68 



THE BIOLOGY OF A PLANT. 



A 





FIG. 32 An entire plant 

 of Pterits. One of the 

 leaves is young and 

 small, and a compari- 

 son of the figure with 

 Fig. 31 will show some 

 of the differences be- 

 tween leaves of differ- 

 ent ages. 



crossed and recrossed in all di- 

 rections, both above and below, 

 by the rhizomes of its neighbors, 

 the whole constituting a coarse 

 network of underground stems 

 loosely filling the upper layer 

 of the soil. 



The aerial part (the frond 

 or leaf] is likewise divisible into 

 a number of parts, comprising 

 in the first place the leaf-stalk 

 or stipe, and the leaf proper or 

 lamina. The latter is subdi- 

 vided like a feather (pinnately) 

 into a number of lobes (pinnae, 

 Fig. 32), which vary in form 

 according to the state of de- 

 velopment of the leaf. In large 

 leaves the two lower pinnae are 

 often larger than the others, so 

 that the leaf appears to consist 

 of three principal divisions, and 

 is said to be " ternate" or triply 

 divided (Fig. 31, A). Each 

 pinna is in turn pinnately sub- 

 divided into pinnules (pin- 

 nulce) or leaflets (Fig. 31, B), 

 each of which is traversed down 

 the middle by a thickened ridge 

 or rod , the in idrib. The leaflets 

 sometimes have smooth out- 

 lines, but are usually lobed 

 along the sides, as in Fig. 31, B. 

 In this case their form is said 

 to bepinnatiftd. Eacli lobe is 

 likewise furnished with a mid- 

 rib. The stipe enlarges some- 

 what just below the surface of 

 the ground, then grows smaller 

 and joins the rhizome. The 



