146 CANNON: STUDIES IN PLAnt HyBrRips 
when pollinated by a plant of the same sort (Rubus caesius x R. 
/daeus). 
Besides the forms of sterility that result from imperfect sexual 
organs an infertile condition may also result from other causes. 
For instance, in unisexual flowers, the male buds may fall away, 
as in the Cucurbitaceae and the Bignoniaceae, or in some hybrids, 
Pelargonium and Digitalis (D. litea x D. purpurea tubifiora Lindl.), 
the anthers may be so transformed that the flowers are hermaphro- 
dite. Finally, sterility may manifest itself in the reluctance with 
which hybrids produce flowers, as in those of Rhododendron, Epilo- 
dium, Cereus and fymenocallis, but Focke (/. c. 477) says that 
these are rare exceptions, since as a rule hybrids flower earlier and 
more profusely than pure species. 
As to the relation between vegetative vigor and relative fruit- 
fulness of hybrids, Swingle and Webber (/. c. 412) say that in 
some cases “the increase in vegetative vigor secured by crossing 
distinct species is at the expense of fertility, but this is by no 
means true in all, Focke says that ‘it was formerly thought that 
the diminished sexual fruitfulness is compensated by a greater 
vegetative luxuriance, a statement the untenableness of which, as 
Gartner showed, is most plainly demonstrated by the fact that 
many of the most fruitful crosses (Datura, Mirabilis) are also dis- 
tinguished by a most gigantic growth.’ On this subject Fritz 
Miller also says: ‘So far as my experience goes, the hybrids 
which grow the most luxuriantly are generally the most fruitful.’ ”’ 
Although other instances illustrating this principle might be given 
it will suffice, I think, to mention the remarkable walnut hybrid 
which Burbank succeeded in producing from the English form and 
the California one. This was referred to above (page 144). In 
this hybrid not only is there a great vigor of growth but the fruit 
is also unusually large and probably very abundant. 
There appears to be no relation between the ease of making 
a cross and the fertility of the resulting form, but as a rule, the 
nearer the relationship of the parental forms the more readily will 
they be crossed. Strasburger,* in commenting on this subject, 
says: ‘‘ Manche Familien neigen leichter dazu (Solanaceen, Caryo- 
phyllaceen, Irideen u. s. w.), andere bilden nur schwierig oder 
" *Strasburger, Lehrbuch der Botanik, Ed. 5, 248. 1902. _ 
