THE LEAF. 71 



is the formation of organic food products out of the crude 

 substances taken in from the atmosphere and F unc tions. 

 soil. In the presence of sunlight starch is Assimilation, 

 produced in the chlorophyll bodies. The materials from 

 Avhich it is formed are carbon dioxide, obtained from 

 the atmosphere, and water brought up from the roots. 

 The starch accumulates in the daytime in the cells where 

 it is formed, and afterwards is conveyed away in a soluble 

 form to the various reservoirs of reserve materials. Simple 

 experiments have shown the conditions under which the 

 formation of starch takes place and the attendant phe- 

 nomena. The rapid evolution of oxygen seen when a 

 water plant is allowed to stand in 'bright sunlight is at 

 once checked when the vessel containing it is brought 

 into the shade. The oxygen is given off in the formation 

 of starch arid this process ceases when light is wanting. 

 Again, if the water in which the plant is growing is boiled 

 so as to expel the carbon dioxide, it is observed that the 

 evolution of oxygen ceases as in the preceding experiment, 

 but for a different reason. The carbon dioxide being 

 wanting, the leaves are deprived of the carbon necessary 

 to the production of starch. 



Water in relatively large quantities is required to carry 

 to the leaf, and to the other parts of the plant, the sub- 

 stances used in the formation of starch and Transpira- 

 other products. The surplus water is evapo- tion ' 

 rated by the leaves. By simply weighing at stated inter- 

 vals a plant arranged so that evaporation can take place 

 from no other part, it is found that large amounts of 

 watery vapor are given off through the leaves. Transpira- 

 tion, then, or the evaporation of water, is another important 

 function of leaves, since the water thus given off is the 

 vehicle of transportation of the various substances used by 

 the plant, 



