INFERTILE HYBRIDS. 183 
INFERTILE HYBRIDS. 
By Joun H. Witson, D.Sce., F.R.S.H., The University, St. Andrews. 
A WIDESPREAD popular belief exists that infertility, or even complete 
sterility, is a very usual characteristic of hybrids. This belief may in 
large measure be traceable by association to the well-known instance of 
the mule amongst animals, and to some the term “ hybrid” is synonymous 
with “ mule.”’ 
Tt is well known that certain plants are much more easily inter- 
crossed than others. It is an equally familiar fact that some hybrids 
are perfectly fertile, some more or less so, and some completely sterile. 
It is probable that sufficient material exists to permit of the elucidation 
of the problems involved in groups where many species and varieties 
have been crossed and observations carefully recorded. The questions, 
Why are certain plants so easy or so difficult to cross? and Why are the 
hybrids so fertile or so infertile? are often asked, and only incomplete 
answers have been given to them. Many problems which seem hopelessly 
obscure may yet be solved by Mendelian methods. In the present paper, 
however, all that is attempted is to describe a number of new hybrids with 
reference to peculiarities of their structure, and their behaviour under 
experiment, their infertility being the chief reason why they were chosen 
for study. 
Dieiratis Hyprips. 
Digitalis lutea 9 x D. purpurea @. 
Records show that Digitalis purpurea and D. lutea have been 
repeatedly hybridised, both spontaneously and experimentally. A plant 
with this parentage was made the subject of careful examination and 
graphic degcription by Henslow seventy years ago.* It appeared 
spontaneously in a garden where the above species grew together. 
Henslow states that “the plant in question was undoubtedly a seedling 
from a specimen of D. lutea,” and adds, “ My plant exactly agrees in most 
particulars with a hybrid procured by Kélreuter in 1768 from seeds of 
lutea fertilised by the pollen of purpurea.” 
Focke mentionst that Kolreuter did not succeed in fertilising 
D. purpurea with pollen of D. lutea. Gaertner succeeded in producing 
the union, but failed in the great majority of attempts. Godron and 
Focke himself, however, found no difficulty in carrying out the cross. 
I did not succeed with it, but my experiments were few. 
It may be taken as certain that it is much easier to effect the cross 
when D. purpurea is used as the pollen parent. My hybrids with this 
parentage bore a close resemblance to those described by other observers 
The leaves were deep green, and smooth on the upper surface. While 
* Camb. Phil. Trans., vol. iv. p. 257. + Die Pflanzen-Mischlinge, p. 315. 
