1922] BUCHHOLZ—VASCULAR PLANTS 273 
correlative “struggle”? between vegetative parts, or flowers in a 
crowded inflorescence. It is evident that he recognized a significant 
similarity in these forms of developmental selection, although 
apparently he did not anticipate the significance of these facts in 
relation to the selective mechanism of evolution. 
A very unique form of developmental selection is represented 
in Welwitschia mirabilis described by PEARSON (32). The female 
gametophyte gives rise to a number of nuclei, potentially eggs, 
which develop prothallial tubes that grow up into the nucellar 
tissue. When such a prothallial tube comes in contact with a 
pollen tube, fertilization takes place. The embryos may be found 
growing down through these prothallial tubes into the female 
gametophyte tissue. Although we have polyembryony, the selec- 
tion is probably in part predetermined by priority of fertiliza- 
tion, which depends upon the pollen tubes and the prothallial 
tubes of the female gametophyte. Apparently the selection 
resolves itself, in part at least, into a competition between eggs, or 
prothallial tubes containing eggs, a form of selection between female 
gametes which is very rare in plants. 
Megaspore tetrad formation and the abortion of the megaspores 
in angiosperms might suggest itself as a form of developmental 
selection, but the selection in this case seems to be largely one of 
position. It is not any megaspore of the group in a linear tetrad 
that may give rise to the embryo sac, but almost always the inner- 
most of the four. This selection is not dependent on the physio- 
logical success of the megaspore, but is morphologically fixed, and 
therefore not properly included among processes of developmental 
selection. 
Among angiosperms the selection between male gametophytes 
or pollen tubes represents the most important developmental 
selection machinery. In the pistil of the ordinary flower an 
excessive number of pollen grains may germinate on the stigmatic 
surface, but usually only a limited number of these can function 
in fertilizing the eggs within the ovules. Only one pollen tube is 
necessary to fertilize the single ovule in the pistil of maize, yet 
hundreds may fall on each stigma and germinate, producing pollen 
tubes of varying lengths. Fig. 28 represents the pistil of an 
