150 GENETICS AND EUGENICS 



2. Forms of flowers. The forms of flowers, no less than 

 their colors, are subject to unit-character variation. In 

 sweet peas the ordinary form of flower with erect standard is 

 dominant over a variation in which the standard lops down 

 at either corner forming what is called a "hood." Symmetri- 

 cal forms of flowers which appear as sports in species having 

 normally asymmetrical flowers are a unit-character variation. 

 Thus a peloric (symmetrical) variation in the snapdragon is 

 recessive to normal (asymmetrical) shape of flower (Baur). 

 Double flowers, those which have an increased number of 

 parts (commonly petals), are in general recessive to singles. 

 This is the case for example in primulas, poppies and lark- 

 spurs. But some cases occur in which the heterozygote is 

 intermediate, as for example in carnations. Here a good 

 commercial double type is found to be regularly heterozy- 

 gous, producing when selfed both singles and extremely 

 double types ("busters"), each of which sorts breeds true, 

 and in addition the unstable but more valuable heterozygous 

 type of the parent (Norton). 



3. Colors of leaves and stems. The colors of leaf and stem 

 often vary abruptly in cultivated plants by unit-character 

 changes. Thus strains variegated with yellow arise from 

 local loss or inhibition of chlorophyl, a change which impairs 

 the assimilative power of the plant but adds to its ornamental 

 value in horticulture. Of course plants largely or completely 

 yellow because of deficiency of chlorophyl would be unable 

 to maintain themselves other than as parasites, such as 

 dodder; hence the yellow of variegated plants is usually 

 limited in amount. Some varieties of cultivated plants pos- 

 sess as a distinguishing character an unusual amount of red 

 coloring matter (anthocyan) in leaf or stem. Examples of 

 this are seen in purple beeches and maples, variations known 



