Chlorophyl 181 



MS in the autumnal change in deciduous leaves. It is well known that 

 plants will germinate in the <l;irk, and will continue to grow until the 

 starch, deposited in the seed, is used up, after which if the}' receive 

 some light they form the yellow coloring matter, and continue to grow 

 in a sickly, ineffectual way, but finally take on the healthy green when 

 exposed to full light. < ' A writer mentions a forest upon which the sun 

 had not shown for twenty days. The leaves, during this period, were 

 expanded to their full size, but were almost white. One forenoon the 

 sun began to shine in full brightness ; the color of the forest absolutely 

 changed so fast that we could perceive its progress. By the middle of 

 the afternoon the whole of this extensive forest, many miles in length, 

 presented its usual summer dress." * 



Diffused light will not start the starch making action of the chloro- 

 phyl cells of water plants, but will keep it up after "it has been started, 

 by direct radiation, f As mentioned, the action stops at night. And to 

 prove how completely it is due to the action of light, observation has shown 

 that in water-plants it is checked by a shadow from a passing cloud. 



The colors of flowers of course depend on the action of light. In 

 man} r cases these colors change from one tint to another, under such ac- 

 tion. Some are white at first, and afterwards yellow or purple. Some 

 change from one day to the next, others during a day. One of the 

 Rose Mallows opens out white in the morning and becomes red during 

 the day. 



By artificially preventing the action of the light, the change of color 

 in flowers can be prevented in some cases. The tints of fruits are also 

 dependent on the action of light, and in this case the molecular change 

 goes below the surface, for the odor and taste of fruit alters greatly un- 

 der the influence of light. 



Certain mechanical movements of plants are due to the action of light. 

 All plants seek to expose themselves to the light. The action of the 

 Sunflower in twisting about to receive the rays of the sun, is familiar. 

 Payer experimented with the young stems of common garden cresses, 

 grown on damp cotton in the dark. The stems curve quickly toward 

 the side in which light is admitted, the top of the stem, being the most 

 flexible, turning first, and the lower part afterwards. He also sowed 

 turnips, and while the plants were small exposed them to the divers 

 rays of the solar spectrum. They all bowed toward the blue from 

 both sides of it, showing that the blue rays influence this action more 

 than the others. 



Darwin concluded that the light affected the action of twining plants. 

 In one case a growing twining plant in a window was observed to grow 

 around an entire turn of its spiral twist in five hours and twenty min- 



* Youman's Chein., 241. P:ipiM>ii, 105. 



