1922] BUCHHOLZ—VASCULAR PLANTS 271 
event does not usually occur absolutely simultaneously even in 
a single ovule. Some embryos are usually produced a little earlier 
than the others, and have a slight advantage, although this differ- 
ence may represent only a few hours or minutes. The competition 
is continued between the several embryos in the ensuing embryonic 
selection, which becomes truly a struggle for existence, so that 
the resulting seed has only one embryo. Here in gymnosperms 
the developmental selection process is a sort of relay race between 
one pollen tube plus its embryo, and other pollen tubes plus their 
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Fic. 26.—Sections of ovules of Sequoia sempervirens, showing selection between 
female gametophytes: A, ovule with six megaspore mother cells; B, numerous 
germinating megaspores; C, older stage showing two young gametophytes whose 
competition has persisted into multinuclear stage; after Lawson (28). 
embryos. The embryo of a gymnosperm seed is therefore the 
survival of a rapidly developing pollen tube combined with a very 
vigorous embryo. 
Among conifers the male gametophytes are not the only indi- 
viduals taking part in this competition. Female gametophytes 
may also undergo competition under normal conditions in some 
species. DupLer (16), in his work on Taxus canadensis, showed 
that the existence of several female gametophytes arising from as 
many megaspores is quite the normal condition. Lawson (28) 
found a similar situation in Sequoia (fig. 26), and apparently also 
