122 



Elementary Biology. 



FIG. 50. PINNA AND 

 PrNNULEs or A FROND 

 OF Pleris aguitina. 



Frond. The leaf or frond is usually a much branched 

 structure of considerable size. It consists of a stout rachis, 

 leaf-stalk, or petiole, and in Pteris of 

 considerable length. From the petiole 

 are given off veins, and from these 

 again veinlets. These strands support 

 the flattened green lamina, which is 

 thus subdivided into pinnae and pin- 

 nules, corresponding to the veins and 

 veinlets respectively. The entire la- 

 mina is covered on both sides by an 

 epidermis of flat green cells, bound- 

 ing and enclosing loose parenchyma 

 plentifully supplied with chlorophyll, 

 starch, &c. The epidermal cells are 

 prolonged into epidermal hairs, but 

 have no intercellular spaces, with the 

 exception of the stomata. A stoma is 

 a minute aperture in the epidermis 

 bounded by two chlorophyll-bearing 

 guard cells, capable of altering the size 

 of the stoma by their contraction or 

 expansion under different hygroscopic 

 conditions of the atmosphere, and according as the frond 

 is or is not exposed to sunlight. The stomata are as a rule 

 more abundant on the under surface of the leaf (fig. 71). 

 Since the histological structure of a fern leaf is fundamentally 

 the same as that of the angiosperm leaf a detailed descrip- 

 tion of that organ is postponed (page 152). 



One important structure developed on the leaf remains 

 to be described, viz. the asexual reproductive organ or 

 sporangium. At certain seasons of the year the edges of 

 the laminae will be found to be curled in towards the under 

 surface, and will be found to enclose a number of small dark 

 brown stalked bodies. These are the sporangia. In many 

 ferns, where the sporangia are produced on the under sur- 



