Sterile Varieties. 93 



mmcuhis acris (Vol. I, Fig. 40, p. 194), Caltha palustris. 

 Anemone nemorosa, Hepatica triloba, Tropaeolum majus 

 flore pleno, Clematis recta, Barbarea vulgaris floribus 

 plenis and many others are alleged in horticultural litera- 

 ture to be perfectly sterile. Double varieties of compo- 

 sites also afford instances such as Achillea P tar mica, 

 Ageratum mexicanum (some varieties), Pyrethriun ro- 

 seiim, etc. Others, such as Anthemis nobilis, are known 

 to bear seed from time to time and so do not belong here. 



Viburnum Opulns, Hydrangea hortensea, Mnscari co- 

 niosnm plnmosum and others have become sterile by the 

 transformation of their reproductive flowers into sterile 

 ornamental ones. Bananas and other seedless fruits 

 have already been dealt with in the first volume (p. 195). 



Many varieties of the sugar-cane never set seed, such 

 as the Cheribon cane which is the richest in sugar. This 

 variety, which extends over vast regions, consists of a 

 single individual ; that is to say, it is derived from one 

 single unknown stock plant and has always been propa- 

 gated by cuttings or so-called Bibits only. 



Robinia P send- Acacia inermis^ is also said never to 

 flower ; and is only propagated by grafting. 



If this sterility affects annual species or such as 

 cannot be permanently reproduced by vegetative means, 

 the sterile form must vanish sooner or later. Such forms 

 hardly deserve the name of variety, and are usually 

 spoken of as monstrosities. But, in regard to their ori- 

 gin, they are perfectly analogous with the sterile forms 

 of which we have already treated. In the first volume 

 (p. 195, Fig. 41), I gave the sterile maize as such an 

 instance. More remarkable still is the unbranched Fir 

 {Pinus excelsa aclada or monocaulis) wdiich Sciiroter 



^De Candolle, Physiologic, II, p. 735. 



