MATURE SPOROPHYTE 17 



an epidermal layer, with a cuticularised external wall, which prevents 

 indiscriminate loss of water by surface-evaporation. But the epidermis 

 is perforated by numerous stomata, the motile guard-cells of which can 

 control, according to circumstances, the width of the pores leading into 

 the intercellular spaces. There is thus a highly organised ventilating 



.**&! < " 



'"^ 



, 



Nephrodium h'ili.\--inas. Rich. Fertile leaf about one-sixth n.itunil si/e, the lower 

 part with the under surface exposed. To the left a single frnik- segment, enlarged about 

 7 times. (After Luerssen.) 



system. Finally, there are also firm, brown, resistant tissues, disposed 

 sometimes near the outer surface, as in the stem and in the leaf-stalk : 

 sometimes more deeply seated, as in the root, while in the leaf they 

 follow the course of the vascular strands. These give to the several parts 

 increased mechanical strength, and power of resistance. 



Thus constituted the Male .Shield Fern is an organism which is 

 capable of leading an independent life on an exposed land-surface : it 



