502 Organisms Are Products of 



resistant to drought and cold and is im- 

 mune to our "rust" disease. The red wheat 

 of Kansas, known as Kanred, famous for 

 its resistance to rust, was developed by 

 hybridizing native wheats with a variety 

 imported from the Crimea. Costly expe- 

 ditions will more than pay for themselves 

 if just one desirable plant is brought back. 



The American breeder, Luther Bur- 

 bank, is famous for the many new forms 

 he produced. He wanted a cultivated 

 daisy that would be hardy and have large 

 showy blossoms. His first step was to 

 cross the smrdy but modest appearing 

 American wild daisy with the less sturdy 

 but large-blossomed English daisy. After 

 hybridizing these two forms and select- 

 ing for several generations, he got the 

 desired combination of genes. He crossed 

 this new plant with a Japanese daisy 

 noted for the whiteness of its flower. In 

 this way another desirable gene was in- 

 troduced, and the result was the "Shasta" 

 daisy named after California's giant 

 snow-capped peak, and now so common 

 in all our gardens. See Exercise 9. 



Producing new varieties that combine 

 more than two desirable characters is 

 common practice. The Conqueror water- 

 melon combines at least five desirable 

 characters: it is disease resistant, is cov- 

 ered with a rind which is thin but tough 

 enough to stand shipping, has flesh of the 

 right quality, and is full of juice. 



Sometimes, in trying to hybridize, ex- 

 periments fail because a desirable gene is 

 linked with an undesirable one; to get 

 one, you must take the other, hi oats, for 

 example, the desirable quality of many 

 seeds on a stem is caused by a gene which 

 is linked with an undesirable gene. Test 

 your understanding by doing Exercise 10. 



Heredity and Environment unit ix 



Plant breeders have one great advan- 

 tage. Hybridization produces hybrids. 

 Also, when a mutation occurs the mutant 

 is often a hybrid. According to Alende- 

 lian inheritance, when hybrids are 

 crossed they produce offspring of dif- 

 ferent kinds. Assuming that the hybrids 

 had desirable characters, there will then 

 be some offspring that are desirable, some 

 undesirable. It takes much crossing and 

 selecting to get an organism that is pure 

 for a desirable character. But a plant hy- 

 brid which has a desirable character or 

 combination of characters can often be 

 propagated vegetatively. If you have 

 forgotten vegetative propagation, turn 

 back to it now. Some plants can repro- 

 duce asexually by means of slips, or bulbs 

 or tubers, or other plant structures. OfiF- 

 spring produced in this way will have ex- 

 actly the same gene combinations as the 

 parent; no new genes are introduced. 

 Whether the desirable new form is hy- 

 brid or pure, therefore, makes no differ- 

 ence; the offspring will have all the de- 

 sirable characters, for it has the same 

 genes as the parent. See Exercise i i . 



Grafting. Besides the methods of vege- 

 tative propagation mentioned above, 

 breeders use a method called grafting. It 

 can be used only with plants that have a 

 certain amount of woody tissue. Graft- 

 ing consists of attaching a small twig to a 

 branch of a growing plant. The small 

 stem that is cut off, known as the scion 

 (sy'on), is placed against or in the stem 

 of the rooted plant called the stock, in 

 such a way that the growing layer (cam- 

 bium) of one is in close contact with the 

 growing layer of the other. To prevent 

 infection and too much loss of water the 

 two parts are bound tightly together and 



