TRANSPIRATION 



41 



is given off from leaves, but only from the surface provided 

 with stomata, consequently the stomata must be regarded as 

 the openings through which the vapor escapes. The term 

 transpiration is apphed to this process by which water in the 

 form of vapor escapes from the leaves. 



85. If the atmosphere surrounding a green plant growing in a 

 closed chamber and exposed to the sunlight be tested from time 

 to time for carbon dioxide and 



oxygen it will be found that the 

 percentage of the former gas 

 decreases, while that of the 

 latter increases. Other tests 

 will further show that the car- 

 bon dioxide is absorbed and 

 assimilated by the leaf, in which 

 process an excess of oxygen over 

 that required by the plant is set 

 free in the leaf, and, if the leaf 

 is immersed in water, the oxy- 

 gen may be seen to collect on ^^^ ^^_^^ Dj^g,^^ of stoma in 



the surface of the leaf in bub- open and closed condition (heavy 



lines represent stoma open). B, C, 

 bles. Ihese gases are not ab- and D, successive stages in the de- 



SOrbed or eliminated through stev^ens'after'' Sachs ) '^''"'^' ^^''''"' 



epidermal surfaces having no 



stomata, consequently we must conclude that the stomata 



give passage to carbon dioxide and oxygen, as well as to water 



vapor. 



86. The rate of transpiration of water vapor is controlled 

 by an automatic opening and closing of the stomata. Excessive 

 transpiration results in the wilting of the leaf, which means that 

 the cells having lost some water are less turgid. The cells which 

 guard the stoma on either side are so constructed that with 

 increased turgidity they open the stoma, while with loss of 

 turgidity the stoma is closed. Of course the rate of absorption 



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