BREEDING 



BREEDING 



553 



mended. The breeder, in general, should hybridize with 

 some definite aim in view and use systematic methods 

 in attempting to accomplish that aim. One cannot 

 ordinarily expect to obtain in hybrids of any two varie- 

 ties any characters which are not present in the parents. 

 The unit-character conception explained in the begin- 

 ning of this article is of fundamental importance in 

 understanding hybrids. The breeder, by a careful 

 study of varieties, determines the good characters and 

 the poor or weak characters of each variety. He may, as 

 an illustration, if working with tomatoes, find all of the 

 varieties with yellow pear-shaped fruits to be large 

 bushy plants, the so-called standards, and he may 

 desire a dwarf type of plant and red fruits. By examin- 

 ing the different races of tomatoes, he would soon find 

 a variety, such as the Quarter Century, which possesses 

 the characters of dwarf plant and red fruit. By crossing 

 these two varieties, he would obtain new combinations 

 of the characters of the two sorts, and if he grew a 

 sufficient number of the second generation of these 

 hybrids, he would be certain to find some plants in 

 which the pear-shape had been combined with the red 

 color and dwarf habit of the Quarter Century variety. 

 The study of the varieties of any crop thus gives the 

 breeder an idea of the characters availabje, and he must 

 then use his originality and judgment in determining 

 what combinations of these characters would form the 

 best commercial variety. If this combination does not 

 already exist, he may start out with considerable con- 

 fidence that it is possible for him to obtain such a 

 combination and thus a valuable, new variety. Plants, 

 however, are not simple in their organization and the 

 reaction of different characters on one another in 

 different combinations may not always be what one 

 expects. Again, in no plant has a complete analysis 

 been made of all characters, and it may be impossible 

 for us evor to reduce all the characters of a plant to a 

 unit-character basis; thus there is always an element 

 of doubt as to the value of any new combination of 

 characters until this combination has been produced 

 and tested. 



Methods of crossing plants. 



Plants, like animals, bear male and female organs, and an act 

 of fecundation is necessary in all ordinary cases to insure the devel- 

 opment of seeds. In probably the larger number of plants, the male 

 and female organs or the stamens and pistils are borne in the same 

 flowers on one plant. In some cases, as in the castor bean, corn, and 

 the like, both sexes are borne on the same plant but in different 

 flowers. In still other cases, as in the date palm, asparagus, hop 

 and hemp, the sexes are on different plants. 



In hybridizing plants, it is necessary to insure that the plants 

 are not fertilized with their own pollen or with pollen from any 

 other source than that desired. If, therefore, the plant to be oper- 

 ated on has the stamens and pistils in the same flower, the stamens 

 must be removed from the buds before they burst and discharge 

 the pollen. This act of removing the stamens, or emasculation, 

 aa the process is called, is necessary in order to prevent self-fer- 

 tilization. In some plants, it is necessary to emasculate the buds 

 very early, as the pollen develops considerably in advance of the 

 pistils. In other cases, the pistils reach maturity or a receptive 

 condition before the pollen is shed. In this latter case, the emas- 

 cuUtiun may be delayed until a time just previous to the normal 

 opening of the flower. 



The process of emasculation may be illustrated by the colum- 

 bine. Here large-sized buds are chosen just before they open 

 normally (Fig. 642). The tips of the petals can then be easily pried 

 apart so that the stamens may be pulled off with small forceps. 

 This process should be performed carefully to avoid crushing or 

 injuring the pistil. The bud should then be inclosed in a small 

 light paper bag in order to prevent pollen from any foreign source 

 being brought to the pistil by insects or wind (Fig. 643). The bud 

 should remain covered until sufficient time has elapsed to allow the 

 pistil to reach normal maturity, when the bag should be removed 

 and the pollen from the desired variety dusted over the pistil. 

 After this act of pollination, the bud should again be covered with 

 the paper bag, which should not be finally removed until several 

 days later, after fecundation has taken place. As soon as a flower is 

 pollinated, it should be labeled with a small tag of some sort 

 which may remain attached to the flower-stem until the fruit is 

 ripe. In some cases, the pollen may be placed on the immature 

 pistil without injury, when the flower is emasculated, and this is a 

 great saving of time when it can be done. However, in most 

 cases, premature pollination is liable to injure the pistil and pre- 

 vent the setting of seed. One should ordinarily attempt to polli- 

 nate the pistil at aa nearly the normal time as possible. Many 



plants are difficult to hybridize and every process must be as 

 natural as possible to insure results. 



Many handy methods have been devised to use in pollination 

 work and are described in breeding literature. In all work fine 

 copper wire is better to attach bags and labels than is string. In 

 emasculation work also, it will often be found convenient when 

 some pollen has accidentally fallen on the pistil to wash it off with 

 water by means of a small dental syringe. In many cases, such as 

 apples, pears and cotton, the best means of emasculation ia to 

 remove the outer floral envelopes by cutting them off, using a 

 sharp scalpel. With a little practice this can be done quickly and 

 with minimum injury to the essential organs (Fig. 644). 



Difficulty is frequently experienced when hybridizing different 

 varieties, in getting plants of each variety to bloom at the same 

 time. This difficulty may be overcome in many cases by keeping the 

 pollen, which can be done for a limited period by slightly drying 

 the pollen without allowing it to become desiccated, and preserving 

 it in a tightly corked bottle. 



After the pollen has been placed on the stigma of the pistil by 

 the act of pollination, each pollen-grain develops a small tube 

 which grows down through the pistil to the ovary. Through thia 

 tube, the male germ-cells pass down and finally a male germ-cell 

 comes in contact with each egg-cell of the different ovules in the 

 ovary (in most plants there are several ovules in each ovary) and 

 fuses with them. This constitutes the act of fecundation or fer- 

 tilization. This fecundated egg-cell is then the beginning of the 

 hybrid and from the seed containing it, when grown, there develops 

 the hybrid plant. The plant developed directly from this hybrid 

 egg-cell is known as the first-generation hybrid (F , ). Seeds from 

 this first-generation hybrid, when grown, give second-generation 

 hybrids (F,). The expressions F F,, and F,, meaning first, second 

 and third filial generations, are used very commonly to designate 

 the first, second and third generations of hybrids. 



Laws of inheritance in hybrids. 



When plants of different pure races are crossed, as, 

 for example, different races of wheat, corn or cotton, the 

 hybrids are usually all very similar to each other in the 

 first generation, exhibiting in general the same char- 

 acters. And this is the case also when different fixed 

 species are crossed. If, however, individuals belonging 

 to unfixed races are crossed, there is usually a consider- 

 able variation in the first generation. This is well 

 illustrated by the crossing of different clons of apples, 

 pears, oranges, and the like, when the different so-called 

 varieties are merely transplanted parts of the same 

 individual seedlings which have not been bred to a 

 purity of type. It is well known that if seeds of an apple 

 variety be planted, the resulting plants exhibit many 

 different variations in the first generation. The parents, 

 themselves, therefore, not being of pure type, when 

 they are hybridized produce progeny which in the first 

 generation is variable. In the crossing of races which 

 nave been bred true to type, w r hether of the same or of 

 different species, the first-generation hybrids, however, 



644. Cotton flowers, illustrating the process of emasculation. 



a, mature bud showing the stage which should be chosen for 

 emasculation; 6, a similar bud with the corolla cut off ready 

 to emasculate; c, a similar bud with the stamens removed, 

 emasculated. 



are nearly uniform in the characters presented, and in 

 such instances it is necessary to secure a second genera- 

 tjon of the hybrids in order to accomplish the segrega- 

 tion of the characters and the production of a large 

 number of variations. Ordinarily, therefore, desirable 

 variations are looked for in the second generation. 

 This, as has been explained above, is true only in the 

 case of hybrids of species and races that are fixed in 

 type. 



