52 



BOTANY OP CROP PLANTS 



xenogamy, or cross-pollination. Insects, wind and water are 

 the chief agents in the spread of pollen. 



Fertilization. — Fig. 23 is a diagram of an ovary with a 

 single ovule cut lengthwise. It shows a stage of develop- 

 ment of the ovule about at the time when the pollen grain 

 has reached the stigma. As has been said, the mature pollen 

 grain consists of a protoplasmic mass surrounded by a rather 

 thick wall. Three nuclei (Fig. 24) constitute 

 the important structures in the pollen grain. 

 It absorbs water and nutrient materials from 

 the stigmatic surface, and grows by sending 

 out a tube, known as the pollen tube. The 

 tube grows downward through the stigma, 

 sometimes in a tubular passage, or when 

 necessary, secreting enzymes which digest 

 (render soluble) the walls of cells that are in 

 its path, at the same time deriving nourish- 

 ment from this digested material. As the 

 tube grows, the three nuclei keep pretty close 

 to the tip, the tube nucleus in the lead, with 

 the two sperm nuclei (male gametes) follow- 

 ing. The tube finally reaches the ovule, 

 takes a course through the micropyle and comes into con- 

 tact with the nucellus. This nucellar tissue is penetrated, 

 and after dissolution of the wall at the tip of the pollen tube, 

 the three nuclei are discharged into the embryo sac. The 

 tube nucleus is reabsorbed. One sperm nucleus unites with 

 the egg nucleus (female gamete) to form the zygote, a nuclear 

 mass which contains both the characters of the plarit fur- 

 nishing the pollen - (paternal characters) and those of the 

 plant fertilized (maternal characters). The union of the 

 male gamete (sperm nucleus of pollen tube) with female 

 gamete (egg nucleus of embryo sac) is fertilization. It is a 



■tube nucleus 



Fig. 24. — Ger- 

 minating pollen 

 grain. {After 

 Bergen and Cald- 

 well.) 



