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 réle 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 
Digitized by Microsoft® 
