512 Introduction to the Study of Science 



poor resistance to disease, frost, or prolonged dryness, or a poor 

 quality of fruit, and to replace the defective with a desirable 

 character. This is one of the most valuable ways of plant 

 improvement. 



Suppose that only one plant in a group of others of its kind, 

 which are infected with a fatal disease, remains immune. This 

 plant, if selected and properly crossed with plants which are 

 susceptible to the disease, may be made to transmit to the de- 

 scendants this character of immunity to the disease. If, more- 

 over, healthy hybrids are selected for propagation, this method 

 is after all the most successful in conquering and eradicating 

 plant diseases. 



Improving wheat. An interesting case may be used to illus- 

 trate this. Cultivated wheat is usually close-pollinated, show- 

 ing therefore very little variation in its progeny ; and it has 

 been improved chiefly by selection of chance or sport varia- 

 tions. It is generally susceptible to rust and other diseases, 

 and is unable to withstand prolonged drought, especially when 

 ready for pollination. These characters restrict its cultivation 

 to fairly definite regions and even to districts within these 

 regions. 



About twelve years ago a wild wheat was discovered in cer- 

 tain arid, stony regions of Palestine. This wheat may be a 

 wild original of the cultivated wheat. It has several characters 

 not possessed by modern varieties of wheat. These are, first, 

 its cross-pollination by the aid of wind, and close-pollination 

 in certain unusual conditions ; second, its ability to grow in 

 a hot, dry climate, owing to its special adaptive structure for 

 checking rapid transpiration and evaporation ; and third, its 

 immunity to rust, and apparently to other common diseases 

 of the cultivated wheat. These characters have been combined 

 with the best characters of cultivated wheat, and the new 

 varieties, by careful selection and control, have been made 

 permanent and adapted to the climate of certain regions that 

 are now, or are to become, wheat-growing. The success of 



