70 BOTANY OF THE LIVING PLANT 



are those of the mesophyll, and it is essential that their thin cellulose 

 walls, saturated with water, shall be exposed for aeration. But if 

 simply and directly exposed to the atmospheric air they would quickly 

 dry up and shrivel. They are therefore protected on either side by 

 the epidermis, which is a continuous layer covered by the impervious 

 cuticle. Access to the atmosphere is still maintained, though under 

 control, through the numerous stomata, and it extends onwards 

 to the individual cells by means of the intercellular spaces. Another 

 point of importance is that the epidermis gives mechanical strength. 

 The mesophyll with its thin walls and spongy texture would by itself 

 be too weak to maintain its form, and resist the impact of winds. It is 

 bound together by the firmer epidermis. The thin expanse of the bhxle 

 is further stiffened by the framework of the midrib and veins. (Fig. 43.) 

 These illustrate in modified form the same methods of mechanical 

 strengthening as the stem itself. (Figs. 42, 44.) Often the blade is 

 strengthened also by a marginal band of hardened tissue, which acts 

 like a hem. Lastly, the whole lamina is attached to the leaf-stalk, 

 which, though sufficiently rigid to support it, will yet yield to the 

 impact of wind, and so avoid the risk of mechanical damage. 



On the other hand the conducting system is continuous from the 

 axis outwards through the leaf-stalk, and on through the midrib and 

 veins to the ultimate branchlets which ramify throughout the meso- 

 phyll. Thus there is efficient provision for the transit of material 

 from the axis to the remotest points of the lamina, and conversely 

 materials may also be transmitted backwards from these to the leaf- 

 base and into the axis. Such transit does actually take place, the 

 first through the woody tract of the vascular strand, the other through 

 the bast. In short, a foliage leaf is fundamentally a structure 

 with adequate provision for mechanical strength, and for the transit of 

 materials backwards and forwards between the axis and its distal 

 points, which is secured by its conducting system : while it exposes 

 as large an area as possible of green tissue to the light, with ready 

 access to the atmospheric air. 





