SECTION 7.j ORDINARY LEAVES. 49 



Section VII. LEAVES. 



118. Stems bear leaves, at definite points (nodes, 13) ; and these are 

 produced in a great variety of forms, and subserve various uses. The 

 commonest kind of leaf, -which therefore may be taken as the type or 

 pattern, is an expanded green body, by means of which the plant exposes 

 to the air and light the matters which it imbibes, exhales certain portions, 

 and assimilates the residue into vegetable matter for its nourishment and 

 giowth. 



119. But the fact is already familiar (10-30) that leaves occur under 

 other forms and serve for other uses, — for the storage of food already 

 assimilated, as in thickened seed-leaves and bulb-scales ; for covering, as in 

 bud-scales ; and still other uses are to be pointed out. Indeed, sometimes 

 they are of no service to the plant, being reduced to mere scales or rudi- 

 ments, such as those on the rootstocks of Peppermint (Pig. 97) or the 

 tubers of Jerusalem Artichoke (Fig. 101). These may be said to be of 

 service only to the botanist, in explaining to him the plan upon which a 

 plant is constructed. 



120. Accordingly, just as a rootstock, or a tuber, or a tendril is a kind 

 of stem, so a bud-scale, or a bulb-scale, or a cotyledon, or a petal of a flower, 

 is a kind of leaf. Even in respect to ordinary leaves, it is natural to use 

 the word either in a wider or in a narrower sense ; as when in one sense 

 we say that a leaf consists of blade and petiole or leaf-stalk, and in another 

 sense say that a leaf is petioled, or that the leaf of Hepatica is three-lobed. 

 The connection should make it plain whether by leaf we mean leaf-blade 

 only, or the blade with any other parts it may have. And the student will 

 readily understand that by leaf in its largest or morphological sense, the 

 botanist means the organ which occupies the place of a leaf, whatever be 

 its form or its function. 



§ 1. LEAVES AS FOLIAGE 



121. This is tautological ; for foliage is simply leaves : but it is very 

 convenient to speak of typical leaves, or those which serve the plant for 

 assimilation, as foliage-leaves, or ordinary leaves. These may first be 

 considered. 



122. The Parts of a Leaf. The ordinary leaf, complete in its parts, 

 consists of blade, foot-stalk, or petiole, and a pair of stipules. 



123. Eirst the Blade or Lamina, which is the essential part of ordinary 

 leaves, that is, of such as serve the purpose of foliage. In structure it con- 

 sists of a softer part, the green pulp, called parenchyma, which is traversed 

 and supported by a fibrous frame, the parts of which are called ribs or veins, 

 on account of a certain likeness in arrangement to the veins of animals. 



