CHEMICAL ACTIONS. 39 



and rank growth of the plants and trees ol 

 those latitudes. The influence of light is equally 

 necessary to promote those peculiar combina- 

 tions of the elementary constituents of plants 

 which are termed their secretions, ad which, 

 in fact, render the vegetable creation of such 

 vast importance to man. Starch, sugar, oils, 

 gum, caoutchouc, acids, dyes, camphor, opium, 

 the vegetable alkalies, and a great variety of 

 other principles, are thus formed ; and if the 

 growing plant be deprived of any portion of the 

 degree of light necessary for it, these secretions 

 will not be formed, or only in diminished 

 quantity. 



A tropical plant of the composite order, 

 nearly allied to our rag-worts and groundsels, 

 ( Cacalia ficoides,) illustrates this power of solar 

 light. Its leaves combine with the oxygen of 

 the atmosphere during the night, and are as 

 sour as sorrel in the morning ; as the sun rises, 

 they gradually lose their oxygen, and become 

 tasteless at noon ; and by the continued action 

 of light they lose more and more, till towards 

 evening they become bitter. The amount of 

 heat and cold which some plants will bear is 

 very considerable. While plants are at rest in 

 the winter they can bear a high degree of cold 

 without injury. In those regions of Siberia 

 where the cold is known to freeze quicksilver, 

 (forty degrees below zero,) the vegetation in- 

 summer is much more luxuriant than that of 

 the North Cape, where cold such as is felt in 

 Siberia is quite unknown. This same amount 



