PHYSIOLOGICAL 305 



photosynthesis of green leafage is the all-sustaining process of life, 

 fundamental (with but practically trifling exceptions) to our whole 

 living world, plant, animal, and human. 



Our agriculture and horticulture have of course always depended 

 on this process; but even our present knowledge, clear, though of 

 course still very incomplete in its chemical and physical detail, 

 dates only from the later eighteenth century. 



We should pause here to recall some of the steps by which a 

 general recognition of the import of photosynthesis was reached. 

 There had been sporadic suggestions from time to time, as when 

 Cesalpino compared the vessels of plants to veins in animals, when 

 Helmont proved that green plants did not get all their food-materials 

 from the soil, when Malpighi suggested that leaves elaborated the 

 crude sap, when Mariotte compared the ascent of soil-water in the 

 plant to the phenomena of capillarity, and so on ; but the beginning 

 of precise vegetable physiology practically dates from Stephen 

 Hales (1677-1761), who, with a rigorous scientific mind, insisted on 

 measurement and measurement. By ingenious experiments he 

 "made his plants themselves speak", and besides his fundamental 

 work on the ascent of sap, he was the first to prove up to the hilt 

 that a great part of the food-material of green plants must be 

 derived from the air. 



The next impulse came from chemistry, which Lavoisier had 

 begun to reorganise. In 1774 Priestley (1733-1804) had discovered 

 or re-discovered oxygen, and five years later he showed that this 

 gas was, in the sunlight, exhaled by green plants. In the same year 

 Ingenhousz (1730-1799) took an even bigger stride, emphasising the 

 facts that the oxygen was given off only by green parts and only in 

 the sunlight; that this is quite distinct from another process (of 

 respiration) in which plants, like animals, give off carbonic acid gas; 

 and that the chief source of the carbon found in the carbon com- 

 pounds of plants, e.g. in wood, was to be found in the carbonic acid 

 gas of the atmosphere. 



In 1800 Senebier (1742-1809) corroborated Ingenhousz's discovery 

 of the decomposition of carbon dioxide by green plants in the light. 

 More important, however, was the work of Theodore de Saussure 

 (1767-1845), son of the famous explorer of the Alps, who introduced 

 the quantitative method of estimating a plant's income and expendi- 

 ture, and thereby showed that the elements of absorbed water 

 become fixed in the plants as well as the carbon of the carbon 

 dioxide; that respiration is as essential to plants as to animals and 

 is related to the internal heat of some plants, as within the spathe 

 of Cuckoo Pint and inside some flowers; that plants are unable to 

 use the free nitrogen of the air (a conclusion now modified by the 

 discovery of some nitrogen-fixing Bacteria and Fungi); and that 

 there is no normal nutrition for plants apart from nitrates and other 



VOL. I X 



