THE CONDITIONS OF PLANT LIFE 21 



bearing leaves. This is one of many instances where 

 in response to similar needs there has been a parallel 

 development in groups which genetically are widely 

 separated. 



It is, of course, among the Spermatopbytes (seed- 

 bearing or flowering plants) that the highest degree 

 of specialization, both of the plant body and tissues, is 

 reached. The plant usually shows a definite main axis 

 or stem, to which are attached a variety of appendicular 

 organs — leaves, roots, and branches. The tissues of 

 which these various organs are composed show much 

 variation in the cells of which they are made up. The 

 green tissue is mainly restricted to the leaves, where 

 it is so placed as to be most favorably situated 

 with reference to the light. As a rule the outer or 

 epidermal cells are not provided with chloroplasts, but 

 serve as a protection for the delicate green cells lying 

 below them, and in case the plant is exposed to great 

 heat or dryness, the epidermal cells become much 

 thickened and almost impervious to water, so that the 

 loss of water from the green cells is effectively checked. 

 Familiar examples of this kind are seen in the leaves 

 of the laurel, oleander, and many other evergreens. 

 As it is necessary, however, for the green cells to have 

 communication with the atmosphere in order to obtain 

 the necessary carbon dioxide and oxygen, this is pro- 

 vided for by the development of the stomata or breath- 

 ing pores always found upon the leaves, and these 

 communicate with the numerous air-spaces between 

 the green cells which are thus brought directly into 

 contact with the atmospheric gases. 



Within the green cells the decomposition of the car- 



