258 



BOTANY 



The early growth of the leaf is much stronger upon the outer side, 

 so that most Ferns show the marked inward rolling of the leaf which 

 is so characteristic of these plants. In the Ophioglossaceae, how- 

 ever, the young leaves are usually folded straight in the bud. 



A few Ferns, e.g. Scolopendrium, Asplenium nidus, etc., have simple 

 leaves, but usually they are pinnately compound, or decompound, the 

 leaves of many Tree-ferns being among the most complex and beauti- 

 fully segmented known. The leaf commonly has a well-marked stalk 

 (" Stipe "), which when young is often covered with thin, chaffy scales 

 or " Paleae," and these sometimes are tipped with a glandular cell. 



A. 



FIG. 223. Polypodium falcatum. Cross-section of a leaf catting across a vein ; st, 

 section of a storaa; m, mesophyll (x 200). B, section of young sorus (X 75). 



Hairs are less common, but occur upon some Ferns. They are espe- 

 cially conspicuous upon the young leaves of Osmnnda cinnamomea. 



Venation. The venation of the leaves is usually pinnate, but the 

 ultimate divisions are generally dichotomous. Sometimes connect- 

 ing veins, enclosing reticulations, are developed (Fig. 222). The 

 venation is of some importance in classification. 



Epidermis. The epidermis of the leaf is composed of flat cells 

 with strongly undulating outline, and, unlike the epidermal cells of 

 most vascular plants, they contain chlorophyll. Stomata are usually 

 developed upon the lower epidermis only, but may occur upon the 

 upper surface in some instances. The Hymenophyllaceae differ from 

 the other Ferns in having the lamina of the leaf reduced to a single 



