510 A?ialogi/ between Vegetables and Animals, 



light, as well as air, is essential to the well-being of plants. It^ 

 is light that decomposes the carbonic acid gas, and thus fur- 

 nishes the plant with carbon ; the substance whence it derives 

 all its colour, its solidity, and strength. If light be withdrawn, 

 the carbonic acid gas which has been absorbed remains un- 

 altered ; the oxygen is not set free ; and no deposition of 

 carbon takes place. Hence it is that plants growing in 

 valleys and dark situations are pale and delicate, while those 

 which grow on mountains, and in sunny regions, are extremely 

 hard, and of a deep green colour. For the same reason, 

 the stem of celery, which is buried in the ground, is white, or 

 etiolated, as it is called ; while the leaves, which appear above 

 the ground, and are exposed to the light, present a green 

 colour. It has been found that red rose trees, if deprived of 

 light, will produce roses almost white. All kinds of seed 

 become more nutritious and perfect in clear than in gloomy 

 weather ; because, in the former, there is a greater intensity 

 of light, and, consequently, a more copious deposition of car- 

 bon. Besides causing a greater assimilation of carbonaceous 

 matter, light promotes the exhalation of vapour from the 

 leaves : if there be a deficiency of it, the plant retains an 

 excess of liquid, and becomes dropsical. 



Animals are as dependent on light, for their growth and 

 well-being, as vegetables are. If an animal be deprived of 

 its influence, it becomes debilitated, unhealthy, and dropsical. 

 The insects of southern regions, and those exposed to the 

 light of the sun, exhibit much more brilliant colours than 

 those which reside in northern regions, or which leave their 

 places of concealment only during the night. The plumage 

 of birds, and the covering of other animals, which inhabit 

 countries near the equator, are brighter and more variegated 

 than the vesture of those which belong to the arctic climates. 

 When a European migrates into some equatorial country, he 

 soon loses his fair complexion, and his skin assumes a dark 

 colour. This change depends upon the agency of light, and 

 the consequent deposition of carbon* The rete mucosum, or 

 second layer of the skin, consists of oxygen, carbon, and 

 hydrogen. The light combines with the oxygen of this sub- 

 stance, and sets it free ; the carbon and hydrogen, the other 

 constituents, are thus allowed to enter into new affinities, and 

 to unite in such proportions as to form a more or less per- 

 fect carbonaceous product, which, being of a bluish grey 

 colour, imparts to the surface its tawny complexion. An in- 

 genious experiment of Mr. Edwards has shown that the per- 

 fect growth of animals is very dependent upon the action of 

 light. This indefatigable and learned physiologist put some 



