176 APPLICATIONS OP THEORY TO PRACTICE. 



are many others, which will readily arise from the knowledge of 

 the character of this function, and of the causes on which it is 

 dependent. Thus we learn from it, that the operation of trans- 

 planting should not be performed in the summer, when the 

 exhalation is most active ; since, the roots being always injured 

 in greater or less degree, the process of absorption is imperfectly 

 performed, and cannot supply the loss by exhalation ; so that 

 the plant is dried up. This evil may in some degree be guarded 

 against, by watering the plant copiously ; but it is much better 

 to make the change either in early spring, or in the latter part 

 of the autumn ; when most plants are destitute of leaves, and 

 therefore scarcely exhale at all ; and when the function is per- 

 formed with much inferior energy, in those which possess them. 



268. Again, we see the reasonableness of the practice, which 

 has been long known as a useful one, of keeping a nosegay, the 

 freshness of which it is desired to preserve, in a dark room ; 

 since the check thus put to the exhalation which takes place, 

 not only from the leaves, but also from the leafy surfaces of 

 flowers, prevents the rapid withering which will otherwise occur. 

 Even the light of lamps and candles is to a certain extent 

 effectual, in maintaining this function ; so that this is to be 

 avoided, where, for any particular occasion, flowers which have 

 been picked are to be preserved as nearly as possible in their 

 previous blooming state. Such a plan, however, would prevent 

 the expansion of any buds which the nosegay might include ; 

 since this (both in leaf-buds and flower-buds) depends upon the 

 vigour with which the process is performed, and is hastened 

 by light. 



269. The water exhaled by plants is very nearly pure ; so 

 that what is furnished by different species, varies extremely little 

 either in taste or odour. It has been remarked, however, that 

 fluid thus obtained becomes foul sooner than ordinary water ; 

 and this is the case wherever organic matter, even in extremely 

 minute proportion, is diffused through the fluid. The quantity 

 of solid matter contained in 40 ounces of the liquid, exhaled 

 from a Vine at the commencement of the summer, has been 

 found to be only two grains ; and no more than this was con- 



