234 LEAVES 



respiration, carbohydrate synthesis, and transpiration. 

 Respiration, involving the absorption of oxygen and the 

 emission of carbon dioxide, takes place in nearly all plants 

 at all times, though it is slight, or even wanting, in resting 

 organs, such as seeds. Carbohydrate synthesis, involving 

 the absorption of carbon dioxide and the emission of oxygen, 

 is confined essentially to chlorophyll-bearing organs in the 

 presence of sunlight. Transpiration, involving the emis- 

 sion of water vapor, occurs in all aerial organs ; principally 

 in the leaves. Transpiration involves much the greatest gas 

 movement, respiration much the least." (Cowles's Ecology.) 



57. Structure. — We are now to consider the inner 

 appearance of leaves. We are to study the construction 

 of that apparatus whereby principally plants are related 

 to air and light, and wherein principally they make their 

 food. We are to note how this apparatus appears to fit 

 these uses. 



A . Epidermis. — The leaf of a tulip or hyacinth or some 

 similar fleshy leaf is best for studying the epidermis. From 

 such a leaf the thin, transparent, whitish skin may be 

 easily peeled off. 



The cells of the epidermis are usually about the same 

 size. Except for those which border the stomates, they 

 do not contain chlorophyll. Their outer walls, those 

 which form the leaf's surface, are usually a good deal 

 thicker than those walls which are not so exposed. (See 

 Figure 79.) It is in connection with this outer wall that 

 there occur those secretions of cutin or wax or those out- 

 growths of hairs or spines which you have already noted. 

 It is evident that the epidermis is a tissue whose use to 

 the plant appears to be for protection exclusively. 



