HORTICULTURE A GREAT GREEN CARPET 425 



The red bud sports of apple and pear are frequently only "skin-deep." The 

 red pigment change may occur only in the outer layer or layers of the fruit, 

 yet completely cover the internal layers of non-red characteristics. Likewise, 

 plants with pubescent or hairy leaves and fruits may give rise to smooth 

 or glabrous forms through a mutation in the outer layer of the plant so that 

 hairs do not develop from epidermal cells. The typically smooth-skinned 

 nectarine is but a bud sport of the typically fuzzy peach. It is not surprising 

 that peach trees give rise to nectarines and that nectarines give rise to peach 

 trees. 



The variegated leaf patterns of the garden and the conservatory are the 

 products of various combinations of layers of cells containing different pig- 

 ments. Thus a section of the outer layer may appear colorless because it con- 

 tains no pigment. Another section contains light green pigment. Beneath may 

 be a layer of dark green pigment or red pigment. The whitish, reddish, green, 

 and shaded segments, blotches, and patches are the result of these overlapping 

 and variously arranged chimaeral tissues. Small wonder we have had difficul- 

 ties in the past in understanding the vagaries of some of our cultivated plants. 



But the plant breeder is no longer ignorant of these situations and he need 

 no longer wait ages for an accident of nature to produce a desired change 

 in chromosome number. By means of a drug (colchicine) derived from a 

 species of Colchicum, he is able to induce an artifical change in one season so 

 so as to provide him with just the plant material he needs for breeding 

 purposes. 



Thus, the southern muscadine grape is a diploid with 40 chromosomes. It 

 does not cross readily with the northern bunch grape which has 38 chromo- 

 somes. By treating both species with colchicine, the chromosome number 

 of each has been doubled (tetraploid). These new forms will now combine 

 to blend the characters from both species. Again, a variety of apple may have 

 three sets of chromosomes (triploid) and may not be useful as a parent in 

 breeding because of the abnormalities which arise during reduction division. 

 Happily, the plant breeder can induce a doubling up of the chromosome 

 number to form a fertile hexaploid which can now be successfully used in 

 breeding. 



The plant breeder has been given additional new tools by the geneticist, 

 cytologist, chemist, and plant physiologists. By performing a Caesarian 

 section on immature fruits, removing the partially developed embryos, and 

 culturing them like incubator babies as with certain species of Prunus, he 

 has succeeded in making heretofore impossible crosses. By the use of certain 

 growth-regulating chemicals he can prevent abscission of the flower as with 

 the lily, until fertilization has been effected. 



The contributions of hybrid vigor have been put to valuable use. In the case 

 of those plants which carry the male flowers and the female flowers in separate 

 parts, as in corn, it has been possible to remove the male parts (tassels) 



