TRANSPIRATION. 237 



In addition there are two processes which may be briefly 

 described 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 parts of 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 temperature 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 

 diminished 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 % nutritive solution, but 688 

 grammes of water in 0'25 % solution." 



The building up of nitrogenous, proteid matter from carbo- 

 hydrates 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 requires the energy of light to 

 bring about the necessary reduction of nitrogen and sulphur 

 from their oxidised compounds.! Experiments made recently, 

 especially in Japan, show that barley, French beans, and pota- 

 toes are able to produce proteids from nitrates in complete 

 darkness, provided they be supplied with a sufficient amount 

 of sugar ; with small quantities of sugar no proteids were 



* Heinrich, Ann. Agron. 1897, 186 ; J.C.S. 1897, abst. ii. 424. 

 t v. (i<lk'\vski. Jour. Chem. Soc. 1897, abst. ii. 583. 



