THE FLOWER AND INSECT-POLLINATION 165 



most common method of producing new and desirable forms- 

 is to " cross " two different kinds of plants; that is, to take 

 the pollen of one kind and apply it to the stigma of another 

 kind. This is artificial pollination, in the sense that the plant- 

 breeder is the agent of transfer, but such crosses are occurring 

 constantly in nature. The plant-breeder, however, makes a 

 cross for a definite purpose, while crosses in nature are in- 

 definite and matters of chance. The plant that is produced 

 by crossing parent plants of different kinds is a hybrid. The 

 purpose of the plant-breeder in producing hybrids is to get 

 in a single plant a combination of desirable qualities belong- 

 ing to the two parents. It must not be supposed that every 

 hybrid shows the desired combination, for usually only one 

 individual out of thousands will show it. For example, if a 

 race of wheat is found to be very resistant to dry weather, 

 this feature would be rsgarded as a very desirable one. This 

 valuable " drought-resistant " character, however, might be 

 associated with a grain of poor quality, so that this kind of 

 wheat could not serve our purpose. One method of pro- 

 cedure in such a case would be to cross a wheat of good qual- 

 ity with the drought-resistant wheat, in the hope that among 

 the hybrids thus produced, some one of them would have the 

 desired combination of good quality and drought-resistance. 

 If this could be the case, the desirable hybrid would be propa- 

 gated, this time cross-pollination being guarded against, 

 and the seed multiplied for use. The use of hybrids in 

 plant-breeding, therefore, is to secure desired combinations 

 of characters, and the securing of hybrids is by artificial 

 pollination. 



101 . Summary. The evolution of the flower has proceeded 

 in many directions, but there are certain general tendencies that 

 can be kept in mind. The -perianth is generally differentiated 

 into two sets, the sepals and the petals, and the latter usually 

 constitute the conspicuous feature of the flower. There is 

 also a tendency to shorten the receptacle, so that the num- 

 12 



