THE LIFE -HISTORY OF PLANTS. 



159 



leaves, whether perfectly developed or mere scales, 

 occur at regular definite intervals called " nodes." 

 The nodes, therefore, are the places where the leaves 

 spring from the stem, and anything that comes from 

 those nodes (not being a stem or a branch) is a leaf. 

 Speaking generally, 

 and disregarding 

 exceptions, a leaf is 

 a flat plate con- 

 tinuous with the 

 bark of the shoot, 

 and having an upper 

 surface looking to 

 the heavens, a lower 

 surface, usually 

 duller in colour, 

 turned in the oppo- 

 site direction. It 

 may be sessile," 

 when it springs 

 from the stem with- 

 out any intervening 

 stalk ; or it is pro- 

 vided with a "pe- 

 tiole " or leaf-stalk, 

 which separates the 

 blade or flat part of 

 the leaf from the 

 stem — the 

 leaf - stalk 

 being inter- 

 mediate, not 

 only in posi- 

 tion but also 

 in structure 

 and func- 

 tion, be- 

 tween stem 

 and leaf. In 

 form, size, 

 and appear- 

 ance leaves 

 vary almost 

 beyond de- 

 s cr iption. 

 These varia- 

 tions are of 

 two kinds : 

 those which 



are, as it were, mere stages or phases of growth, and 

 as such may occur in all plants, and those which are 

 peculiar to certain plants and serve as distinguishing 

 characteristics— such as the cylindrical hollow leaves 

 of an Onion, or the pinnate foliage of a Rose. For 

 an account of the principal forms of leaves reference 

 must be made to botanical text-books. 



Fig. 44.— Partite Leaf of Hemp. 



Fig. 45.— Compound (Pinnate) Leaf of Robinia. 



Forms of Leaves. — The functions of a fully- 

 developed leaf are essentially the same whatever its 

 form, and therefore in this place it is not necessary to 

 enter into much detail concerning the myriad sbapes 

 they assume. For our purpose it is enough to know 



that some leaves, 

 such as scale-leaves, 

 are imperfectly de- 

 veloped or degene- 

 rate organs, of great 

 interest morj)holo- 

 gically and histori- 

 cally, but of no 

 special use to the 

 plant ; while other 

 leaves, despite their 

 variations in dif- 

 ferent cases, never- 

 theless fulfil the 

 same duties. So far 

 as form goes, it is 

 enough for us here 

 to recognise the 

 three main groups 

 of leaves : (1) simple 

 leaves, when they 

 are in one piece, 

 which may be quite 

 "entire," or 

 in which the 

 margin may 

 be variously 

 indented, 

 but always 

 to a very 

 slight ex- 

 tent, so that 

 the sub- 

 stance is still 

 undivided 

 (Fig. 42) ; 

 (2) divided 

 leaves, where 

 the blade of 

 the leaf is 

 branched or 

 lobed in va- 

 rious ways 

 and to a 



various extent (Figs. 43, 44) ; and (3) compound 

 leaves, where the branching of the blade of the leaf 

 goes to such an extent that each subdivision is com- 

 pletely distinct from its neighbour, and generally 

 "articulated," that is, separated from the common 

 stalk by a distinct constriction or joint (Fig. 45). 

 Of these three types the modifications are infinite. 



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