232 TKANSPIKATION 



up in respiration and tissue formation, and consequently that levulose 

 enters the stem of the plant in larger quantities than dextrose. 



Usher & Priestley l have adduced evidence that the interaction of 

 carbon dioxide and water in the presence of chlorophyll yields, first of 

 all, formaldehyde and hydrogen peroxide : 



C0 2 + 3H 2 = HCHO + 2H 2 2 



that the hydrogen peroxide is quickly destroyed by enzymes which 

 they showed were present in all green leaves, evolving oxygen, and that 

 the formaldehyde is as rapidly polymerised, by the living protoplasm, 

 into carbohydrates. 



The formation of carbohydrates, though perhaps the most im- 

 portant function of the leaves, is by no means their only one. 



In addition there are two processes which may be briefly de- 

 scribed transpiration of water and formation of nitrogenous organic 

 substances. 



Transpiration is effected, as already stated, chiefly through the 

 stomata ; but exhalation of aqueous vapour occurs from almost all 

 the exposed parts of plants. The activity of transpiration depends 

 chiefly upon the temperature, the humidity of the air, and the amount 

 of light received by the plant. It is increased by a rise of tempera- 

 ture or by brighter light, but diminishes with greater humidity of 

 the air around. It is also regulated by the size of the apertures 

 the stomata through which the greater quantity of the transpiration 

 is effected. These openings are altered in size according to the greater 

 or less turgidity of the guard cells. 



In consequence of this escape of water from the leaves, a dimin- 

 ished pressure is often set up in the upper parts of a plant, so that 

 the root pressure is aided in driving water from below. Transpiration 

 is thus active in producing the rise of sap and the consequent bringing 

 up of the mineral matters absorbed by the roots into the leaves, there 

 to be elaborated into nutritive materials. 



The amount of water evaporated by a plant increases if the soil 

 water or culture fluid is very dilute. Oats were found to evaporate 

 515 grammes of water for each gramme of dry produce when grown 

 in a 3 per cent nutritive solution, but 688 grammes of water in 0'25 

 per cent solution. 2 



Direct determinations by the author of the water transpired by a 

 potato plant growing in soil, gave 467 grammes for each gramme of 

 solid matter formed. This was on unmanured soil in the very dry 

 atmosphere of the Transvaal in 1904. A parallel experiment, on soil 

 well manured with basic slag and potassium nitrate, gave 284 grammes 

 of water transpired for each gramme of dry matter formed. 



The building up of nitrogenous, proteid matter from carbohydrates 

 and nitrates or ammonium salts is not thoroughly understood. It is 

 generally stated that the presence of light is essential to this process 

 and that the production of proteids from nitrates and sulphates re- 

 quires the energy of light to bring about the necessary reduction of 



1 Proc. Koy. Soc., 1906, 77, 369 ; Jour. Chem. Soc., 1906, Abstracts, ii. 299. 

 2 Heinrich, Ann. Agron., 1897, 186; Jour. Chem. Soc., 1897, Abstracts, ii. 424. 



