20 LANG, Morphology of the Stock of IsoeJes lacustris. 



vertical lines, the lowest roots in each line being the 

 oldest. 



These points will all require to be examined in detail 

 and they undoubtedly suggest a relation between the 

 growing line of the rhizophoric region and the secondary 

 meristem of the shoot. They do not however prove that 

 the rhizophoric region is merely a peculiar secondary 

 growth, nor do they suggest any explanation of some 

 striking features of the lower region of the stock, especially 

 of the orientation of the roots to the growing line. It is 

 difficult on this view to understand either the mode of 

 growth of the rhizophoric region or its comparative 

 morphology. 



The following features, on the other hand, support the 

 interpretation of the lower region of the stock as a distinct 

 downwardly growing rhizophore, with a deeply sunken 

 and extended growing line. Growth proceeds from this 

 as if the apical region was not merely depressed (as the 

 stem-apex is), but enclosed by the congenital coherence 

 of the opposed sides of the depression ; the root-bearing 

 surface is later exposed by the splitting process at the 

 groove. 



a. The acropetal succession of the roots in relation to 

 the growing line. 



b. The fact that the roots, which develop close to the 

 growing line, do not penetrate the cortex, but are carried 

 outwards until exposed by the splitting process as practi- 

 cally exogenous organs. 



c. The direction of the young root-tips to the plane 

 of the future split, and the peculiar curvature of the 

 traces of the young roots. 



d. The regular orientation of the structure of the 

 roots in relation to the growing line, the protoxylem 

 always being turned towards this. 



