Chap. ii Photosynthesis 281 



changes involved ; the nature of the chlorophyll apparatus ; 

 the stages of the process of what was till lately known 

 as the assimilation of carbon dioxide, but which is now 

 spoken of as photosynthesis, and the supply and utilization of 

 the energy needed for the latter. These must be examined 

 separately. 



The Gaseous Interchanges. The early observations 

 to which attention has already been called>fforded_' proof 

 that a great deal of the substance of the green plant is 

 furnished by the carbon dioxide which the environment 

 affords it. De Saussure indeed concluded that the greater 

 part of this carbon dioxide is derived from the air, though 

 he thought a certain amount may be taken from the soil. 

 He was acquainted also with the fact that the quantities 

 of carbon dioxide absorbed and of oxygen exhaled by the 

 green plant are approximately equal. 



The problem of the mode of absorption of the gas 

 attracted considerable attention during the whole of the 

 period with which we are concerned. The larger question 

 of the whole of the gaseous interchanges between the green 

 plant and the air was in most cases the subject of 

 investigation, but for the present discussion we may con- 

 veniently postpone the consideration of respiration. 



Two views were prominently before the minds of physio- 

 logists during the whole period, and a somewhat vehement 

 controversy was continued down to almost the end of the 

 century. They may be referred to as the stomatal and 

 the cuticular theory respectively. 



The stomatal theory followed naturally some observations 

 and deductions made by Dutrochet as long ago as 1832. 

 He called attention to the continuity of the intercellular 

 spaces in the leaf, and their connexion with its exterior 

 by means of the stomatal apertures, and suggested for 

 them the function of a lung so far as the exhalation of 

 carbon dioxide is concerned. In 1850 Garreau also con- 



