Nov., 1923] 
DAVIS-POLLEN- AND SEED-STERILITY 
463 
any plant showing high proportions of shriveled pollen may very justly be 
an object of suspicion. There is a form of pollen sterility due to malnutri¬ 
tion, and this condition may be brought on experimentally by operations 
seriously affecting the vegetative activities of a plant, as, for example, 
stripping the leaves from stems. In this connection should be mentioned 
a form of abortion presented in heterosporous plants when one or more 
megaspores, perhaps through more favorable position or the good fortune 
of a better start, are able to develop at the expense of their neighbors which, 
giving up their substance, are sacrificed for the good of those megaspores 
that survive. This is fundamentally a form of sterility due to malnutrition, 
but it is not of course peculiar to hybrids. 
Sterility which results from failure to develop gametes is one thing, and 
sterility due to the inability of gametes to unite is quite another. The 
latter form of sterility is much more difficult of study in plant material than 
in animal because the fusion of gametes in higher plants takes place in 
structures and tissues difficult of examination. Zoologists recognize the 
phenomenon and it may be expected to be present in plants, but forms 
most profitable for study seem rather more likely to be found in groups of 
the thallophytes than among the higher plants. Sterility of this character 
need not be due to particular physical or chemical conditions that prevent 
the union of gamete protoplasts. Pollen grains may not find the secretions 
of the stigma favorable for their germination. There is probably a very 
large amount of sterility which results from the inability or slowness of some 
pollen tubes to penetrate certain lengths of style. We know something of 
this matter through the studies of East on Nicotiana, of D. F. Jones with 
pollen mixtures in maize, and from observations of Buchholz and Blakeslee 
on pollen-tube growth in Datura. It is probably a factor of importance in 
determining some of the results of Oenothera breeding. 
Zygotic sterility is very common among plant hybrids. It means that 
the zygote is either unable to develop at all or that it produces an embryo 
which dies early in the production of a seed-like structure. In either case 
this form of sterility may be suspected from the presence of shriveled ovules 
or shriveled seed-like structures of various sizes, but generally smaller 
than normal seeds. Sometimes there may be structures as large as seeds 
and externally like them but without embryos. Zygotic sterility is therefore 
conveniently recognized by seed sterility, only care must be taken to make 
sure that seeds are really sterile because of internal conditions and not merely 
slow of germination. In plant genetics it is necessary to know these facts 
before conclusions can be drawn on the significance of so-called “percentages 
of germination.” Conditions must be arranged to force seeds to complete 
germination, and examinations of the residue of ungerminated seed-like 
structures must be made to make sure that germination is really complete. 
Of course it is impossible to draw a fast line between the embryo which 
dies in the seed and the embryo which comes forth a weakling unable to 
