204 Plant Physiology 



pacity to absorb light, that is, radiant energy to which the 

 retina of the eye is sensitive. The relation of the chloro- 

 phyll and of its main constituents to the absorption of 

 light of different wave lengths, as shown by a spectro- 

 scopic examination, is discussed later. The radiant energy 

 absorbed by the chlorophyll is the force operative in photo- 

 synthesis. 



109. The factors essential in photosynthesis. We 

 may now review briefly the essential features of the process 

 whereby chlorophyll-containing plants in the presence of 

 light are able to construct organic food-materials. The 

 process is termed photosynthesis. In order that photo- 

 synthesis may proceed in the cells of healthy plants, it is 

 necessary that light shall fall upon chlorophyll bodies in 

 the presence of aqueous carbon dioxid. Temperature and 

 other factors are important, the exact relation to tem- 

 perature being especially difficult to analyze, and in 

 general the process is possible only within a certain range 

 of physiological conditions. Nevertheless, under ordinary 

 conditions of growth we may regard as the primarily 

 essential factors: (1) chlorophyll, (2) light, and (3) carbon 

 dioxid, the last two of which will receive further consider- 

 ation later. 



110. The course of photosynthesis. Briefly stated, 

 the gas exchange and the actual phases (several of which 

 are more or less simultaneous) of the process of photo- 

 synthesis as commonly conceived are as follows : 



1. Gas exchange between the green tissues and the 

 surrounding air, whereby carbon dioxid may be absorbed 

 by the cell-sap and reach the protoplasm. 



2. The absorption of radiant energy, as light, by means 

 of the chlorophyll bodies. 



