CH. IV.] THE LEAVES. 107 



three feet high, perspired lib. 14 oz., and spearmint 

 exhales 1^ times its weight in the same period. But 

 of all the plants the diumal perspiration of vrhich 

 has been ascertained, the coiTieHan cheriy f Coniiis 

 masculaj transpires the most ; the exhalation amount- 

 ing to nearly twice the weight of the plant in twenty- 

 foui' hours. This aqueous expiration takes place 

 chiefly duiing the day ; is much promoted by heat, 

 and checked by rain, or a reduction of temperature. 



On the free-performance of this function of plants, 

 their health is dependent in a very high degree ; and 

 I believe that half the epidemics to which they are 

 subject arise from its derangement. That consequence 

 of the clubbing of the roots of the brassica tribe called 

 fingers and toes arises, I consider, entu*ely from it. 

 In the drought of summer, when the moisture sup- 

 plied to a club-rooted cabbage by its root does not 

 nearly equal the exhalation of its foliage, to supply 

 this deficiency the plant endeavours, by forming a 

 kind of spurious bulbous root, to adapt itself to the 

 contingency ; in the same manner that in dry situa- 

 tions, the fibrous roots of Phleum pratense, Alopecu- 

 rus geniculatus, &c., acquire a tuberous form, because 

 bulbous or tuberous-rooted plants, it is well known, 

 will exist in a soil so deficient in moisture as to de- 

 stroy all fibrous-rooted vegetables. 



Evergreens transpire less moisture than deciduous 

 plants ; which would lead to the expectation that 



