LESSON XXXI 



FERNS 



\\ h .saw ill the previous lesson that in mosses there is a 

 certain though small amount of histological differentiation, 

 some cells being modified to form sclerenchyma, others to 

 form axial bundles. We have now to consider a group of 

 plants which may be considered to be, in this respect, on 

 much the same morphological level as Polygordius, the 

 adult organism being composed not of a mere aggregate of 

 simple cells, but of various well-marked tissues. 



A fern-plant has a strong stem which in some forms, such 

 as the common Bracken {Pteris aquilina) is a horizontal 

 underground structure called a rhizome, often incorrectly 

 considered as a root : in others it creeps over the trunks of 

 trees or over rocks : in others again, such as the tree-ferns, 

 it is vertical, and may attain a height of three or four metres. 

 From the stem are given off structures of two kinds — the 

 leaves, which present an almost infinite variety of form in 

 the various species, and the numerous slender roots. In 

 some cases, such as the tree-ferns and the common Male 

 Shield-fern {Aspidium filix-mas), the plant ends distally in a 

 terminal bud, consisting, as in Nitella and mosses, of the 

 growing end of the stem over-arched by leaves : in others 



