64 JOURNAL OF THE 



Plants should not be watered on the leaf unless the soil is likewise 

 moist. The small amount of extra transpiration caused by de"^ can 

 do no harm, as it is almost certain that the ground will be suffi- 

 ciently moist to supply the requisite amount of water. The action 

 of rain is more beneficial still, for then the supply of plant food is 

 most rapid.'' 



The same author notes (Bied. Centr. 1884, 43,) that in most 

 plants the leaves transpire moisture in larger quantities than the 

 flowers, and as a rule cut flowers wither more slowly than leaf twigs. 

 If the transpiration of the leaves is arrested the cut flowers will re- 

 main a long time fresh as when severed, so that it would seem the 

 flowers are deprived of moisture by the leaves. 



Johnson says that the wilting of a plant results from the fact' that 

 the leaves sutler water to evaporate from them more rapidly than 

 the roots can take it up. The speedy revival of a wilted plant on 

 the falling of sudden rain or on the deposition of dew depends not 

 so much on the absorption by the foliage of the water that gathers 

 on it as it does on the suppression of evaporation, which is a conse- 

 quence of the saturation of the surrounding air with moisture. 



Bochen, as quoted by Johnson, (How Crops Feed, p. 204,) has ar- 

 rived at the conclusion that transpiration absolutely ceases in air 

 saturated with aqueous vapor. Yet Unger has shown that plants 

 lose weight in air saturated as nearly as possible with vapor when 

 their roots are not in contact with soil or liquid water, and Duchartre 

 has shown that plants do not gain, but sometimes lose, weight when 

 their foliage only is exposed to dew or even to rain, although they 

 increase in weight when rain is allowed to fall upon the soil in w^hich 

 they are planted. 



Hoffman says that dew entirely prevents transpiration. No crit- 

 icism ot these is possible without reference to the originals. I will 

 merely recount some of my own experiments. 



To see if transpiration ceased in a saturated atmosphere, weighed 

 leaves of Morning Glory (Convolvulus Major) were suspended in a 

 bell-jar over a saucer of water. The ground edge of the jar fltted 

 closely to a ground glass plate. The jar was fltted with glass tubes 

 so that air could be sucked in. This air first passed through .3 — .4 

 metre of water and was used to fill the jar with saturated air imme- 

 diately after putting in the leaf. Two experiments were made, the 

 leaves remaining 8 anrl 9 hours at a temperature of about 25°C. 



I. Weight of leaf 2.8490 ; loss .0870, or 3.02 per cent. 

 II. Weight of leaf 1.1820; loss .0090, or .76 per cent. 



The loss is, then, a decided one. The leaves of the convolvulus 



