270 



GENETICS IN RELATION TO AGRICULTURE 



seedlings indicate that, as in animals, germinal mutations usually occur 

 just before or during the maturation process. The strongest evidence 

 for this conclusion is the fact that, so far as known, new dominant char- 

 acters appear first in only one or two individuals. The following cases 

 illustrate this point. The red-leaved evening primrose, (Enothera 

 rubricalyx (Fig. 118) has been known to occur but once in all (Enothera 

 cultures and then in a single plant. The red sunflower, Helianthus 

 lenticularis coronatus, as reported by Cockerell, first appeared as a single 

 plant which proved later to be a heterozygous 

 dominant. A purple-leaved mutation in hemp, 

 Cannabis saliva, is reported by Dewey to have 

 first appeared in two pistillate plants in a closely 

 inbred strain of normal green plants. Had these 

 mutations occurred at some preliminary stage 

 in germ-cell formation, the change in chemical 

 constitution would have been transmitted to 

 several or many gametes and a considerable 

 number of individuals would have appeared 

 instead of only one or two. 



Factor mutations in meristematic cells, or 

 vegetative mutations, as distinguished from 

 those originating in the germ cells, give rise to 

 simple bud sports or to chimeras according to the 

 location of the mutating cell. A bud sport is a 

 shoot or branch which differs genotypically in 

 one or more characters from the remainder of the 

 plant. Here the factor mutation must occur in 

 one of the undifferentiated cells of the very- 

 young shoot. Just as in the case of factor 

 mutations in germ cells, so in vegetative muta- 



half red and half white; 

 a sectorial chimera (see 

 Chapter XXII). 



Fia. 111. Bud sport 

 from a white flowered 

 gladiolus bearing red flow- 

 ers on ore side of the stalk 



and showing one flower tions the somatic effects range from single visible 

 character differences to manifold effects in which 

 many structural details are different. An example 

 of bud sports in which the factor mutation 



induced a single character difference is shown in Fig. 111. The early 

 gladiolus known as "The Bride" is a white variety of Gladiolus colvillei, 

 a red-flowered form, and doubtless originated from it as a seed or bud 

 mutation. In 1915 there appeared in a row of "The Bride" a single 

 stalk bearing partly red and partly white flowers. That this grew from 

 a corm which was an offshoot from a typical white-flowering corm is 

 certain. Furthermore, that the mutation occurred very early in the 

 development of this corm and not sometime during the growth of the 

 flower stalk is proved by the following observation. In the autumn 



