POLLINATION AND FERTILIZATION. 



205 



respond to the spermatozoids, though they are not motile. 

 Sometimes the sperm cells are formed within the pollen grain. 

 At other times they are only formed 

 after the pollen grain has germinated. 

 In fig. 184 is a germinating pollen 

 grain of peltandra, showing three 

 nuclei. The generative cell has di- 

 vided to form the two sperm cells. 



336. The embryo sat? is the female 

 prothallium. — Now while the small Germinating spores 



, , . (pollen grains) of pel- 



Spore (= the pollen gram) escapes tandra ; generative 



,. r . , . , nucleus in one undl- 



usually from the anther, the larger vided, in other divi- 



, . , . ded to form the two 



spore (= embryo sac), borne in the sperm nuclei ;vegeta- V^j3 



. , ,, tlve nucleus in each V^J^ 



OVUle On the piStll, never escapes COm- near the pollen grain. 



pletely from the ovule, and only rarely protrudes part way. 

 Inside of the nucellus, which is the central part of the ovule, a 

 sac is formed which contains several nuclei. It is the embryo 

 sac, or large spore, as shown in the diagram. It is also the 

 female prothallium. One of these nuclei is the egg nucleus, 

 but the prothallium is so reduced that there is no archegonium 

 wall. The egg itself is perhaps the reduced archegonium. 



337. Fertilization. — When the pollen tube grows down the 

 pistil and into the embryo sac in the ovule, as shown in the 

 diagram (fig. 182), one of the sperm nuclei which it bears unites 

 with the egg nucleus of the embryo sac. This is fertilization. 

 The fertilized egg now grows to form the embryo. So the em- 

 bryo is formed inside of the ovule. This is what makes the seed. 

 The ovule with its coats contains the embryo. Since the embryo 

 sac containing the egg does not escape from the ovule, the sperm 

 cell must in some way be brought to it. This necessitates the 

 transportation of the pollen from the stamen to the pistil. 

 This transportation of the pollen from the stamen to the pistil 

 is pollination. Botanists now usually distinguish in this way 

 between pollination and fertilization. 



338. Difference between organ and member. — While it is 



