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1922] BUCHHOLZ—VASCULAR PLANTS 253 
a differential effect on the competing individuals. The foregoing 
outline is suggestive of the general relation of the several forms of 
developmental selection to the general process of natural selection. 
Developmental selection expresses itself in some form or other 
in the sexual reproductive cycle of practically all vascular plants. 
It is probably also involved in the life cycle of most of the crypto- 
gamic forms, and is a factor to be reckoned with among animals as 
well. The main purpose in this discussion is to describe in a general 
way the various expressions of the principle of developmental 
selection as it applies to vascular plants. 
The ordinary details of conifer embryogeny have been described 
(11) and may be assumed to be fairly well understood by botanists. 
It is generally known that in cycads or in such conifers as the spruce, 
for example, there are several embryos that engage in an intense life 
and death competition during their development. Only one 
embryo reaches its full term of growth to become the seed embryo, 
while the weaker individuals are aborted in the earlier stages. 
Only in extremely rare cases may two embryos be matured together 
in the conifer seed. In Ginkgo this happens rather more frequently, 
about 2 per cent of the seeds having been found with equally 
developed “twin” embryos (13). Even if several embryos should 
occur within the same testa, as in citrus seeds and several other 
angiosperms, these must necessarily come up so close together 
that a close competition between them will be inevitable after the 
seeds germinate. This competition which occurs after seeds 
germinate in the soil is environmental, however, and belongs to 
the categories of natural selection, where it remains as an intense 
intraspecific form of natural selection. 
When pollination is successful enough to provide a plurality of 
male gametophytes, making polyembryony possible, practically all 
gymnosperms possess the feature of embryonic selection. Here the 
female gametophyte tissue is well formed before the embryos 
begin to develop, is somewhat firm and resistant, and it is only by 
a vigorous growth and rapid suspensor elongation, together with 
an abundant secretion of digestive ferments, that the successful 
embryo matures at the expense of its fellows and brings about their 
destruction. In angiosperms the endosperm within which the 
