33.] CHAPTER II. THE TISSUES. 171 



analogous condition is sometimes presented by schizostelic mem- 

 bers, where the schizosteles are developed in partial continuity 

 {e.g. stem of Equisetum, see Fig. 116). The resulting vascular 

 mass is generally a solid cylinder, but it is occasionally a hollow 

 cylinder. 



With regard to the occurrence of vascular tissue in the gametophyte 

 generation, and in the sporophyte of the lower plants, it may be stated that 

 liguified vascular tissue (i.e. wood) does not occur in any gametophyte, nor in 

 the sporopbyte of any plant below the Pteridophyta. However, in the stem 

 of the gametophytic shoots of some Mosses there is a solid central stele cou- 

 tistiug of tissue which is functionally vascular tissue ; the same is true of the 

 stem (seta) of the Moss-sporophyte in certain cases. Sieve-tissue has been 

 found in some of the larger Brown Seaweeds. 



The primary vascular tissue-system extends continuously 

 throughout the body of the sporophyte of the higher plants ; the 

 vascular bundles of root, stem, and leaf are all in direct com- 

 munication. 



The arrangement and course of the vascular bundles are in- 

 timately connected with the morphology of the plant and with the 

 differentiation of its members. In elongated members (stems, 

 petioles, roots) the bundles run longitudinally, so that a transverse 

 section of such a member shows transverse sections of its vascular 

 bundles. 



In the primary root the longitudinal course of the bundles is 

 simple ; there is an axial vascular cylinder, either solid or hollow, 

 consisting of straight more or less distinct bundles of wood and 

 bast, and extending from the growing- point backwards to where 

 the root merges into the stem ; from this cylinder there arise 

 lateral offsets, which constitute the steles of the lateral branches 

 of the root. 



In the stem the course of the bundles is more complicated, on 

 account of the fact that the stem bears lateral members, leaves, 

 which differ from itself or from its branches. In some cases, the 

 bundles of the stem, when traced upward toward the growing- 

 point, are found to terminate in the young leaves ; whilst in other 

 cases the bundles end (like those of the root) in the plerome of the 

 growing-point ; bundles of the former kind are distinguished as 

 common (i.e. common to stem and leaf), and, in their course in the 

 stem, are termed leaf-traces ; bundles of the latter kind are distin- 

 guished as cauline (i.e. confined to the stem). 



Stems with common bundles are generally moiiostelic ; the leaf 



