J90 PHYSIOLOGICAL BOTANY. PART II. 



Many effects, popularly ascribed to the action of air, are 

 in fact due to the agency of light. Thus trees which 

 grow in elevated or in isolated situations, are more 

 vigorous than others of the same species which grow in 

 forests or in shady places; and those on the skirts of a 

 wood are finer than those in the interior. When fields 

 are arranged into alternate strips of fallow and crop, 

 the produce is much greater from a given portion of 

 land than where the whole field is regularly sown, and 

 this effect must be attributed to the increased in- 

 fluence of light in such cases. The loss of light in 

 stoves and green-houses, by diminishing the effects of 

 exhalation, renders plants more liable to be frozen than 

 others of the same description which are growing in the 

 open air. 



(176-) Organisable Products. When we proceed to 

 inquire in what form the carbon appears after it has be- 

 come fixed, the subject assumes a degree of uncertainty, 

 which it seems almost hopeless to get rid of in the pre- 

 sent state of our knowledge. Since this fixation is effected 

 by the leaf and other green parts of the plant, it is con- 

 sequently in them that we may expect to find the organ- 

 isable product, whatever it be, which is the primary 

 and immediate result df this action. Now unluckily 

 for our inquiry, there are so many different compounds 

 contained in solution among the sap and various juices 

 of plants, such as gums, sugars, resins, oils, acids, 

 alkaloids, &c., all of which are composed of different 

 modifications of the same three elements, carbon, oxygen, 

 and hydrogen, that it becomes a task of the greatest 

 delicacy to determine which of them ought to be con- 

 sidered as the immediate result of the process of fixation. 

 If we may presume that this result is the same in all 

 plants, or so nearly the same that we may designate it 

 (like the blood of animals) by some name which em- 

 braces all the subordinate modifications, we must ex- 

 pect to find it among those products which are the 

 most generally dispersed in vegetables, and which are 



