AERIAL AND UNDERGROUyD PARTS. 109 



below the surface, and brandies widely in various directions. 

 It may often be followed for a loui^ distance, and in snch casfs. 

 reveals a surprisingly complicated system of undori^round 

 branches. At Urst sight, the underground porti(»n of the fern 

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

 really the stem or axis of the plant, which ditfers frcjm ordinary 

 stems chiefly in the fact that it lies horizontallv uiidvr the 

 ground instead of rising vertically above it. The aerial }K»rtion, 

 which is often taken for stem and leaf, is reallv leaf onlv. The 

 true roots are the line fibres which spring in great abundance 

 from the underground stem. Underground stems more or less 

 like that of Pteris are not uncommon — occurrini^, for iii>t;iiice, 

 in the potato, the Solomon' s-seal, the onion, etc. In /'A/'/.v, 

 and in certain other cases, the underground stem is technically 

 called the rootstock or rhizome^ and in this plant it constitntes. 

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

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

 long and bore two leaves. It was dug out of sandy soil on tlie 

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

 surface. It w^as crossed and recrossed in all directions, both 

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

 constituting a coarse network of underground stems loosely till- 

 ing the upper layer of the soil. 



The aerial portion {t\\Q frond or leaf) is likewise divisible 

 into a number of parts, comprising in the first place the h-af- 

 stalk or stipe^ and tlie leaf proper or lamina. The latter is suImH- 

 vided like a feather {pinnately) into a number of lobes {j^'n/hBj 

 Fig. 44), which vary in form according to the state of de- 

 velopment of the leaf. In large leaves the two lower i)inna^ are 

 often larger than the others, so that the leaf appears to consist 

 of three principal divisions, and is said to be " t^'rfHftr " or tri]» 

 ly divided (Fig. 44, A). Each pimia is in tnrn pinnately sub- 

 divided into pinnules (pinnuke) or leaflets (Fig. 44, />), eadi of 

 which is traversed down the middle by a thickened ridge or 

 rod, the midrib. The leaflets sometimes liave smo»>th «>ntlines, 

 but are usually lobed along tlie edges, as in Fig. 44. />. In 

 this case their form is said to be pinnatifid. Each lohe is like- 

 wise furnished with a midrib. The stipe enlarges somewliat 

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



