AERIAL AND UNDERGROUND PARTS. 109 



below the surface, and branches widely in various directions. 

 It may often be followed for a long distance, and in such cases 

 reveals a surprisingly complicated system of underground 

 branches. At first sight, the underground portion of the fern 

 appears to be the root, but a closer examination shows it to be 

 really the stem or axis of the plant, which differs from ordinarv 



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stems chiefly in the fact that it lies horizontally under the 



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ground instead of rising vertically above it. The aerial portion, 



which is often taken for stem and leaf, is really leaf only. The 



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true roots are the fine fibres which spring in great abundance 

 from the underground stem. Underground stems more or less 

 like that of Pteris are not uncommon occurring, for instance, 

 in the potato, the Solomon' s-seal, the onion, etc. In Pteris, 

 and in certain other cases, the underground stem is technically 

 called the rootstoek or rhizome, and in this plant it constitutes 

 the larger and more persistent part of the organism. In the 

 specimen shown in Fig. 45 the rhizome was about eight feet 

 lon" and bore two leaves. It was dusr out of sandy soil on the 



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edge of a woodland, and lay from one to six inches beloAv the 

 surface. It was crossed and recrossed in all directions, both 

 above and below, by the rhizomes of its neighbors, the whole 



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constituting a coarse network of underground stems loosely fill- 

 ing the upper layer of the soil. 



The aerial portion (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 (pitwce, 

 Fig. 44), 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 ' ' tern-ate ' or trip- 

 ly divided (Fig. 44, A). Each pinna is in turn pinnately sub- 

 divided into pinnules (pinnules) or leafiets (Fig. 44, B), each of 

 which is traversed down the middle by a thickened ridge or 

 rod, the midrib. The leaflets sometimes have smooth outlines, 

 but are usually lobed along the edges, as in Fig. 44, B. In 

 this case their form is said to be pmnatiftd. Each lobe is like- 

 wise furnished with a midrib. The stipe enlarges somewhat 

 just below the surface of the ground, then grows smaller and 



