THE LEAF 81 



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

 walls, saturated with water, shall be exposed for aeration. If simply 

 and directly exposed to the atmospheric air they would quickly dry up 

 and shrivel, but they are 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 : by means of the intercellular spaces that 

 permeate the mesophyll it is extended onwards to the individual cells. 

 The epidermis also 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 blade is further stiffened by 

 the framework of the midrib and veins. (Fig. 46.) These illustrate in 

 modified form the same methods of mechanical strengthening as the 

 stem itself. (Figs. 45, 47.) 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 

 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 water 

 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 or forwards between the axis and its distal 

 points, which is secured by its conducting system : at the same time 

 it exposes a large area cf green tissue to the light, and affords to each 

 cell ready access to the atmospheric air. 



b.b. h 



