Sterile Varieties. 93 
nuncidus acris (Vol. I, Fig. 40, p. 194), Caltha palustris, 
Anemone nemorosa, Hepatica triloba, Tropaeolum ma jus 
florc plena, Clematis recta, Barbarea vulgar is 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), Pyrethriim ro- 
seum, etc. Others, such as Anthemis nobilis, are known 
to bear seed from time to time and so do not belong here. 
Viburnum Opulus, Hydrangea hortensea, Muscari co- 
mosum plumosum 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 Pseud-Acacia inermis 1 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) which SCHROTER 
1 DE CANDOLLE, Physiologic, II, p. 735. 
