LIGHT. 137 



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 tlif leaves, while the parts above and around them are per- 

 fectly white. That these are exceptions to a general rule is 

 manifest from innumerable 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 are exposed soon become green. Plants, 

 which in their natural situation are white, by accidental ex- 

 posure become green ; the side of a potato 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. 

 All parts of the solar spectrum are not equally efficient in the pro- 

 duction of vegetable colors. The yellow rays, according to Dra- 

 per and Hunt, are the most powerful in the production of colors. 



259. Light, Rasp ail says, influences plants to produce vascu- 

 lar tissue, and to make them combine with earthy bases; while 

 in darkness they produce the cellular tissue, and combine with 

 ammoniacai bases. That light exercises an important agency 

 over the growth of vegetables and their secretions, 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. This is seen exhibited 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. 



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

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

 sence 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 equality, just in 

 the same proportion should we expect to find the products of 

 such regions harder, smaller, and less symmetrical. This is the 

 exact state of vegetable products in high latitudes. Trees be- 

 come harder, smaller, and less luxuriant the higher the latitude, 

 for 'luring the period of their growth, the sun is a great part of 

 the time above the horizon. That this is owing to the action 

 <>f light, is proved by the fact, that by transporting vegetables 



Are all parts of the solar spectrum equally active \ Which most ? — 259. 

 • ? What seems the best proportion? When ex- 

 hibited \— 200. What should we be led to conclude from these facts i 



