114 BOTANY PART i 



mainly passive, since they represent places where the resistance to nitration is 

 least. The tracheides terminate in this epithema, and in the overlying epidermis 

 there is a peculiarly -constructed stomatal apparatus in the form of WATER- 

 PORES (Fig. 131), which are of larger size than ordinary stomata. The guard 

 cells usually lose their living contents and the pore then remains permanently 

 and widely open. The thickened ridges so characteristic of the guard cells of 

 ordinary stomata are usually lacking. The excreted liquid frequently contains 

 calcium carbonate, which may remain as a white incrustation over the water- 

 pores, as, for example, on the leaf margin in many species of Saxifraga. 



At the tip of young leaves and of their marginal teeth such water-pores and 

 epithemata frequently occur, but are dried up on the mature leaf. Water-pores 

 also are found at the leaf-tips of submerged plants from which ordinary stomata 



are absent. They tend to perish 

 early, breaking down with the ad- 

 joining tissue to leave open pits by 

 which water and dissolved substances 

 may be expressed. 



Functions of the Leaf- 

 blade. The leaf -blades, as 

 already mentioned, are the 

 most important organs of nutri- 

 tion, i.e. assimilation, and also 

 of transpiration in cormo- 

 phytes. Their form and struc- 

 ture, their arrangement, and 

 the position they assume with 

 regard to the direction of the 

 light, correspond to this. Since 



FIG. 131. Water-pore from the margin of a leaf of ,, , ' ., . r -, 



Tropaeolum majus, with surrounding epidermal the decomposition of Carbon 



ceils, (x 240. After STRASBUROER.) dioxide is dependent both on 



light and on the presence of 



chlorophyll, the green colour of the lamina, the large surface exposed 

 by it, its relative thinness and dorsiventral construction are readily 

 understood. The large surface enables a greater number of cells con- 

 taining chlorophyll to be exposed to the light without shading one 

 another ; it also enables the carbon dioxide to be obtained from the 

 small proportion in the atmosphere, and at the same time facilitates 

 the loss of water vapour in transpiration. Since the passage of light 

 through a few layers of cells filled with chlorophyll renders it 

 ineffective for decomposing carbon dioxide in the deeper layers, the 

 assimilatory tissue is placed towards the upper surface of the leaf- 

 blade. The carbon dioxide is mainly taken into the leaf through the 

 stomata of the lower surface. It can thus diffuse rapidly through the 

 wide intercellular spaces of the spongy parenchyma, which is essentially 

 a ventilating tissue, to the active assimilatory tissue of the upper side. 

 This will take place more rapidly the thinner the leaf is. 



The extensively-branched network of vascular bundles ensures the 



