474 JOUENAL OF THE ROYAL HORTICULTURAL SOCIETY. 



infrequent!}^ the case also with the Carnation (Dianthus Caryophyllus), 

 though less constantly so than in the Sweet William. But though 

 fertile anthers occur more frequently in D. Caryophyllus, the supply of 

 pollen is nevertheless often so scanty that the pure breeding of Carna- 

 tion doubles on a large scale is a troublesome matter. 



By far the largest class of double-flowered plants, however, 

 includes those in which both reproductive organs remain capable of 

 functioning, even despite a considerable degree o'f doubling, as, e.g., 

 in the Hollyhock (Althaea). Some double-flowered plants, however, 

 cannot be rigidly classified on this basis, owing to the fact that con- 

 siderable grading occurs among the different flowers on one individual. 

 This occurs to^ some extent in the Hollyhock itself (fig. 173), but 

 here even in the most fully double flowers both pollen and ovules 

 appear always to be present. The difficulty sometimes in this case 

 is to draw the line between a true single and the individual in which 

 the first stages of doubling are evident in some of the flowers but not 

 in all, and may therefore easily be overlooked. A more striking case 

 is that of the Welsh Poppy (Meconopsis cambrica), where the flowers 

 on some individuals, although all of the fully double type, may vary 

 to the extent that some will show a few functional stamens but have 

 no ovary, others may have an ovary capable of yielding a small 

 quantity of seed but be destitute of pollen, while others again lack 

 both reproductive organs. Classification according to sterility or 

 otherwise of the sexual organs is here not practicable. 



To sum up the various morphological points that have been touched 

 upon we find : 



(1) That the double condition is due to several distinct mor- 



phological causes. 



(2) That different strains of the same species may exhibit quite 



distinct types of doubleness. 



(3) That in some species almost every intermediate grade may be 



found between the single and the extremely double type, and 

 further that in some cases even the different flowers on one 

 individual may show distinct grading. 



When we come to investigate the inheritance of doubleness we find 

 in the same way considerable variability in the genetic relationship of 

 double to single. In some forms singleness is found to be dominant 

 and doubleness recessive, and in others the reverse is the case. Thus, 

 for example, in the Welsh Poppy (Meconopsis camhrica) and the 

 Carnation (Dianthus CaryophylliLs) the single condition is recessive to 

 the double; in other words, when pure-breeding doubles and singles 

 are intercrossed the resulting crossbreds are more or less double. In 

 both cases doubling results from a petaloid metamorphosis of the 

 stamens, either alone or combined with a somiewhat irregular splitting 

 0^ these structures. There is some evidence that the same relation of 

 dominant to recessive between double and single also holds good in 

 the case of Lobelia. On the other hand, when similar matings are 



