1922] BUCHHOLZ—VASCULAR PLANTS 275 
especially in plants with long styles. While this pollen tube selec- 
tion in angiosperms is morphologically very different from’ the 
embryonic selection in gymnosperms, it is physiologically a very 
similar process. In the case of the embryonic selection the embryo 
sporophytes are digesting their way and pushing forward into 
gametophytic tissue, while pollen tubes represent gametophytic 
structures penetrating sporophytic tissue by what appears to be a 
very similar method. 
An important difference, however, between the gymnosperm 
embryonic selection and the pollen tube competition of angiosperms 
should be noted. The embryos concerned in the competition are 
diploid individuals, while the pollen tube and other forms of 
gametophytic competition take place between haploid individuals. 
The recent work of geneticists shows that lethal factors may be 
present in one member of a pair of chromosomes apparently without 
serious consequences, as long as the same lethal is not present at 
the same time in both chromosomes. Factors lethal to the gameto- 
phyte could not be protected in this way by a homologous chromo- 
some. 
INTEROVULAR AND INTERFLORAL SELECTION.—Another form of 
selection, belonging more or less completely to the categories of 
developmental selection, is that occurring in angiosperms between 
the ovules within the same ovary. In species of Quercus, for 
example, there are six ovules within the ovary, although normally 
only one ovule with one embryo is found developed in the acorn 
which matures from this ovary. In Fraxinus and in the olive 
there are two ovules in each pistil, yet only a single seed with one 
embryo is matured. There are numerous similar instances in the 
plant kingdom, and whether the elimination of the unsuccessful 
ovules in such cases occurs as early as the stage when the mega- 
spores or female gametophytes are developing, or only among 
the ovules containing zygotes after fertilization has taken place, 
remains to be determined. According to the published accounts 
of the morphology, the latter is probably what happens in Quercus. 
The selection between reproductive organs during early vegeta- 
tional stages, as that previously described for Selaginella, should 
be included in this category of developmental selection. 
