44 



BIOLOGY AND HUMAN LIFE 



Fig. 19. Fertilization in a flower 



When a pollen grain, p, alights on the moist sur- 

 face of a stigma, s, it absorbs water and puts 

 forth a thread of protoplasm, or a pollen tube, 

 pt, which grows down the style into the ovary. 

 The tip of the pollen tube finds its way to the 

 inside of the ovule, 0, through a small passage- 

 way, the micropyle, m. The large cell in the 

 middle of the ovule, called the embryo sac, es, 

 undergoes a number of changes which result in 

 producing several nuclei. One of these nuclei at 

 the end nearest the micropyle corresponds to an 

 egg cell. Similar divisions take place in the 

 nucleus of the pollen grain, and one of the re- 

 sulting nuclei corresponds to a sperm cell. The 

 cell walls separating the pollen tube and the em- 

 bryo sac dissolve, and the pollen nucleus unites 

 with the egg nucleus. The newly formed joint 

 nucleus, or fertilized egg, begins to divide. 

 Thus it develops into a new plant, or embryo; 

 the ovule containing it becomes a seed; the ovary 

 becomes a fruit 



which can be seen only 

 with the microscope (see 

 7, Fig. 32). This pollen 

 tube grows through the 

 style and into the hol- 

 low of the ovary, then 

 through a small hole in 

 the ovule, the micropyle, 

 which means "small gate- 

 way" (see in, Fig. 19). 

 Finally it reaches the 

 central space of the 

 ovule, where there is a 

 special mass of jelly like 

 living stuff, the embryo 

 sac (see es, Fig. 19). 

 Here a portion of the 

 matter in the pollen tube 

 unites with a portion of 

 the matter in the em- 

 bryo sac, and from this 

 united mass a new plant 

 begins to develop. The 

 uniting of the two masses 

 of living matter is now 

 called fertilization. This 

 is the real act of repro- 

 duction, for its result is 

 in fact a new individual 

 (see Fig. 19). 



36. Seed and fruit. 

 After fertilization takes 

 place the mass in the em- 

 bryo sac absorbs food in 

 large quantities from the 

 parent plant and becomes 



