THE PHYSIOLOGY OF NUTRITION 121 



a portion of a green plant, be placed under a bell jar arranged 

 over mercury, and containing a measured mixture of air and 

 carbon dioxide, and be then ex- 

 posed to light for a few hours, the 

 volume of the gas under the jar 

 will remain unaltered. 



If after the experiment the gas 

 is analysed, there will be found to 

 be less carbon dioxide, but more 

 oxygen than at the commencement 

 of the experiment. This shows 

 that the plant had taken in carbon 

 dioxide and given out as much FJG I44- _ Stem and leaves of 



Oxygen as the Carbon dioxide Water Cress giving out oxygen 



1 in water. The leaves and stem 



taken in. have been cut to allow of the 



If a leaf which possesses no escape of the oxygen. 



stomata on the upper surface has 



a portion of its lower surface coated with wax or vaseline 



so that no air can enter through the stomata, no starch can 



be found in the covered area, while the adjacent parts of 



the leaf become rich in starch. This seems to point to 



the conclusion that the carbon dioxide enters through the 



stomata. 



Recent research shows that : 



1. Under normal conditions, practically the sole pathway for 

 carbon dioxide into or out of the leaf is by the stomata. 



2. Under abnormal conditions, when the stomata or inter- 

 cellular spaces are blocked, and the pressure of the carbon 

 dioxide is great enough, it may pass through the cuticle. 



Assimilation. This term is used for all the nutritive 

 processes which go on in animals, but in botany it is restricted 

 to the taking in of carbon dioxide by the chloroplasts and sub- 

 sequent changes which they produce in it. All the other 

 processes of the plants depend upon the assimilation of carbon 

 dioxide. 



Conditions for Assimilation. Assimilation by green 

 plants can only take place under the following conditions : 



i. A certain intensity of light (either sunlight or electric 

 light will do). 



2. A certain temperature, at least a few degrees above the 



