v.] BOTANY. 



of water as already explained (Par. 15, c\ the substance 

 known as starch is formed. 



27. Transpiration. The sap, on reaching those 

 surfaces of plants that are exposed to the light, parts 

 with a great deal of its water as vapour, either 

 through minute pores in the leaves, or through the 

 walls of the superficial cells, as in the case of plants 

 that have no leaves. These pores are called stomates 

 (Par. 72) they exist in very great numbers, chiefly on 

 the underside of the leaves; an apple-leaf presents 

 upwards of 100,000 of them. This process of evapo- 

 ration, called transpiration, keeps plants cool in the 

 hottest weather, and is so rapid that a sunflower plant 

 has been found to give off a quart of fluid in twenty- 

 four hours, and an oak or beech-tree must give off 

 many gallons in the same space of time. 



28. Assimilation. The process by which the 

 carbonic acid absorbed by the leaves and the water 

 absorbed by the roots are combined together in the 

 leaves under the influence of sunlight to form starch, 

 free oxygen being at the same time given off, is called 

 assimilation. The starch so formed appears to be 

 dissolved in the cell-sap during darkness, and to be 

 distributed from cell to cell all over the plant. It is 

 used up wherever growth is taking place, furnishing 

 the material for the formation of the cellulose of the 

 new cell-walls, or it is stored up again in the solid 

 form as a reserve of material for future use, as in seeds. 

 Besides conversion into cellulose, starch is capable of 

 being transformed under the influence of protoplasm 

 into oily and fatty matters, and also into sugar. 



The soluble starch in its downward passage through 

 the tissues of the stem, meets with various saline 



