VI. 



THE BRYALES 



223 



Bastit ((i), p. 295), 

 who has made a compar- 

 ative study of the subter- 

 ranean and aerial stems of 

 P. jimiperiniim, divides 

 the outer tissue of the lat- 

 ter into epidermis, hypo- 

 derma, and cortex. In 

 the subterranean stems he 

 finds the construction 

 quite different from that 

 of the leafv branches. 

 The section of the former 

 is triangular, and its epi- 

 dermis provided with 

 hairs which are absent 

 from the epidermis of the 

 aerial parts. Rudimen- 

 tary scales, arranged in 

 three rows, are present, 

 and corresponding to 

 these are strands of tissue 

 that represent the leaf- 

 traces of the aerial stems. 

 The central cylinder is 

 much larger relatively 

 than in the leafy branches, 

 and its cross-section is not 

 continuous, but is inter- 

 rupted by three "pericyclic 

 sectors," composed of 

 cells whose walls are but 

 little thickened. The 

 point of each sector is at 

 the periphery of the me- 

 dulla, or central cylinder, 

 and the broad end toward 

 the centre. As might be 

 expected, intermediate con- 

 ditions are found where 

 the rhizome begins to grow upward to form a leafy branch. 



Fig. 121. — A, Transverse section of the leaf of 

 Leucohryum; B, similar section of the leaf of 

 Polytrichum commune; cl, chlorophyll-bear- 

 ing cells (after Goebel). 



