CH. IX] OF PTERIS. 123 



In a transverse section of the rhizome we have ex- 

 ternally the epidermic layer, which presents no special 



PIG. 53. 



KHIZOME OF Pteris, transverse section slightly magnified. 

 p.sc, peripheral sclerenchyma. Z.I, lateral line. 



p, parenchyma. 

 sc, sclerenchyma. 

 sc.st, scattered strands of sc. v.b, vascular bundle. 



points of interest, and from a physiological standpoint it is 

 unimportant, for the protective function is practically 

 taken over by a layer of hard-walled sclerenchyma. This 

 layer makes a dark-coloured border round the section as 

 seen with a low power (fig. 53); it is wanting at two 

 opposite points where the subjacent tissue comes to the 

 surface. These places are visible as streaks running down 

 two opposite sides of the rhizome and are known as 

 lateral lines. Their function is believed to be connected 

 with aeration. In the cork of the oak stem and of the 

 potato tuber a,re certain spots known as lenticels, where the 



