INFLUENCE OP EXTERNAL AGENTS ON VEGETATION. 4U 



changes its color from white to green. It may then be hiid 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 production of vegetable colors. The yellow 

 rays, according to Draper and Hunt, are the most powerful in the production of colors. 



Light, Raspail says, influences plants to produce vascular tissue, and to make them 

 combine with earthy bases ; while in darkness they produce the cellular tissue, and com- 

 bine with ammonlacal bases. That light exercises an importaot 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. 



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 generat- 

 ed, 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 pro- 

 ducts in high latitudes. Trees become harder, smaller, and less luxuriant the higher the 

 latitude, for during the period of their growth, the sun is a great part of the time 

 above the horizon. That this is owing to the action of light, is proved by the fact, 

 that by transporting vegetables into higher latitudes, from equatorial regions, and 

 keep them in an atmosphere, at the temperature of their natural situations, by means 

 of the hot-house, they flourish during the summer ; but during the short days, and 

 long nights of winter, they droop, exhibiting their suffering from the due influence of the 

 Bolar rays. 



Baspaii's theory above noticed receives confirmation from the fact, that those vegetables 

 which consist entirely of cellular substance, are produced only in the absence of the light 

 of the sun, such as mushrooms, &c., their growth ceasing at the coming of light. And it 

 is a common notion among gardeners that melons, cucumbers, and like pulpy fruits, in- 

 crease much more at night than during the day. Although Fungi grow only in dark- 

 ness, they will never produce spores capable of germination, without the action of 

 the sun's light, and in cases where the light of day never enters, there may be Fungi, 

 but they never increase or perpetuate themselves by the production of spores. 



It is during the direct action of the sun's rays, and by their agency, that the most 

 important vegetable products are generated. It is by their influence that water and 

 carbonic acid are decomposed, the oxygen being mostly liberated, and the elements com- 

 bining in other proportions, for the formation of the various oils, resins, &c., including 

 the most important and abundant of the vegetable products. What is generally termed the 

 sleep of plants, that is, the folding up of compound leaves, and the closing of flowers, is, 

 no doubt, in most instances, occasioned by the want of the stimulating action of the solar 

 rays; for we see leaves and flowers, that were folded up' during the night, expand with 

 the first rays of the morning sun. 



We have upon record many instances of the singular phenomena of flowers during 

 twilight emitting flashes of light. It is said the daughter of Linnaeus first observed this 

 emission, exhibited by the Tropceolum Mcijus or Garden Nasturtium. The flashes occur 

 only during twilight, in the morning or evening; those of the evening being much the 

 brilliant. The plants, from whose flowers these flashes have been observed to i 

 frequently, are the Marigold, Calendula officinalis, Orange lily, Lilitim bnlbifeni 





