Phenomena and Problems of Self-Sterility. 205 
Dnsycladus clavceformis, or a heterothallic Mucor or Phycornyces 
should be unable to conjugate. 
It may be suggested that this is in reality a simple case of 
self-sterility. In a plant like Cardamine pratensis self-sterility may 
depend, it seems, on one or more of several hereditary factors, and 
consequently there are a large number of distinct self-sterile races. 
The fewer the factors the fewer the races; with one factor only 
two kinds of individuals are possible, viz., the heterozygote, and 
the homozygote for absence of the inhibitor: such a species would 
consist partly of individuals which could only be cross-fertilised, 
and partly of those which could both be self-fertilised and crossed ; 
a condition possibly occurring in some species of Spirogyra, though 
here as in all other isogamous Algae our knowledge is quite 
insufficient to make this suggestion either plausible or the reverse. 1 
In the Mucorinae the peculiar so-called sex-differentiation may 
quite conceivably be associated with true phenomena of self-sterility 
of a simple type: homothallic species would be those without 
inhibitory factors, heterothallic species would possess perhaps two 
such inhibitors. 
It is clear that these suggestions break down at certain points, 
or require modification—just as do those of Correns on which they 
are founded. The chief conclusions that must be drawn are that 
our knowledge of the facts is insufficient to bear the weight of any 
but the most tentative hypotheses; and that the new view of self- 
sterility which we now possess as a result of Correns’ work is likely 
to be of the greatest value in solving many of the most interesting 
problems in the whole range of Plant Biology. 
' Experiments are in progress which it is hoped may throw light on this 
problem. 
Botany School, 
Cambridge. 
April, 1913. 
SELECT BIBLIOGRAPHY. 
1 Backhouse, W. O. “Self-Sterility in Plums.” Gard. Chron., 1911, 
p. 299. 
2 Baur, E. “ Einfiihrung in die experimentelle Vererbungslehre.” 
p. 212, Berlin, 1911. 
3 Bernard, N. “ L’evolution dans la symbiose.” Ann. Sci. Nat. (Bot.), 
ser. ix, t. IX, p. 1, 1909. 
4. Biffen, R. H. “Studies in the Inheritance of Disease Resistance.” 
Journ. Agric. Sci., II, p. 109, 1907. 
