MOKPHOLOGY OF LEAVES. 85 



of ancestral generations. As in a genealogical tree, only the 

 later ramifications are among the living. The tree differs from 

 the coral structure in that, as it ordinarily imbibes its nourish- 

 ment mainly from the soil through its roots, it makes a downward 

 growth also, and, by constant renewal of fresh tissues, maintains 

 the communication between the two growing extremities, the 

 buds and the rootlets. Otherwise, the analogy of the two, as to 

 individuality, is well-nigh complete. 



SECTION IV. OF LEAVES. 

 1. THEIR NATURE AND OFFICE. 



157. Leaf (Lat. Folium, in Greek form Phyllum), as a botani- 

 cal term, has on the one hand a comprehensive, on the other a 

 restricted sense. In its commonest sense, as used in descriptive 

 botany, it denotes the green blade only. Yet it is perfectly 

 understood that the footstalk is a part of the leaf, and therefore 

 that the phrase " leaves cordate," or the like, is a short wa}' of 

 saying that the blade of the leaf is cordate or heart-shaped. 

 Moreover, two appendages, one on each side of the base of the 

 footstalk, when there is any, are of so common occurrence that 

 they are ranked as a proper part of the organ. So that, to the 

 botanist, a typical leaf consists of three parts : 1, BLADE or 

 LAMINA ; 2, FOOT-STALK or LEAF-STALK, technically PETIOLE ; 

 3, A pair of STIPULES. (Fig. 142.) 



158. The blade, being the most important parf of an ordinary 

 leaf, may naturally be spoken of as the whole. Petiole and 

 stipules are indeed subsidiary when present, and are not rarely 

 wanting. Yet sometimes they usurp the whole function of foli- 

 age, and sometimes there is no such distinction of parts. 



159. Physiologically, leaves are green expansions borne by the 

 stem, outspread in the air and light, in which assimilation (3) 

 and the processes connected with it are carried on. Vegetable 

 assimilation, the most essential function of plants, being the 

 conversion of inorganic into organic matter, takes place in 

 all ordinary vegetation only in green parts, and in these when 

 exposed to the light of the sun. And foliage is an adaptation 

 for largely increasing the green surface. But stems, when green, 

 take part in this office in proportion to the amount of surface, 

 sometimes monopolize it, and in various cases increase their 

 means of doing so by assuming leaf-like forms. (126-129.) 

 Leaves, especially in such cases, may lose this function, appear 

 only as useless vestiges, or may be subservient to various wholly 



