THE LEAF. 71 



is the formation of organic food products out of the crude 

 substances taken in from the atmosphere and Fnnotiona. 

 soil. In the presence of sunlight starch is Assmilatioa. 

 produced in the chlorophyll bodies. The materials from 

 which it is formed are carbon dioxide, obtained from 

 the atmosphere, and water brought up from the roots. 

 The starch accumulates in the daytime in thfe cells where 

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

 form to the various reservoirs of reserve materials. This 

 is shown by such experiments as have been described in 

 the practical exercises and by microscopical examination. 

 The local nature of the process of assimilation is easily 

 demonstrated by excluding the light from a part of a leaf 

 and leaving the rest exposed to its action. The assimilat- 

 ing cells of the part acted upon by light are afterwards 

 found to contain starch, while those not thus acted upon 

 are destitute of it. A cell of a filamentous alga, such as 

 Spirogyra, kept in the dark until its starch has wholly dis- 

 appeared, shows in the course of a minute or two the pres- 

 ence of starch in its chlorophyll band after it has been 

 brought into sunlight. Other forms of experimental proof 

 have been employed, but are hardly necessary. 



Water in greater or less 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 iranapira- 

 other products. The surplus water is evapo- *^™' 

 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. 



