148 LIGHT. 



bonic acid, and depositing the carbonaceous matter. In most ca- 

 ses, certainly, light is absolutely necessary for the deposition 

 of the green coloring matter, since most plants become per- 

 fectly colorless by growing in situations in which they are 

 deprived of light. There are cases, however, in which plants 

 deposit the green chromule, when excluded from the light. 

 Green vegetables have been found in caves of the earth, from 

 which the light of day was excluded, and we have seen the 

 cotyledons of the Mustard, and the Impatiens balsamina green, 

 when the seeds have germinated within the perfectly closed 

 pericarp ; and I have now before me a large onion in which 

 several of the central layers are as green as tlie leaves, while 

 the parts above and around them are perfectly white. That 

 these are exceptions to a general rule, is manifest from innu- 

 merable examples to the contrary, constantly occurring within 

 the observation of every one. If a board lies upon the grass for 

 a short time, the grass becomes blanched ; Plants growing in a 

 dark cellar are colorless ; the interior of the cabbage is white, 

 while the other leaves are green, and if these are removed, those 

 that aie exposed soon become green. Plants which in their nat- 

 ural situation are white, by accidental exposure become green ; 

 the side of a potatoe from which the soil has been by chance 

 removed, soon changes its color from white to green. It may 

 then be laid down as a general principle, that light is the 

 great agent in the production of vegetable colors. 



215. Light, Raspail says, infiacnces plants to produce vas- 

 cular tissue, and to make them combine with earthy bases; 

 while in darkness, they produce the cellular tissue, and com- 

 bine with ammoniacal bases. Tliat light exercises an im- 

 portant agency over the growth of vegetables, and their se- 

 cretions, cannot be doubted. An equal amount of light and 



^darkness seems to be the proportion in which the greatest 

 amount of vegetable vigor is attained. Tiiis is seen exhibi- 

 'ted in the equatorial regions, where the days are uniformly 

 twelve hours long, and the nights of equal length, and there 

 we find the most luxuriant vegetation. 



216. If according to the hypothesis, light acts in producing 

 the firmer and more compact parts of vegetables, and in its 

 absence, the more yielding and succulent parts are generated, 

 we should be led to suppose, that where these periods were 

 equal, the perfection of vegetable products would be found ; and 

 if the light is in much greater proportion than that of equal- 

 ity, just in the same proportion should we expect to find the 

 products of such regions, harder, smaller, and less symmetri- 

 cal. This is the exact state of vegetable products in high 



