136 



INTRODUCTION TO BOTANY 



supposed that the pollen tube is nourished by the cells that 

 are broken down in the path of the tube. The time required 

 for the pollen tube to reach the ovule varies in different plants, 



ranging from a few hours to 

 more than a year. Usually 

 the tube finds its way into 

 the ovule through a minute 

 opening known as the micro- 

 pyle (fig. 119, m), but in 

 some plants it grows directly 

 through the substance of the 

 ovule. 



127. Fertilization. On en- 

 tering the ovule, one of the 

 male cells unites with the egg 

 nucleus of the embryo sac, 

 which is within the ovule, and 

 the other male cell in some 

 cases, or perhaps usually, 

 unites with the central nu- 

 cleus of the embryo sac to 

 form the endosperm nucleus 

 (fig. 119, en). The nature 

 and function of the latter 

 union are not as yet per- 

 fectly understood. The fusion 

 of egg nucleus and male cell 

 is, however, a very common 

 and most important phenom- 

 enon, occurring in many of 

 the simpler plants as well as 

 in the higher ones. In general it may be said that fertilization 

 consists in the union of the nucleus of a male cell with the nucleus 

 of an egg cell Other illustrations of this will be given in 

 Chapters XV-XVIII. After fertilization the egg rapidly 

 grows into the embryo of a new plant. The ovule also grows 



-m 



FIG. 119. Diagram to illustrate course 

 of the pollen tube during fertilization 



p, pollen grains ; t, pollen tube ; n, nucel- 

 lus, or body of the ovule ; a, antipodal 

 cells of embryo sac ; en, endosperm nu- 

 cleus of embryo sac ; egg, the egg ap- 

 paratus, consisting of the egg cell and 

 two cooperating cells ; m, the micropyle, 

 or small opening, through which, in most 

 ordinary flowering plants, the pollen 

 tube makes its way to the egg at the tip 

 of the embryo sac 



