40 



THE GABDENER'S ASSISTANT. 



and which are called "guard-cells". The guard- 

 cells become turgid or flaccid according to the 





Fig. 51.— 1. Felted Hairs on the surface of a plant. 2. Stellate Hairs on the 

 Epiderm of Deutzia. 



amount of moisture in the leaf or in the air, 

 and thus open or close the openings according 

 to circumstances. The stomata are usually most 



cellular substance of the leaf, consisting of various 

 layers of cells (figs. 53, 54), sometimes all 

 alike, but more generally differing 

 in form, colour, and function. Thus 

 the cells immediately beneath the 

 epiderm on the upper side are 

 closely packed and lengthened ver- 

 tically, or at right angles to the sur- 

 face, so that they resemble palings, 

 and hence they are spoken of as 

 palisade cells (fig. 54). They are 

 generally full of chlorophyll, and, 

 being exposed to the light, are the 

 seat of great constructive activity. 

 Beneath the green palisade cells 

 there are usually several layers of 

 more or less spherical cells with little 

 or no chlorophyll, and so loosely 

 packed that interspaces are often left 

 between them (figs. 53, 54). These cells are 

 not so immediately dependent on sunlight, but 

 serve as magazines to store up water, to contain 

 what is formed in the palisade cells, or to act as 

 reservoirs for various secretions. 



The midrib is a reduced copy, so far as struc- 

 ture goes, of the stem, and the "veins" have the 



Fig. 52. -Stomata. 

 Surface view of a portion of the Frond of a fern, Wephrodium FUix-maa. 



abundant on the lower surface of the leaf, but 

 sometimes they arc equally numerous on either 

 surface, in which case the colour of the two sur- 



V??l 





Fig. 53.— Vertical section through a Leaf, showing Epidermis, 

 Palisade, Spongy Tissue, and a Stoma cut through. 



faces is alike. Stomata are not confined to the 

 leaf, but occur in all or any of the green parts 

 of plants. Within the skin or epiderm is the 



Fig. 54.— Vertical section through Leaf of Franciscea eximia, showing 

 Epidermal, l'alisade, and Spongy Tissue, and a Stoma cut through. 



same essential structure as the midrib from 

 which they spring. 



Uses of the Leaves.— Whatever the posi- 

 tion and form of the perfect leaf, its functions 

 are the same — to inspire and expire, to absorb 

 and to evaporate, according to circumstances, 

 to fabricate nutrient matters under the influ- 

 ence of sunlight and heat. A leaf, in fact, per- 

 forms duties which in an animal are carried on 

 in separate organs, as the skin, the lungs, the 

 digestive and the secreting organs. No wonder, 

 then, that gardeners attach so much importance 

 to healthy foliage. The watery fluid absorbed 

 by means of the root-hairs passes up the stem, 

 and at length reaches the leaves. Here such 



