204- now <jRops FBEa 



lieved that tlie air surrounding tlie plants in these experi- 

 ments was saturated Avith vapor of Avater, and concluded 

 that heat was developed within the plant, which caused 

 vaporization. More recently, Boelun [Sltzungsherichte 

 der Wiener Akad., XLVITI, 15) has made probable that 

 the air was not fully or constantly saturated Avitli moist- 

 ure in these experiments, and by taking greater precau- 

 tions has arrived at the conclusion that transpiration abso- 

 lutely ceases in air saturated with acpieous A\apor. 



d. The condition of the tissues of the x)lant^ as depend- 

 ent upon their age and vegetative activity, likewise has a 

 marked effect on transpiration, Lawes* and Knop both 

 found that young plants lose more water than older ones. 

 This is due to the diminished power of mature foliage to 

 imbibe and contain Avater, its cells becoming choked up 

 with growth and inactive. 



e. The character of the tnedium in xohich the roots are 

 situated also remarkably influences the late of transpira- 

 tion. This fact, first observed by Mr. Lawes, in 1S50, loc. 

 cit., was more distinctly brought out by Dr. Sachs at a 

 later period. ( Vs. jSt., I, p. 203.) 



Sachs experimented on A'arious i)lants, amz. : beans, 

 squashes, tobacco, and maize, and observed their transpi- 

 ration in weak solutions (mostly containing one per cent) 

 of nitre, common salt, gypsum, (one-fifth per cent solu- 

 tion) and sulphate of ammonia. He also experimented 

 with maize in a mixed solution of jihosphate and silicate 

 of potash, sulphates of lime and magnesia, and common 

 salt, and likewise observed tlie effect of free nitric acid 

 and free j^otash on the squash ])lant. The young plants 

 were either germinated in the soil, then removed from it 

 and set Avith tlieir rootlets in the solution, or else Avere 

 kept in the soil and Avatered with the solution. The glass 



* Experirnental Investigation into the Amount of Water given ojf by Plant$ 

 flaring tlieir Growtfi, by J. B. Lawes, of Rothauistead, London, 1S50. 



