Chapter 2 



The Nature of Photosynthesis as Determined by 



Observations of Gas Interchange and the 



Formation of Organic Matter 



1. The Gaseous Interchange 



The phenomenon of photosynthesis was discovered as a result of 

 investigations on the composition of the atmosphere and the influence 

 of plants on the same. Thus the study of the problems of gaseous inter- 

 change or the efifect of green plants on the composition of the air sur- 

 rounding them has been so intimately associated with the development of 

 our conceptions of the phenomenon of photosynthesis, that a consideration 

 of this subject is of primary importance both on account of its direct 

 bearing and because of its historical significance. In fact it can be 

 said that historically considered the subject of plant physiology had its 

 real inception as a recognized branch of experimental science with the 

 study of the interchange of gases by de Saussure. 



a. The Path of Gaseous Exchange. 



In submerged aquatic plants the carbon dioxide reaches the interior 

 of the plant by means of diffusion of the dissolved gas through the outer 

 walls of the epidermal cells. This is probably also the case in many of 

 the lower land plants. In the leaves of the higher land plants, on the 

 other hand, there are special differentiated organs, the stomata, through 

 which gaseous carbon dioxide passes to the chlorophyll-bearing cells and 

 although the existence of stomata was known for a long time their func- 

 tion in photosynthesis was a subject of great dispute for many years. 

 It had to be determined what portion of the carbon dioxide used by the 

 plant actually passed through the stomata and what portion diffused 

 directly through the outer cuticle of the leaf. 



In brief, the stomata are minute mouth-like openings in the surface 

 of the leaf. They are usually on the under side of the leaf though in 

 some species the upper surface also has stomata. There exists consider- 

 able variation in the structure of the stomata according to species and 

 habitat of the plant.' The cross diameter of the stomata is exceedingly 

 small, 0.0006-0.02 mm. Brown and Escombe ^ found the average area 



' Haberlandt, G., Physioloqische FHanseiumatomie . Leipzig, 1904, p 395 

 "Brown and Escombe, Phil. Trans. Roy. Soc. B., 193, 275 (1900). 



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