THE MALE GAMETOPHYTE 177 



The Pollen Tube. Germination of the pollen grain on the stigma occurs 

 after a period of time that varies greatly: from almost immediately — 

 Sacchariim, Sorghum; five to ten minutes — Taraxacum, Zea; two hours; 

 three hours; two days. The grain swells, probably by absorption of the 

 secretions of the stigmatic surfaces; the shape may be greatly modified; 

 furrows are filled out and flat surfaces rounded. The inner wall pro- 

 trudes at one or more of the thin areas in the outer wall — in the fur- 

 rows or apertures. In triangular pollen grains, tlie apertures are at the 

 corners of the grain, and the tube develops from one of these corners — 

 Proteaceae. By rapid apical growth, one of the protrusions becomes the 

 pollen tube. Rate of elongation of the tube varies greatly — with the 

 taxon, and with conditions of temperature and humidity. 



In early stages of tube development, the contents of the pollen grain, 

 or most of them, pass into the tube, where further development of the 

 cells may occur. The tube and its contents — together with any part of 

 the contents that may remain within the pollen-grain wall — constitute 

 the male gametophyte. The "pollen tube" — the tube plus its contents — 

 is commonly called the male gametophyte; the contents alone are some- 

 times described as the gametophyte. Objection has been raised to the 

 inclusion of the tube itself as a part of the gametophyte, but the tube is 

 a development of the microspore wall, and the spore is the first cell of 

 the gametophyte. Both the pollen tube and the embryo-sac wall are en- 

 larged and modified spore walls (walls of the spore mother cell or 

 dyad in tetrasporic and disporic sacs). The contents of the mature 

 pollen grain and of the young pollen tube, commonly called the male 

 gametophyte, represent the immature gametophyte. (The generative 

 nucleus may not yet have formed the male nuclei; the cytoplasm of 

 the tube continues to increase in volume as the tube develops.) The 

 male gametophyte of the angiosperms remains enclosed within the 

 greatly modified spore wall until the male cells are formed. In this en- 

 closure of the male gametophyte within the spore wall, the angiosperms 

 resemble other major heterosporous taxa. 



The pollen tube itself, as a male-cell carrier, is characteristic of the 

 highest gymnosperms and angiosperms only; by its aid, the male gametes 

 reach the egg. Accompanying this high specialization of the spore wall 

 has been loss of motility in the gametes. Only in the highest seed plants 

 is fertilization accomplished by nonswimming male cells. The pollen 

 tube represents one of the most difficult adaptations to life on land, the 

 step from fertilization in a fluid by free-swimming sperms — the ancient 

 method— to fertilization by nonswimming sperm cells, carried to the 

 egg by the gametophyte. No animals, and only the highest plants, have 

 made this advance. 



The pollen tube is probably an example of parallel development. The 



