HYBRIDIZATION 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 moneecious 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. 
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