FERNS 



333 



areas of stony tissue (sclerenchyma) which give the chief mechan- 

 ical support (fig. 310). 



503. Structure of the fern leaf. In many respects the 

 leaves of ferns are similar in structure to the leaves of the flower- 

 ing plants (paragraphs 140-145). The blade of the leaves is 

 thin and expanded, and 

 shows the same light rela- 

 tion as the leaves of the 

 higher plants. There is a 

 layer of epidermal cells on 

 either side of the leaf, 

 which are quite regular as 

 seen in cross section of 

 the leaf, but meet by very 

 irregular edges in surface 

 view. There are numerous 

 stomates which are pro- 

 tected by two crescent- 

 shaped guard cells as in 

 the higher plants. The 

 mesophyll of the leaves 

 consists usually of a pali- 

 sade layer next the upper 

 epidermis, while the rest 

 is made up of the loose 

 parenchyma or spongy tis- 

 sue with large intercellular 

 spaces which communicate 

 with the stomates. The 

 disk-shaped chlorophyll 

 bodies lie in the cells of the loose parenchyma, the palisade and 

 guard cells, and sometimes in cells of the epidermis. In the 

 veins are the vascular bundles. In the arrangement of these 

 veins is shown one of the differences in structure from that of 

 the higher plants. The veins branch in a forked manner, which 

 sometimes is very striking. The vascular system strengthens 



Fig. 311. 



Rhizome with bases of leaves, and roots of the 

 Christmas fern. 



