H YBRID1ZA TION 343 



has its own morphological and physiological peculiarities and general 

 methods will need to be modified to some extent in almost every case. 



General Method. Some results of value have come from promis- 

 cuous crossing of varieties and species that appeared to give promise of 

 desirable combinations. Considerable hybridization has been done in 

 this way especially in establishments where large collections are main- 

 tained and by seedmen and nurserymen who have undertaken such work 

 as a side issue. Some of the most important results of such work have 

 been the accidental discovery of unforeseen possibilities or limitations in 

 crossing. But many important results have come from carefully 

 planned and executed experiments and the demands of modern agri- 

 culture necessitate systematic procedure in the employment of hybridi- 

 zation in plant improvement. Such procedure includes six steps. 



1. Choice of Parents. This involves two important matters: first, 

 decision regarding the object to be attained which implies thorough 

 familiarity with existing conditions affecting crop production; second, 

 comparative study of existing varieties or of species that may yield the 

 desired result. 



2. Culture of Parent Plants. Hybridization is painstaking work and 

 when carried on extensively it is time-consuming and, therefore, expensive. 

 While it is sometimes necessary to use certain plants, especially shrubs 

 and trees, wherever they happen to be growing, yet it is always advisable 

 to concentrate materials so far as possible and to grow them under protec- 

 tion in the breeding garden or greenhouse. Arrangement of the details 

 of culture should include consideration of the optimum conditions for 

 normal fruitfulness of the intended mother plants. These plants in 

 some cases must be kept under observation and prepared for crossing by 

 reducing vegetative growth and restricting blooming and the setting of 

 fruit. 



3. Protection of Pollen. Flowers on intended male parents should be 

 guarded in order to prevent contamination with pollen of other plants. 



4. Castration of Hermaphrodite Flowers. This must be accomplished 

 before anthesis and is usually done shortly before the flower opens in 

 order to avoid needless mutilation. But in some close-pollinated species 

 it is necessary to emasculate very young buds. The operation consists 

 of removal of the stamens and can usually be accomplished easily by 

 using a pair of fine pointed forceps or scissors. The castrated flower is 

 then protected with some sort of covering until ready for pollination. 

 In monoecious plants it is necessary to guard the young pistillate flowers 

 which are to be pollinated. 



5. Pollination. The transfer of pollen from guarded flowers of the 

 male parent to the prepared flowers of the mother plant should be 

 accomplished before or just at the time the stigma becomes receptive. 



