5o8 A TEXTBOOK OF THEORETICAL BOTANY 



Anatomy of the Stem 



The stem consists of an outer limiting zone of brown sclerenchymatous 

 cells, inside which is a cortex of thin-walled parenchyma filled with starch. 



Vessel 



Xylem parenchyma 



Fig. 502. — Pteridium oquilinum. Longitudinal section 

 of meristele showing perforated end wall of vessel. 



There are two concentric steles present, each having the form of a 



tubular network of meristeles (Fig. 499). Unlike the 

 meristele of Dryopteris, the meshes of the network 

 all correspond to the insertions of leaves, that is, 

 they are leaf gaps. In Pteridium there are also 

 leaf gaps, but as the leaves are far apart there 

 are numerous additional gaps, not related to the 

 leaves, which are called perforations. Between 

 the two concentric steles there lie two thick 

 bands of sclerenchyma, the dorsal band arched 

 and the ventral {i.e., the lower) one more or 

 less flat. Sometimes their edges unite to form 

 a tube, like the steles. 



The two steles are independent of each other, 

 except at the nodes, where leaf traces originate 

 from both inner and outer steles and accessory 

 branches may link them together. The condition 

 found in Pteridium is called polycycly and is 

 not uncommon in thick Fern stems. It is no 

 doubt useful in improving the vascular supply 

 to a massive tissue system (Fig. 500). 



The structure of the individual meristeles 



Fig. 1503. — Pteridium aqui- . • -i ^ ^i ^ • >^i txt i t^ /t^- \ 



liniim. Longitudinal IS very Similar to that in the Male Fern (Fig. 501), 

 section of sieve tube but there is an important peculiarity in that 



showing numerous , i n r ^i i i -r i .. 



lateral sieve areas. the end walls ot the large scalantorm elements 



