

EXCITEMENT OF LIGHT. 373 



hydrogen, nitrogen, and carbon are absorbed as air or 

 moisture by the leaves and through the roots, and the 

 great phenomenon of vegetable life is the conversion of 

 these to an organic condition. Sugar and gum are 

 constantly produced, and from these, by combination 

 with atmospheric nitrogen, a proteinc compound is 

 formed, which is an essential element in the progress of 

 development.* 



Plants growing in the light are beautifully green, the 

 intensity of colouring increasing with the brilliancy of 

 the light. Those which are grown in the dark are etio- 

 lated, their tissues are weak and succulent, their leaves 

 of a pale yellow. It is, therefore, evident that the for- 

 mation of this chlorophylle as the green colouring 

 matter of leaves is called results from some action 

 determined by the sun's rays^ 



Chlorophylle is a carbonaceous compound formed in 

 the leaves, serving, it would appear, many purposes in 

 the process of assimilation. In the dark the plant still 

 requires carbon for its further development, and growing 

 slowly, it removes it from the leaves, decomposing the 

 chlorophylle, and supports its weak existence by preying 

 on parts of its own structure, until at length, this being 

 exhausted, it actually perishes of starvation. 



Plants always turn towards the light : the guiding po^er 

 we know not, but the evidence of some impulsive or at- 

 tracting force is strong and the purpose for which they 

 are constituted to obey it, is proved to be the dependence 

 of vegetable existence upon luminous power. 



Light is not, however, alone sufficient to perfect the 

 plant : another agent is required to aid in the production 

 of flowers and fruits, and this power is proved to be 

 heat : and heat, perhaps, in some peculiar condition. 

 Having reached that point of development when the 



* Consult Rural Economy, by J. B. Boussingault; The Chemical 

 and Physiological Balance of Organic Nature, by Dumas and 

 Boussingault ; and Agricnllural Chemistry, by Liebig*. 



