3 oo THE POPULAR SCIENCE MONTHLY. 



color in the sun. The tints of fruits in the same way develop under 

 the healthy action of daylight, and the rule extends to those principles 

 of every nature which give taste and odor to the different parts of the 

 the plant. 



Flowers, fruits, and leaves, then, are elaborated by the help of 

 luminous vibrations. Their tissue holds the sun's rays. Those 

 charming colors, those fragrant perfumes, and delicious flavors, all the 

 innocent pleasures the vegetable kingdom yields us, owe their creation 

 to light. The subtle working of these wonderful operations eludes us, 

 as does that which guides the fleeting diffusion and thousand-fold re- 

 fractions displayed by the imposing spectacle of the dawn ; but is it 

 nothing to gain a glimpse of those primal laws, and to possess even a 

 twilight ray upon these magnificent phenomena ? 



II. 



Light exerts a mechanical influence on vegetables. The sleep of 

 flowers, the bending of their stems, the nutation of heliotropic plants, 

 the inter-cellular movements of chlorophyll, offer proofs of an ex- 

 tremely delicate sensitiveness of certain plants in this respect. Pliny 

 mentions the plant called the sunflower, which always looks toward 

 the sun, and steadily follows its motion. He notices, too, that the 

 lupin always follows the sun in its daily movement, and points out the 

 hour for laborers. Tessier, at the end of the last century, took up the 

 study of these phenomena, and inferred in a general way that the 

 stems of plants always turn toward the light, and bend over, if neces- 

 sary, to receive it. He noted, too, that leaves tend to turn toward the 

 side whence daylight comes. Payer made more exact experiments. 

 He tried them with young stems of common garden cresses grown on 

 damp cotton in the dark. These stems have the property of curving 

 and turning quickly when placed in a room lighted only from one side 

 or iu a box receiving light on one wall only. The upper part of the 

 stem curves first, the lower part remaining straight. By a second 

 movement the top erects and the bottom bends over, so that the plant, 

 though leaning, becomes almost rectilinear again. "When put in a 

 room receiving light from two windows, the following results are no- 

 ticed : If the openings are on the same side admitting light equally, 

 the stem bends in the direction of the middle of the angle formed by 

 these two beams ; if one of the two windows admits more light than 

 the other, the stem leans toward it ; if the windows are opposite each 

 other, the stem stands erect, when light comes equally from both sides, 

 and, if it does not, turns toward the stronger rays. Payer discovered, 

 moreover, that the part of the irradiating light most active in its 

 effects corresponds in this case to the violet and the blue. The red, 

 orange, yellow, and green rays, do not seem to produce any movement 

 in plants. Gardner carried the investigation still further. He sowed 

 turnips, and let them develop in the dark to two or three inches in 



