310 



GENETICS IN RELATION TO AGRICULTURE 



particular features of form, color, and habit have arisen but we know 

 that there was only one original form, and fragments of the history 

 (Beal and Hurst) are sufficiently clear to give us assurance in advanc- 

 ing this explanation of the role of hybridization in the creation of varieties 

 of sweet peas. There is no authentic instance of a variety having origi- 

 nated from hybridization of the sweet pea proper, Lathyrus odoratus, 

 with any other species of Lathyrus, consequently the possibility of such 

 germinal diversity is precluded. Similarly in the case of selection for 

 more obscure characters such as number of blossoms in the cluster, size 

 of flower, and vigor of growth, apparently the things that have been 

 utilized in cases of improvement are mutations and new combinations of 

 mutant factors. 



FIG. 126. Four types of rose: a, typical modern Hybrid Tea rose, b, typical Hybrid 

 Perpetual rose; c, the Damask rose, which was popular in old gardens; d, the old single 

 Rosa gallica. (Reproduced from The Garden Magazine by permission.) 



Creation of Varieties of the Rose. No finer examples of the origin 

 of horticultural varieties by means of hybridization could be found than 

 the garden roses of today. The genus Rosa is widely distributed in the 

 Northern Hemisphere and contains several hundred species of which, 

 according to Wilson, twenty-six have been utilized in the production of 

 our garden roses. But these twenty-six species fall into fifteen distinct 

 groups, and in habitat they represent Asia, Europe, and North America. 



The most important group of modern roses are the Hybrid Teas for 



