

 as a sheath, the endodermis (see Fig. 116 End.), 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,. Trichomanes, Lygodium, Gleichenia, Osmunda, 

 Todea, lower part of the stem of the Ophioglossaceae, Salvinia, 

 Azolla, Lycopodium, Isoetes, some species of Selaginella). 



The variations from the primitive monostelic structure are 

 polystely (p. 146) and scliizostely. In polystely, the whole of the 

 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 pi erome- strands. Polystely is rare in stems 

 which have a growing-point without an apical cell, whilst it is 

 common in stems where the growing-point has an apical cell : 

 hence it is rare in Phanerogams (occurs in Auricula and Gunnera), 

 and is common in Pfceridophyta (especially Leptosporangiate 

 Ferns, and some Selaginellas). 



An important modification of the polystelic structure is that 

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

 distinct for any considerable distance in their longitudinal course; 

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

 frequent intervals for longer or shorter distances ; this is common 

 in Ferns, Marsilea, etc. 



In schizostely, the single primitive stele breaks up into as many 

 distinct strands as it possesses vascular bundles; and since each 

 such strand may consist of but little more than the vascular 

 bundle with a pericycle, the chief part of the conjunctive tissue 

 of the primitive monostele is excluded from the schizosteles, 

 and becomes, in fact, extra-stelar ground-tissue (see Fig. 116 B), 

 each schizostele being invested by its own segment of the en- 

 dodermis. This obtains, among Phanerogams, in the Nymph geacese, 

 Hydrocleis, Limnocharis, some species of Ranunculus (JK. 

 aquatilis and Lingua) ; among Pteridophyta, in the upper part of 

 the stem of the Ophioglossaceee, and in Equisetum. The schizo- 



