832 ECOLOGY 



flowers of the same or nearly related species, but which have either 

 no effect or a detrimental effect upon other pollen. 



The pollen tube. — Under suitable conditions pollen grains adherent 

 to the stigmatic surface germinate, and the developing pollen tube, 

 which is the bearer of the male cells, penetrates the style and enters 

 the ovary; ultimately it may reach the female gametophyte in- 

 side an ovule, where the fusion of the gametes takes place. Usually 

 the pollen tube enters the ovule through the micropyle {m, fig. 594), 

 which is a narrow channel at the ovule apex, where the enveloping 

 integuments have not quite grown together. In some species the 

 pollen tube penetrates the pistil so rapidly that the gametes fuse 

 a few hours after pollination, while in other species a number of 

 months elapse between pollination and gamete fusion (as in the oaks 

 and pines). 



The secluded position of the female gamete and the usual non-motility 

 of the male gametes make the pollen tube an organ of the first impor- 

 tance in the facilitation of sexual reproduction in most seed plants, since 

 it bears the male cells (sometimes for an almost incredible distance) 

 in its fungus-like course through the pistil tissues, from which it derives 

 food parasitically. This method of bringing the male gametes into the 

 proximity of the egg seems especially suited to land plants, since it elim- 

 inates the necessity of a liquid medium, such as is required by motile 

 sperms. Pollen grains germinate readily in various liquid media, swell- 

 ing rapidly and sending out tubes for a short distance. In respect to 

 conditions favoring germination, pollen grains show wide diversity, 

 especially in their osmotic relations with the medium. The pollen of 

 a number of species germinates readily in distilled water, but in other 

 cases this medium causes the grains to burst; Canna grains, for example, 

 burst in water, but not in a 2 per cent cane sugar solution. Most pollen 

 germinates in cane sugar solutions, that of some species requiring high 

 concentration, while that of others germinates readily in solutions of low 

 concentration. Pollen grains that are difficult to germinate (as those of 

 the grasses) send out tubes if they absorb water slowly. Some pollen 

 (as in certain umbellifers and composites) has never been seen to germi- 

 nate except on stigmas. Probably because of the presence of the proper 

 stimulating substance at the proper degree of concentration, germina- 

 tion usually takes place more readily on stigmas than in artificial media, 

 and complete development does not occur unless germination has taken 

 place on the stigma of the proper plant {viz. of the same or of a closely 



