LIGHT. 147 



modified by the petals; after this time the emanations of odor 

 took place at the same periods as before." 



212. Many other cases might be cited of singular phenom- 

 ena, properly coming undor tliis head. The odors in these ca- 

 ses are certainly developed or emitted on dillerent principles, 

 in the ditTcrent ca^es under this variety. The explanations 

 are entirely beyond our reach. There seems to be a specific 

 action of the organs for the production of the odors, as there 

 can be no glands discovered by whicii the odorous fluid is secret. 

 ed. That the odorous fluid is emitted as it is generated, which of 

 course must bo periodically, is rendered probable by the fact, 

 that cmissioM of car'>onic acid took place in the same manner 

 from the flower of (lie Ccreus. 



213. Odors have also been classed from their similarity of 

 effect on the human system into aromatic, stimulating, penetra- 

 ting and sweet, but the difliculty of fixing definite limits to 

 the application of these terms, renders the classification of 

 little use. 



CEIAPTER VI. 



INFLUENCE OF EXTERNAL AGKNTS ON VEGETATION. 



The infliences, to which we shall direct our attention as 

 exercising a decided influence on vegetation, are light, heat, 

 teater and rarth. Many of the influences exerted by these 

 agents on plants, have already been incidentally noticed un- 

 der the description of organs, and their functions, in the pre- 

 ceding chapters ; but we deem it expedient to give a more 

 connected view of the subject in a separate chapter. 



The concurrent influence of all these agents is absolute- 

 ly required for the perfection of vegetable products ; and ac- 

 cording as some of them exist in excess, or in diminished 

 quantity, is the functional operation of the vegetable organs 

 injured or destroyed. 



Section 1. Light. 



214. We have made several remarks on the eflTects of light, 

 in the section of the function of the leaves, lo which the student 

 is referred, in connection with what is stated here. 



The most obvious effect of liglit on vegetation, is the 

 production of colors, and this it eflects by decomposing car- 



