160 IMPROVEMENT OF PLANTS 



only by electricity. The change in light desired may be effected 

 by using globes or bulbs of different colors. 



Another interesting study is the effect of different kinds of 

 light on plant growth. Experiments show that plants will grow 

 under the influence of electric light and make satisfactory develop- 

 ment. Acetylene gas has also been tried with some success. 

 Natural gas and ordinary illuminating gas give off many products 

 that are very injurious to plants, and they cannot be satisfactorily 

 used in our experiments with plants unless arranged and placed 

 in such a position that none of the injurious products of the gas 

 can come in contact with the plants. Many scientists have sup- 

 plemented the action of the sunlight by subjecting plants to the 

 action of electric light at night, and in practically every instance 

 the plants made greater and more rapid growth. This discovery 

 is likely to be of little value until some cheaper and better method 

 of getting electricity is found. 



The amount and intensity of light produce a very marked effect 

 on the growth of plants. Those grown under the action of sun- 

 light in the open air generally make the most satisfactory progress. 

 Interesting experiments showing this may be made by growing 

 plants in a cellar or dark room, in rooms where only diffused 

 light and no direct sunlight is present, and in open-air plats where 

 the sunlight is unobstructed. If plants are grown in beds some- 

 what shaded by being covered with colored glass, we find that the 

 growth of the plants is less than the growth of those grown in ordi- 

 nary sunlight, so far as perfect development is concerned. If we 

 compare plants grown in the dark with the same kinds of plants 

 grown in sunlight, we find that their stems are longer and more 

 slender than the latter. From this it is seen that light has a re- 

 tarding influence on the elongation of the stem. In general we 

 may say that the action of light on plants is manifested in the 

 following ways: 



(1) In the elongation of the stem; (2) the direction of the stem; 

 (3) heliotropism, or the tendency of the plant to direct its stem 

 and growth towards the source of light (This is very marked in 

 the sunflower; in the morning its leaves and flowers are directed 

 towards the east and in the afternoon they are directed westward 

 to the sun ) ; f4) diaheliotropism, or the tendency of leaves of some 



