FOLIAGE AND SCALE LEAVES. 65 



vascular bundles or veins, which, as we have seen, may be dis- 

 tributed in various ways. 



The epidermis, which can be conveniently examined both in 

 sections and by peeling off pieces, is composed, as seen under the 

 microscope, of flattened cells, the boundaries between which may 

 be straight or curved (fig. 7, F). The outer walls of the cells are 

 covered by a cuticle, and besides this, especially in leathery leaves 

 like those of the holly, are often much thickened and cuticularized, 

 i.e., their cellulose is more or less completely converted into 

 cutin (cf. p. 30). The prickles upon many leaves owe their 

 firmness to the thickness of the epidermic cell-walls. Protoplasm 

 and cell-sap are contained in the cells, but, as a general rule, 

 chlorophyll granules are absent. In the under epidermis of a 

 leaf, and often to a less extent in the upper epidermis as well, a 

 large number of minute openings are found. These stomata 

 (fig. 7, G) are intercellular spaces formed as small splits between 

 adjacent cells. 1 Each of them is bounded by two kidney-shaped 

 guard-cells, the concavities of which face one another. These two 

 cells not only differ in shape from the ordinary epidermic cells, 

 but also in the possession of chlorophyll granules. In many 

 cases some of the neighbouring cells are of different character 

 from those making up most of the epidermis. Stomata are not 

 confined to the foliage leaves, but occur with more or less fre- 

 quency in the epidermis of all organs except roots. They are 

 also absent in entirely submerged leaves and stems, while the 

 stomata of floating leaves only occur in the upper epidermis. 



The hair structures, developed from the epidermis of many 

 leaves, present the same variety as in the case of the stem 

 (p. 28). The structure of the stinging hairs of the nettle will 

 be described further on (p. 72). 



The ground-tissue (fig. 7, G) or mesophyll, like the epidermis, 

 is not constructed in exactly the same manner above and below. 

 An ordinary horizontal foliage leaf is, in fact, bifacial in struc- 

 ture. The upper part of the mesophyll is composed of one or 

 more layers of cells elongated at right angles to the surface, 

 and termed from their appearance palisade parenchyma. Their 

 cellulose walls are thin, and they contain a great many chlorophyll 

 granules. The lower part of the mesophyll is made up of cells 

 differing from the preceding in their very irregular shape. 

 This spongy parenchyma is traversed in all directions by inter- 

 cellular spaces which communicate with one another, and, by 

 means of the stomata, with the exterior. Under each stoma 

 there is a specially large intercellular space known as a respira- 



1 The stomata of the white lily are favourable objects for study. They are 

 large enough to be seen with a lens. 



