262 



AGRONOMY 



partly through admitting more light to the plant, and partly 

 through checking growth processes. Injury to the plant may 

 also result in variation. It is a remarkable fact that when the 

 type has once been induced to " break," or vary, the tendency 

 for the resulting forms to continue to do so is strong. A 



notable instance is 

 found in the plant 

 known as the Boston 

 fern, which is fre- 

 quently grown in the 

 window garden. A 

 few years ago a sport 

 with much-divided 

 leaves was put on 

 the market, but it 

 was soon eclipsed by 

 numerous much finer 

 forms that had been 

 developed from it. 

 When once a desira- 

 ble variation has been 

 secured, its value 

 need not be jeop- 

 ardized by further 

 breeding. It may 

 then be multiplied 



vegetatively ; in fact, many improved plants will not come 

 true from seeds, and their number must be increased in this 

 way. Most of our fruits, flowers, and garden vegetables have 

 arisen through variations from less desirable types. 



Hybrids and hybridizing. Another way in which new plants 

 may be obtained or variations started is by crossing, or hybridiz- 

 ing. In this process pollen from the flowers of one species, or 

 variety, is applied to the stigmas in the flowers of another, 



Photograph by W. A. Terry 



FIG. 191. Two leaf sports from the common 

 Christmas fern (Polystichum) 



