IMPROVEMENT OF PLANTS 159 



another ear on a neighboring stalk may have small grains of a 

 darker color. In the whole field we shall not find any two 

 plants exactly alike. These slight differences give us the start- 

 ing point for the improvement of the parent plant and for the 

 origination of new varieties. When variation does not occur 

 naturally to any marked extent, it may be brought about by 

 (a) change in the food supply, (6) the action of light, (c) prun- 

 ing, (d) cross fertilization, (e) bud variation. 



The Food Supply. Many experiments show that the food 

 supply of plants is an important factor in bringing about variation. 

 Japanese and Chinese gardeners for ages past have dwarfed many 

 well-known plants by giving them a scanty food supply. In this 

 way many varieties of forest trees have been dwarfed and so re- 

 duced in size that they might be grown in ordinary flower pots 

 without difficulty. This is accomplished by starting the plants 

 in sand which contains only a limited amount of the food elements 

 needed by the plants. This method of starting and growing plants 

 is called sand culture. In some cases water is used in place of 

 sand, but in this case some kind of artificial support must be 

 provided for keeping the plant in position. Hyacinths and many 

 bulbs of like nature, as well as cuttings of rose bushes and of the 

 so-called wandering Jew, may be made to grow in water contain- 

 ing the necessary food elements in solution. This is called water 

 culture. 



Experiments also show that excess of food elements causes much 

 variation in the growth and development of plants. The greatest 

 variation is due to the nature and kind of food elements supplied. 

 One of the most important elements is nitrogen, which is very 

 necessary to the successful development of all plants of heavy 

 foliage and luxuriant growth. If we desire plants of small size 

 with less foliage and more fruit, we find it necessary to use fertilizers 

 containing potash and phosphorus. From this it appears that 

 we may alter the character of the crop, and the quantity of the 

 crop, by changing certain conditions of food supply. 



The Supply of Light. Another very important factor in 

 producing variation in plants is the kind and quantity of light 

 supplied. Interesting experiments to show the effect of colored 

 lights on the growth of plants may be made in hothouses lighted 



