180 MORPHOLOGY OF THE ANCIOSPERMS 



megaspores; in this, it resembles the tubes of the araucarian conifers, 

 which destroy the contents of immature archegonia in their course. 



The Tube Nucleus. The nucleus of the vegetative cell, the tube nucleus, 

 commonly shows signs of degeneration as the generative cell matures, 

 and may change in shape. It has been generally stated that, as the de- 

 velopment of the pollen tube begins by extrusion of the intine of the 

 pollen-grain wall, the tube nucleus passes into the tube and, as the 

 tube grows, maintains a position in tlie distal part, where it "governs 

 apical growth." But the supposed function of this nucleus has been 

 questioned. The nucleus may remain within the spore wall; it is often 

 degenerate before the beginning of tube growth; it may not enter the 

 tube until the tube is well developed; it often follows, rather than pre- 

 cedes, the generative cell or the gametes (the relative position of the 

 two or three nuclei is perhaps a matter of chance); it may not progress 

 far, and the male cells may pass it. The tube may branch, even re- 

 peatedly — Fagiis and other Amentiferae — and the branches which have 

 no nuclei develop as well as that with the nucleus. The vegetative cell 

 has also been called a vestigial cell, the last survivor of the vegetative 

 tissue of an ancestral, free-living gametophvte. As such, it would be 

 homologous with the one or two prothallial cells of the cycads, Ginkgo, 

 and the conifers. 



Male Gametes. Division of the generative cell, forming two male 

 gametes or sperms, may follow soon after it is mature or may be de- 

 layed until the cell has passed into the pollen tube. Within the tube, 

 division may occur close to the base or far along, even well down in the 

 style — LiJitim. In some species, it apparently may occur in either place. 

 Division of the generative cell in the ungerminated pollen grain as it 

 lies on the stigma also occurs. The pollen grain is either 2- or 3-nucleate 

 — 3-nucleate if the male gametes have been formed before shedding. It 

 is uncertain whether the two-celled or the three-celled pollen grain is 

 more common. The male gametes are definite cells, with limiting cyto- 

 plasmic sheaths, not naked nuclei, as often formerly believed. In shape, 

 they are spherical, ellipsoid, lenticular, or vermiform. Their shape may 

 change as they pass through the pollen tube. The two gametes are usu- 

 ally alike in size. DiflFerences in size and staining quality — the gamete 

 that unites with the egg, often smaller and staining less deeply — have 

 been reported in a few species. 



The question of possible motility of the male cells has received con- 

 siderable attention. In the lower gymnosperms, the sperms are ciliate 

 and actively motile; in the higher gymnosperms, the pollen tube is a 

 "male-cell carrier," as in the angiosperms, and independent movement 

 seem„s to have been lost. It has been claimed that the sperms of angio- 

 sperms have some capacity for "active movement" independent of 



