FOLIAGE LEAVES. 47 



lecture to turn our attention to some other points in the organisation of the foliage- 

 leaves. 



The physiologically important part of any foliage leaf is the blade (lamina), 

 a lamella, consisting of several layers of tissue, in the typical case however always 

 very thin, a few tenths of a millimeter thick, in which the cells containing chlorophyll,' 

 the so-called mesophyll, play the chief part. Upon the activity of this layer con- 

 taining chlorophyll the structure of the foliage-leaves depends. 



All other arrangements in a foliage leaf have as their object the presenting of 

 this very thin layer of tissue, extended flat, to the light, of moderating and regu- 

 lating according to requirement the too rapid evaporation of the water out of it, 

 of bringing about the flow of new assimilable matters to the cells containing 

 chlorophyll, and making possible the return flow of the products of assimilation 



FIG. 43.— Venation of the foliage leaf of ^sarum Europaiim (after Ettinghausen). 



to the shoot-axis, as well as, finally, of protecting these thin lamellae of tissue against 

 being torn under the influence of the wind. All the relations of organisation of the 

 foliage leaves are intelligible from this point of view. 



The epidermis of foliage leaves is the most completely organised which is 

 found on plants. On their upper and lower sides, it envelopes the thin lamella 

 of tissue containing chlorophyll: it protects it by means of its elasticity and 

 solidity: its strongly developed cuticle prevents a too rapid evaporation of the 

 water flowing to the mesophyll : its innumerable stomata, the openings of which 

 widen or close according- to need, allow to the watery vapour arising in the inter- 

 cellular spaces of the mesophyll a passage out into the atmosphere, regulated 

 according to need, and at the same time facilitate the entrance of the carbon 

 dioxide, and, after its decomposition, the exit of the oxygen. Hairy coverings of 



