THE FOOD OF GREEN PLANTS 77 



that sunlight is necessary to their health. In the age 

 of Galileo a Belgian physician and chemist, Van 

 Helmont, endeavoured to pursue the subject by experi- 

 ment. He planted the stem of a live willow in furnace- 

 dried earth, which was enclosed in an earthen vessel. 

 Rain-water or distilled water was supplied when neces- 

 sary, and dust excluded by a perforated lid. The loss 

 of weight due to the falling-off of leaves was neglected. 

 In the course of five years the tree was found to have 

 increased to more than thirty times its original weight ; 

 Van Helmont concluded that this increase was due to 

 water only. Malpighi (1671), being guided mainly by 

 his microscopic studies of the anatomy of the stem and 

 leaf, taught that moisture absorbed by the roots ascends 

 by the wood, becoming (apparently at the same time) 

 aerated by the large, air-conducting vessels ; that it 

 enters the leaves, and is there elaborated by evapora- 

 tion, the action of the sun's rays, and a process of 

 fermentation ; lastly, that the elaborated sap passes 

 from the leaves in all directions towards the growing 

 parts. It will be seen that this explanation, though 

 incomplete, makes a fair approximation to the beliefs 

 now held ; for more than a hundred years after 

 Malpighi's day less instructed opinions were commonly 

 held. Hales (1727) recognised that green plants are 

 largely nourished at the expense of the atmosphere ; 

 he dwelt also on the action of the leaves in drawing 

 water from the soil, and in discharging superfluous 

 moisture by evaporation. 



Joseph Priestley, who had been proving that air is 

 necessary both to combustion and respiration, made an 

 experiment in 1771 to discover whether plants affected 

 air in the same way that animals do. He put a sprig of 

 mint into a vessel filled with air in which a candle had 



