THE LEAF— THE COTYLEDONS. 



39 



variations; thus, leaves are spoken of as "en- 

 tire " when the margin is continuous (fig. 49), 

 "toothed" (as in fig. 47), "lobed" or "divided" 

 when the leaf branches more or less deeply so 

 as to form "lobes" or "segments", and "com- 

 pound" (figs. 48, 49) 

 when the branching is 

 carried to such an ex- 

 tent that the leaf divides 



Fig. 47— Cherry— Simple Leaf. 



p, Petiole, with Stipules at the 

 base; 1, midrib; 2, 3, branches 

 of the midrib. 



Fig. 48.— Horse-chestnut— Compound 

 Palmate Leaf. 



into numerous eventually separate pieces or leaf- 

 lets attached to the central stalk by a distinct 

 little joint. When the subdivisions or leaflets 

 diverge or radiate from the top of the leaf-stalk, 

 the term palmate (fig. 48) is employed, as in 

 vines, fan -palms, or strawberries; when they 

 spring at inter- 

 vals from the sides 

 of the leaf, then 

 the word pinnate 

 (fig. 49) is made 

 use of, as in the 

 rose-leaf. It is re- 

 quisite also to pay 

 attention to the 

 "base" of the leaf, 

 that portion near- 

 est to the petiole; 

 to the "apex", that 

 portion farthest 

 from the petiole; 

 and to the "mar- 

 gin ", which, as has 

 been pointed out, is 



entire, or variously toothed or lobed. The "mid- 

 rib", or direct continuation of the petiole, must 

 also be noticed, as well as its subdivisions or 

 "veins". In most Dicotyledonous plants there 

 is one prominent midrib, from the sides of which 

 branch off numerous veins arranged usually in a 

 close, irregular network, so that Dicotyledonous 



Fig. 49.— Robinia— Compound Unequally- 

 pinnate Leaf. 



plants may often be recognized by the mere 

 inspection of their leaves, without seeing their 

 cotyledons, their wood, or their flowers. Net- 

 veined leaves are, with few exceptions, charac- 

 teristic of Dicotyledons, as straight-veined leaves 

 are of Monocotyledons. It must be remembered 

 that the terms ribs, veins, nerves, are employed 

 in botany without any reference to their use. 



The surfaces also demand attention, being 

 glabrous, that is, free from hairs; glaucous, 

 bluish -green; or more or less and variously 

 hairy, appropriate terms being given in each 

 case. The lower surface of the leaf is generally 

 duller in colour than the upper. 



The internal structure, as revealed by the mi- 

 croscope, has reference to the varied duties of 





Fig. 50—1. Shows Epiderm with thickened upper wall and palisade 

 cells beneath, tilled with chlorophyll. 2. Epidermal Cells thickened 

 on oue side, with Cellular Tissue beneath. 



the leaf. Speaking generally, the leaf is made 

 of a cellular substance which consists of an 

 expansion of the cortex, stiffened hy a frame- 

 work of veins and fibres which all, or some of 

 them, act as conduits for the transmission of 

 fluids from one part of the plant to another. 

 Outside all is a protective skin or epiderm. 

 The epiderm usually consists of a single layer 

 of flattened cells destitute of chlorophyll, but 

 often thickened on one side for protection sake 

 (fig. 50), and is covered with "cuticle", with a 

 waxy coating, or with thickly -felted hairs of 

 various shapes, all provisions against excessive 

 evaporation of watery vapour or injurious ra- 

 diation of heat (fig. 51). The epiderm on the 

 lower surface is usually thinner and is generally 

 perforated by a number of small openings called 

 stomates (fig. 52). These are bounded by two 

 or more cells of different shape from the rest, 



