70 LESSONS IN CHEMISTRY. 



100. The little openings or pores, by which the membrane 

 that covers the leaves is perforated, and the spongy ex- 

 tremities of the rootlets are the channels through which, in 

 the form of carbonic acid, ammonia, and water, the growing 

 plant is supplied with the carbon, nitrogen, hydrogen, and 

 oxygen, which compose the bulk of its structure, and in the 

 water sucked up from the soil it also receives the saline and 

 earthly substances that we discover in its ashes (42). When 

 our cultivated crops, at this stage of their growth, find in the 

 soil an abundant supply of the peculiar matters which they 

 require, they rapidly increase in size, and produce more 

 numerous leaves and rootlets, and are thus enabled to appro- 

 priate more of the food which the air affords, but when these 

 matters are not present in sufficient quantity in the soil, or 

 are locked up in an insoluble form, it is impossible for them to 

 come to perfection. 



101. The gaseous and inorganic matters which, dissolved in 

 water, the roots pump up from the soil, are conveyed by means 

 of the vertical tubes, which compose the wood of the stem 

 (86), to the leaves, and being there diffused through the net- 

 work of branching vessels that cover the surface of the leaf, the 

 solution (the sap) experiences an important chemical change, 

 and also loses a considerable amount of water by evaporation. 

 Thus altered, the sap descends by the vessels which run 

 along the under smface of the leaf to the inner bark, where 

 it contributes to the development of the plant. The quan- 

 tity of water which is separated from the leaves of a plant 

 by evaporation, must powerfully influence its growth, for in 

 proportion as it escapes into the air, more water will be 

 sucked up by the roots from the soil, and thus new supplies 

 of the dissolved gases and inorganic matters be conveyed to 

 the leaves to be converted into food. When from a continu- 

 ance of dry weather the soil becomes incapable of supplying 

 the water evaporated, the leaves are seen to shrivel up, and 

 the growth of the plant is retarded. Dr. Hales calculated 

 that the water converted into vapour by the leaves of a plant, 

 was seventeen times more than that given off in perspiration 

 from the human body. He found by experiment, that a sun- 

 flower lost one pound four ounces and a cabbage one pound 

 three ounces a day by evaporation. 



102. It is in the leaves that the sap undergoes those 

 changes in its composition by which it becomes capable of 



