800 PHYSIOLOGY. 



which the crude sap undergoes in the leaves by the action of 

 light and air, and by which it becomes transformed into the 

 elaborated sap, have been already alluded to in treating of 

 the Functions of Leaves ; it will be here, therefore, only neces- 

 sary to state in what these changes consist. They are: — 

 1st. The transpiration of the superfluous fluid of the crude sap 

 in the form of watery vapour, by which it becomes thickened. 

 2nd. The absorption and decomposition of carbonic acid, by 

 which carbon — that most important constituent of plants— is 

 added to the crude sap whilst oxygen is evolved (Bssinration). 

 3rd. The formation out of the various inorganic elements present 

 in the crude sap, of the numerous organic products and secre- 

 tions, which process is properly termed Assimilatmi. The crude 

 sap being thus altered, contains in itself all the various azotised 

 and unazotised substances which are required for the develop- 

 ment of new tissues {Development), and the different secretions 

 {Secretion), It is then termed Elaborated Sap. 



The important influences which these changes have in Nature, 

 in promoting the purity of the atmosphere we breathe (p. 753), 

 the healthiness or otherwise of a particular country (p. 7-1:9), 

 and the fertility or barrenness of a soil (p. 750), &c., have been 

 already noticed. We have also seen, that in order that these 

 changes may be properly performed, the leaves mnst be freely 

 exposed to light ; and from this dependence of assimila- 

 tion on light it follows, as we haA'^e seen (p. 755), that when the 

 secretions of particular plants which are otherwise agreeable, 

 are injurious, or of unpleasant flavour, they can by growing 

 them in darkness or in diminished light, be made fit for 

 the table, as is the case with Cel^ery, Sea Kale, Lettuce, 

 Endive, and others. For the same reason the plants of warm 

 and tropical regions, where the light is much more intense than 

 it is in this country or in other cold and temperate regions, 

 are commonly remarkable for the powerful characters of their 

 secretions, as is well illustrated by the strong odours of their 

 flowers, and the rich flavours of their fruits, as contrasted with 

 those of cold and temperate climates. 



Again, as the production of secretions depends upon the 

 intensity of light, it frequently happens that a plant of a warm 

 or tropical region which naturally produces a secretion which 

 may be of great value as a medicinal agent, or useful in the 

 arts, when transported to this or any other climate in which 

 the intensity of the light is much less than it is in its native 

 country, that secretion is not formed at all, or in diminished 

 quantity. Even if such plants be placed in our hot-houses, 

 where they may be submitted to the same degree of heat as they 

 obtain naturally in their native countries, their secretions are 

 not formed at all, or in diminished amount, because light is the 

 main agent concerned in their formation, and we cannot increase 



