TUB FKEE WATEK OF THE SOIL. 207 



although there be no ascending .-Kiueous curi-ent. (H. C. 

 G., ijp. 288 and 340.) 



In accordance Avith these views, vegetation grows as Avell 

 in the confined atniosplierc of green-houses or of Wardian 

 Cases, Avhere tlie air is for tlie most part or entirely satu- 

 rated with vapor, so that transpiration is reduced to a mini- 

 mum, as in the free air, where it may attain a maximum. 

 As is well known, the growth of field crops and garden 

 'Tegetables is often most rapid during damp and showery 

 weather, when the transpiration must proceed with com- 

 parative slowness. 



While the above considerations, together with the asser- 

 tion of Knop,that leaves lose for the first half hour nearly 

 the same quantities of water under similar exposure, 

 whether they are attached to the stem or removed from 

 it, whether entire or i:i fragments, would lead to the con- 

 clusion that transpiration, which is so extreinely variable 

 in its amount, is, so to speak, an accident to the plant and 

 not a process essential to its existence or welfare, there 

 are, on the other hand, facts which appear I > indicate the 

 contrary. 



In certain experiments of Sachs, in which the roots of 

 a bean were situated in an atmosphere nearly saturated 

 with aqueous vapor, the foliage being exposed to the air, 

 although the plant continued for two months fresh and 

 healthy to ajjpearance, it remained entirely stationary in 

 its development. ( Vs. St., I, 237.) 



Knop also mentions incidentally {Vs. St., I, 192) that 

 beans, lupines, and maize, die when the whole pU^it is 

 kept confined in a vessel over water. 



It is not, however, improbable that the cessfition of 

 growth in the one case and the death of the plants in the 

 other were due not so much to the checking of transpii-a- 

 tion, which, as Ave have seen, is never entirely suppressed 

 under these circumstances, as to the exhaustion of oxygen 

 or the undue accumulation of carbonic acid in the narrow 



