VEGETATIVE AND SEXUAL REPRODUCTION 165 



Mlcropyle 



Polar 

 Egg nuclei Antipodal* 



Integuments 



Micropy 



gametes are formed in the pollen grain on the stigma and the 

 female gamete is developed in the embryo sac, the plant has 

 before it the problem of getting the gametes together. This 

 difficulty has been solved in the seed plants by the production 

 of the pollen tube, which forms a canal down which the male 

 gametes move toward the egg located in the embryo sac. Very 

 soon after the pollen 

 grain is deposited on 

 the stigma its outer 

 coat ruptures and its 

 thin elastic inner coat 

 begins to extend in 

 the form of a tube, 

 the pollen tube, which 

 grows down into the 

 tissues of the style. 

 The tube nucleus and 

 the male gametes pass 

 out of the pollen grain 

 soon after the pollen 

 tube starts to elon- 

 gate, and move down 

 the tube with its 

 growth and extension 

 through the style. 



FIG. 86. Ovules, fertilization, and seed 

 of the mandrake 



a, young ovule with contained spore (megaspore) ; 



b, mature ovule with an embryo sac containing an egg 

 and synergidae at the left, two polar nuclei in the center, 

 and three antipodal cells at the right ; c, fertilization 

 with the entry of the pollen tube into the embryo sac ; 



When the pollen tube d > the seed with an embryo (light) and endosperm 



(dotted). All diagrammatic 



reaches the ovary 



cavity, it is apparently attracted by some chemical substance 

 secreted from the micropyle of the ovule, since it turns sharply 

 and makes its way into this pore between the ovule coats. 

 When it reaches the embryo sac, it comes in contact with the 

 egg apparatus and the egg cell, which always lies at the base of 

 the micropylar canal. The pollen tube then ruptures at its thin 

 end, and the tube nucleus and male cells enter the embryo sac 

 in the immediate vicinity of the egg, as illustrated in Fig. 86, c. 

 Fertilization. One of the male gametes, or its nucleus, then 

 unites with the female gamete to form the zygote, or fertilized egg 



