122 



PART II. ANATOMY AND HISTOLOGY. 



[33 



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-po i n_t ; 

 bundles of the former 

 kind are distinguished 

 as common (i.e. com- 

 mon to stem and 

 leaf), and, in their 

 course in the stem, 

 are termed leaf- 

 traces ; bundles of the 

 latter kind are dis- 

 tinguished as cauline 

 (i-e. confined to the 

 stem). 



Stems with common 

 bundles are generally 

 monostelic ; the leaf- 

 traces do not, how- 

 ever, follow a uniform 

 course in all cases. 

 Thus, they may pro- 

 ceed to the centre of 

 the stem and form a 

 solid vascular cylinder 

 (e.g. Isoetes among 

 Pteridophyta ; and 

 certain aquatic Mono- 

 cotyledons, such as 

 species of Potamoge- 

 ton, etc.). Or they may 

 form a hollow cylinder. 

 In the simplest case 



FIG. 99. Diagram of the course of the vascular 

 bundles in stems. A Longitudinal section through the 

 axis of a Palm-stem, showing a transverse section of half 

 of it. The leaves (cut off above the insertion) are hypothe- 

 tically conceived of as distichous and amplexicaul, and so 

 are seen on both sides of the stem, 1 m 2 m 3 m being the 

 median line of each. B Outside view and transverse 

 section of Cerastium (hypothetically transparent, to show 

 the internal bundles). The decussate leaves (1, 2, 3) are 

 cut off. The bundle proceeding from each leaf divides into 

 two above the leaf immediately below it, and the branches 

 of all the bundles unite to form the four thin bundles 

 which alternate in the Hection with the thicker ones. In 

 the section, m is the pith, r the cortex, v the medullary 

 ray. The xylem in the nbro-vascular bundles is indicated 

 by shading. 



