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 
ih 
Fic. 2. 
Nephrodium Filix-mas, Rich. — Fertile leaf about one-sixth natural size, the lower 
part with the under surface exposed. ‘lo the left a single fertile 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 
B 
