EVAPORATION. 337 



when the tree finds itself in a new situation, is to create new 

 mouths by which to feed when the season for growing again 

 returns. 



EVAPORATION. 



Evaporation takes place in plants to an inconceivable de- 

 gree in certain circumstances. It is known by the experi- 

 ments of Dr. Hales, that a sunflower plant will lose as much 

 as 1 Ib. 14 oz. by perspiration in twelve hours ; and that in 

 general, " in equal surfaces and equal times, a man would 

 perspire ^ , the plant yJ-^, or as 50 : 15 ;" and that taking 

 all things into account, a sunflower perspires 17 times more 

 than a man. The same most accurate observer found 

 that a cabbage perspired in twelve hours 1 Ib. 9oz. ; a Pa- 

 radise Stock in a pot, 11 ounces ; and a Lemon Plant 8 oz. 

 Guettard states that he found Cornus Mascula perspire twice 

 its own weight in a day ; and Mr. Knight has remarked a 

 Vine in a hot day losing moisture with such rapidity that a 

 glass placed under one of its leaves was speedily covered 

 with dew, and in half an hour the perspiration was running 

 off the glass. In damp or wet weather this evaporation is 

 least ; in hot dry weather it is greatest. This loss has all 

 to be supplied by the moisture introduced into the system 

 by the spongioles ; and hence, if the spongioles are destroy- 

 ed, and evaporation takes place before they can be replaced, 

 a plant must necessarily die. This is the reason why decidu- 

 ous trees cannot be transplanted when in leaf; it is impossi- 

 ble to remove them without injuring their spongioles, and it 

 is equally impossible to hinder the evaporation by their 

 leaves : but if they are kept in pots, it matters not at what 

 season their removal takes place, because as their spon- 

 gioles are then uninjured, even excessive evaporation would 

 be made good by their action. It is well known that cer- 

 tain evergreens, such as Hollies, Laurels, &c., can be 

 transplanted in almost all months ;* this arises from their 

 perspiration being much less copious than in deciduous 

 trees, wherefore the spongioles have less difficulty in sup- 

 plying the loss occasioned by it; yet even evergreens 

 cannot be removed in the hottest months in the year, be- 



* Not exactly'correct : Evergreens, no more than deciduous trees can he trans- 

 planted at all periods, nor " in almost all months." In their growing season, they 

 generally grow very rapidly, and at these periods removal of them would be very 

 dangerous, if not fatal. j m . Ed. 



29 



