96 



a protection against excessive transpiration (Fig. 10,1; Fig. 9, i, 

 2, 4). By Ihe disorganization of these cells the large intercellular 

 spacesare formed. Below the vascular bundle, which contains but 

 very little stereom, a tissue occurs consisting of transparent cells 

 (Fig. 10, 2) which have far greater power of offering resistance 

 to external factors than those in the spongy parenchyma and 

 which resemble those 

 that occur in Andro- 

 meda^ but are not 

 so compact and 

 have not the inter- 

 spersed thick-walled 

 cells, occurring either 

 singly or in trabeculae. 

 The leaf varies great- 

 ly in thickness and 

 breadth , sometimes 

 it is thick and narrow 

 [palustre]^ sometimes 

 thin and broad [groen- 

 landicum] , but be- 

 sides this there exist 

 hardly any differences 

 of importance. 



The stem. The primary cortex consists of a zone of 

 small cells which occur just within the epidermis, which is 

 hairy like the leaf and fairly thick-walled ; internal to these are 

 transparent, thin-walled cells mixed with a few with thicker 

 walls, and trabeculae of such thick-walled cells; while most 

 internal are one to several rows of cells of the same form as 

 the outermost ones. The pith, consists partly of transparent, 

 thin-walled cells and partly of thick-walled cells, the latter 

 occurring as trabeculae between the former. 



The secondary cortex, which according to Segerstedt is 



Fig. 12. Ledum palustre. 



Stem in transverse section ; the limits of two annual 

 rings are discernable ; X 180. (Greenland). (Phot.) 



