784 PHYSIOLOGY. 



here, therefore, only necessary, to state in what those changes 

 consist. They are: — 1st. The transpiration of the super- 

 fluous fluid of the crude sap in the form of watery vapour, 

 by which it becomes thickened. This transpiration is not 

 a distinct function, but is simply an accompaniment of the 

 second change which occurs in the crude sap, and which is com- 

 monly termed Respiration. This consists essentially, and per- 

 haps entirely, in the absorption and decomposition of carbonic 

 acid gas, by which carbon — that most important constituent of 

 plants, is added to the crude sap. The third change is the 

 formation out of the various inorganic elements present in the 

 crude sap, of the numerous organic products and secretions, 

 which process is properly termed Assimilation. The crude sap 

 being thus altered, contains in itself all the various azotized 

 and unazotizcd 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. 739), 

 the healthiness or othemase of a particular country (p. 736), 

 and the fertility or barrenness of a soil (p. 736), &c., have 

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

 these changes may be properly performed, the leaves must 

 be freely exposed to light (p. 741), or otherwise no proper 

 assimilation of the various inorganic substances which they con- 

 tain takes place, and hence the formation of the various organic 

 substances necessary for the free development of the tissues, and 

 for the production of the secretions, is more or less interfered with. 



As light is thus seen to be essentially necessary for the for- 

 mation of the secretions of the plant, it follows, as we have seen 

 (p. 741), that when the secretions of particular plants which 

 are otherwise agreeable, are injurious, or of unpleasant flavour, 

 that, by growing them in darkness or in diminished light, 

 they can then be used as vegetables for the table, as is the case 

 with Celery, Sea Kale, Lettuce, Endive, &c. 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 illus- 

 trated by the strong odours of their flowers, and the rich 

 flavours of their fruits, as contrasted with those of cold and 

 temperate chmates. 



Again, as the production of secretions depends upon the 

 intensity of light, it frequently follows, — 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, 

 &c., when transported to this or any other climate in which 

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



