IMPROVEMENT OF PLANTS. 253 



II. Variation Furnishes the Starting-point. 



1. J'iij'ialwu of Srcd/iui^s. — When the charac- 

 teristics of the desired variety have been defi- 

 nitely determined, then if one will diligently and 

 carefully search among- his plants, he may find 

 — owinor to variation — individuals which possess 

 these characters in a more marked degree than 

 do the others. Rut if such individuals are not 

 found, then 



2. ] 'aviation /nay be indnced hy (i) Environ- 

 mental Changes. 



Important among these is (a^ a change in 

 food-supply. Darwin says: "Of all the causes 

 which induce variability, excess of food, whether 

 or not changed in nature, is probably the most 

 powerful." 



If heavy foliage and rank-growing plants rep- 

 resent the " ideal," they should be given a liberal 

 supply of nitrogeneous food (see " Effect of 

 Nitrogen," Chapter IV.) If dwarf size and 

 fruitfulness are the desired characters, then 

 foods containing potash and phosphorus should 

 be substituted. 



Experiment 26. — [a) To show variation induced bv 

 change of food supply. Secure one-half bushel of pure 

 white sand, and sterilizej it by tlioroughly baking it in 

 a hot oven. 



(/') The tomato, geranium, etc., are suitable plants for 

 tliis experiment. Select three small, similarly developed 

 plants grown from cuttings of the same stock (see page 

 220). 



