8 STRUCTURAL BOTANY 



structure among the species is so great that a general 

 description, even of the genus as a whole, is impossible. 

 In an elementary book, we cannot enter into all these 

 variations ; we can only give a short description of two 

 or three forms, which may serve to give some idea of the 

 peculiarities of the genus and of the range of variation 

 among its species. 



a. The Stem 



In each of our types of Flowering Plants we found 

 that the stem was traversed by one central cylinder, 

 consisting of the vascular bundles and conjunctive tissue 

 (see Part I. pp. 47, 152, 236). We learnt further that 

 the bundles of the stem are directly continuous with 

 those of the leaves. These facts hold good, with certain 

 exceptions, for the Phanerogams generally. 



In the Selaginellas the arrangement is totally different. 

 The number of cylinders or steles varies from one up to 

 five or more, not only in different species, but some- 

 times even in different parts of the same plant. The 

 conjunctive tissue is very little developed, and pith 

 is almost always absent, the whole interior of the 

 cylinder being occupied by a solid strand of wood. 

 Consequently it is generally impossible to distinguish the 

 limits of the individual vascular bundles in the stele, 

 or, to be more accurate, the stele in the stem is not 

 differentiated into distinct bundles. Lastly, the vascular 

 system of the stem is not built up entirely of leaf -trace 

 bundles. The greater part of the xylem and phloem 

 can be traced continuously through the whole stem, and 

 only certain portions of the vascular tissue are directly 

 connected with the bundles of the leaves. 



It will be convenient to begin with a short description 



