THE ORIGIN OF CULTIVATED PLANTS 153 



possible, in order to have a greater number from which to select 

 desirable — and often novel — combinations of traits. 



Controlled hybridization makes possible the combination of 

 several desirable traits, and means bringing into existence a 

 plant with probable (if not certain) specifications. A drought- 

 resistant plant with poor fruit, for example, crossed with a 

 drought-sensitive plant bearing high quality fruit, may result in 

 a hybrid combining drought resistance with good fruit. Luther 

 Burbank during his lifetime "created" many new varieties in 

 this way, especially among plums and peas. Hybridization is also 

 of value because it results in plants with more rapid growth, 

 larger parts and generally more vigorous constitution than either 

 parent; such hybrid vigor is often of great importance. Burbank 

 crossed a California walnut with the New England black walnut 

 and produced a hybrid which grew twice as rapidly as either 

 parent and greatly exceeded them in stature. Corn hybrids have 

 been known to yield 50% more corn than the yield of the parents. 



Thus by combining a judicious amount of hybridization with 

 controlled selection of the offspring in every generation, patient 

 plant breeders have been able to direct the evolution of cultivated 

 plants. The size and color of fruits, the starch or sugar content of 

 roots, the length of seed hairs in cotton — these are but a few of 

 the plant characters that have been modified to man's advantage 

 since scientific knowledge of plant heredity has made such con- 

 trol possible. Every year hundreds of workers are busy in 

 genetics laboratories, in state agricultural schools and in the 

 federal department of agriculture, with but a single aim — to 

 improve existing varieties of cultivated plants and to create new 

 ones. 



