12 INTRODUCTORY 



alone existed on the earth, all the oxygen in the 

 atmosphere^ would have been long ago exhausted 

 and replaced by carbon dioxide. But the whole 

 economy of nature depends on the fact that in plants 

 the process, in the main, is just the reverse. It is 

 true that plants respire exactly like animals, absorb- 

 ing oxygen and giving off carbon dioxide ; but in the 

 case of a green plant, we can only detect this process 

 in the dark. This is because, in the presence of 

 sunlight, it is masked by the opposite, and far more 

 vigorous, process of assimilation, in which oxygen 

 is given out and carbon dioxide absorbed. 



One further essential function of plants remains to 

 be discussed. All plants immersed in the atmosphere 

 are constantly losing water by evaporation — a loss 

 which has to be made good by absorption from the 

 soil by means of the roots. This process is known 

 as transpiration. It is not simply due to evaporation, 

 but is controlled by, and intimately connected with, 

 the vital processes of the plant. 



With Alpine plants the tendency to excessive 

 transpiration in summer is very marked, owing to 

 the high day temperature and the dryness of the 

 atmosphere. The water vapour passes out through 

 the pores or stomata which occur on the leaf (Plate 

 XXIX., Fig. 1) and sometimes on the stem and 

 flowers also. Consequently we shall find in many 



^ The atmospliere consists approximately of 23 parts by weight of 

 oxygen and 77 parts of nitrogen and a very small quantity of carbon 

 dioxide, about 3 to 6 parts in 10,000 measures. 



