3 68 TEXTBOOK OF BOTANY 



its grains are planted, will give rise to a small-headed race. 

 To prevent a lowering of the average of the variety, the new 

 small-headed race must be weeded out by selection. This 

 explains why selection must be continued, even after a valu- 

 able pure variety has been established. 



On the other hand, mutations sometimes give rise to races 

 that are improvements on the variety from which they come. 

 Among the members of a large-headed variety a plant with 

 heads larger than the average may appear, which will pass 

 on to its offspring a tendency to bear especially large heads. 

 By selecting the grains of such a plant and of its offspring, 

 we may obtain from the large-headed race that was first se- 

 lected a new race with still larger heads. Thus, by taking 

 advantage of mutations that lead in the right direction, a 

 variety may be improved step by step. However, the 

 amount of improvement that can be made in any particular 

 direction is limited. We could hardly hope by selection, no 

 matter how long continued, to produce wheat twenty feet 

 high, or with heads a foot long. When an attempt is made 

 to breed for a particular quality, by either the older or the 

 newer method of selection, improvement may be rapid for a 

 few. generations ; but then it becomes slower and slower, and 

 finally a point is reached beyond which no perceptible ad- 

 vance can be made. After this time, about all that selection 

 can do is to remove undesirable mutations when they appear 

 in order to keep the variety at the level it has reached. 



As a rule, a mutation results in only a small difference 

 between parent and offspring ; but now and then the dif- 

 ference is great enough to be conspicuous, as when a double 

 flower appears on a plant belonging to a race with single 

 flowers. A plant that shows such a striking mutation is 

 often called a sport. Sometimes a sport gives rise to a valu- 

 able new variety. The double-flowered varieties of roses, 

 carnations, violets, and many other cultivated plants, and 

 the seedless varieties of oranges and other fruits have been 



