CHAP. II. FUNCTION OF NUTRITION. 185 



of this character, will even push considerable shoots 

 whilst placed under pressure, when preparing for the 

 herbarium : such specimens should first be killed by 

 immersion for a few seconds in scalding water. As 

 juicy plants require most light to secure for them a 

 regular discharge of moisture, we may mention as a 

 piece of practical information, the propriety of exposing 

 as many leaves as possible in the melon frame to the 

 action of the sun's rays, at the same time providing 

 against the accumulation of moisture in the confined 

 situation in which such plants are placed. 



The operation of transplanting should be carried on 

 either in the spring or autumn, when plants are des- 

 titute of leaves ; otherwise the exhalation is too strong 

 at a time when the absorption has been checked, owing 

 to injury sustained at the root. Provided the plants 

 are well watered, the latter inconvenience may to a 

 certain extent be obviated. The water exhaled is so 

 nearly pure, that scarcely any trace of foreign matter is 

 discoverable in it, certainly not more than would be 

 found in distilled water prepared with the greatest care. 

 Even that which is exhaled by aromatic plants is scarcely 

 tainted by any odour. The stomata are in fact the 

 most perfect and delicate stills to be met with in the 

 laboratory of nature. 



(171.) Retention of Sap. About two thirds of the 

 fluid imbibed by the spongioles is thus exhaled by the 

 stomata, and consequently about one third must be 

 still retained in the plant. As this portion now in- 

 cludes all the saline, earthy, carbonaceous, and other 

 materials, which happened to be dissolved in the sap 

 when it was first absorbed, the obvious effect produced 

 by the exhalation is to condense these matters, so 

 that the sap becomes a comparatively denser fluid 

 than it was before. As many of the materials thus 

 introduced are not adapted to the purposes of nutrition, 

 they are deposited in those parts where the exhalation 

 has been going on ; but the various carbonaceous ma- 

 terials, furnished chiefly by decomposing animal and 



