THE LEAF 115 



must be looked upon as a water-storing protective layer. 

 Some few cells of this layer contain curious structures of 

 cell-wall substance {cellulose), which are incrusted with car- 

 bonate of lime. The part played by these grape- like 

 crystals, which are termed cystoliths (c), in the economy of 

 the leaf is at present still unknown. Such protrusions of 

 the cell-walls with crystallised deposits have only been found 

 in a few natural orders {Vriicacem, Ficoidem, Accmthacecc, Cucur- 

 hitacece). 



The firm covering of the upper surface of the leaf is not 

 favourable for the passage of gases, for the passage of which 

 the under surface is peculiarly adapted. Here we notice the 

 large intercellular spaces in the tissue, which on that account 

 has been termed the spongy parenchyma. The air contained 

 in the intei'cellular spaces (i) communicates with the air con- 

 tained in the respiratory chamber (a), and through the stomata 

 {st) with the outer air, which can in its turn freely pass into 

 the tissues of the leaf 



The water is supplied to the leaf through the veins, a small 

 one of which is seen cut through transversely {g). The letters 

 r represent the vessels of the wood, the actual canals along 

 which the water flows. The path taken by the organised 

 food material, which has been formed in the leaf, and of 

 which there is a superfluous supply, is along the bundle-sheath 

 {sell) down into the stem, k indicates the point where the 

 cells begin to round themselves off abnormally, owing to the 

 excessive supply of water ; the intercellular spaces become 

 obliterated, and they press the still more irritated tissue {int) 

 towards the outside. 



A structure similar to that of the leaf of Ficus would 

 be presented by the leaves of most of our cultivated 

 plants. Many of them, especially the herbaceous ones, have 

 stomata on the upper side of the leaf, as well as on the under 

 surface. 



The functioning of the stomata will call forth our astonish- 

 ment and admiration still more, if we reflect upon the fact 

 that the guard cells, which are considerably thickened on their 

 inner side, become more bent, and cause therefore an opening 

 of the stomata, when they become turgid. But they only 



