BRIEF OUTLINE OF VEGETABLE PHYSIOLOGY 235 



the walls of the cells, and then diffuses throughout the green tissue. 

 Its goal is the chlorophyll granules. 1 Here, in sunlight, its particles 

 are torn apart, and the carbon atoms are combined with the atoms of 

 hydrogen and oxygen derived from the decomposition of water, to 

 form a carbohydrate. This carbohydrate, if not starch, is shortly 

 turned to starch as a rule, appearing as minute granules in the chloro- 

 plastids sometimes within five minutes after exposure of the plant to 

 light. These granules increase in size while assimilation continues; 

 but when assimilation ceases, as at night, the starch begins to be 

 dissolved, and is finally conveyed away in the form of a soluble 

 carbohydrate. Assimilation of carbon by aid of light is termed photo- 

 synthetic assimilation. 



556. The conditions that must be fulfilled before assimilation will 

 take place are these : Carbonic acid gas must be present in the atmos- 

 phere, there must be light and a certain amount of heat, and the 

 chloroplastids must contain chlorophyll. 



557. The atmosphere normally contains about .04 of one per cent 

 of carbonic acid gas, by weight. Increasing this proportion hastens 

 the rate of assimilation slightly; but if the gas is increased two 

 hundred fold, the formation of starch becomes only four or five times 

 greater. Ordinary variations in the amount of carbon dioxide would, 

 therefore, not perceptibly aid assimilation. 



558. Light furnishes the energy of assimilation. Of the different 

 components of white light, the red, orange, and yellow rays are the 

 most effective. 



559. Liberation of oxygen. In the act of assimilation, when 

 carbon is taken into the material of the plant, the oxygen of the 

 carbon dioxide is given off. In the case of water plants this may be 

 seen. Let a cut branch of such a plant be exposed to light under 

 water. Bubbles of oxygen will be seen escaping from the cut end. 

 The rapidity with which these bubbles are given off may be taken as 

 a convenient measure of the activity of assimilation in the given plant 

 under the given circumstances. If, for example, the plant is exposed 

 to one sort or one intensity of light for a period, and the number of 

 bubbles rising from it per minute is found, the conditions as to light 

 may then be varied, and the number of bubbles per minute ascer- 

 tained anew ; compared with the former result, the later count will 

 show whether the assimilative activity of the plant is greater, or less, 

 under the new conditions. 2 



560. The action by which substances like starch and protein gran- 

 ules, insoluble in the sap, are converted into soluble compounds is 

 digestion. In digestion, starch is changed to sugar. In the latter 



1 See Fig. 382, Chap. XVII. 



2 See Goodale, " Physiological Botany," p. 305, for more explicit direc- 

 tions. The experiments are most interesting. 



