122 RHIZOME [CH. IX 



a sunflower in being a place where cells are manufactured 

 by cell-division, but it differs from it in certain details 

 which need not be considered. 



The leaves come off right and left from the rhizome 

 and bend up to emerge above ground. A rhizome, dug up 

 in autumn, will show leaves in various stages ; at the basal 

 end, i.e. away from the growing point, are the dead and 

 withered stalks of last year's leaves, and nearer the apex 

 come the present year's leaves, nearer still are very young 

 leaves which will remain dormant through the winter and 

 shoot up in the following spring. The most noticeable 

 point about the young leaves is that the stalk is strongly 

 developed while the lamina is small and folded down on 

 the top of the stalk. Two other facts must be noted: 

 namely, that buds are formed on the leaf-stalks ; and that 

 the rhizome bears adventitious roots the original true 

 roots having long ago disappeared. 



Pteris. Histology. 



The histology of the rhizome is interesting because it 

 supplies a type of vascular bundle differing from anything 

 previously described. 



The characteristics of the bundle in the sunflower and 

 oak are two : (i) it possesses in its cambium the power 

 of increase in thickness : (ii) xylems and single strands of 

 phloem run side by side; these features are technically 

 expressed by calling the bundle open and collateral. 

 In Pteris the bundle is closed, i.e. the cambium is absent, 

 and there are two layers of phloem running with the 

 xylem and almost surrounding it. 



