442- THE STRUCTURE OF ECONOMIC PLANTS 



fertilization, there are three antipodal nuclei at the chalazal end 

 of the gametophyte, two centrally located polar nuclei, and the 

 megagamete with two synergids at the micropylar end. At this 

 time, the megagametophyte is surrounded by nucellar tissue which 

 is 2.0 to 15 layers in thickness except at the chalaza. The antip- 

 odal nuclei are ephemeral and disappear by the time the flower 

 opens. 



Fertilization. — Various estimates have been made as to the 

 length of time required between pollination and fertilization. 

 These range from approximately 15 to 36 or 40 hours. The insect- 

 borne pollen grains reach 

 the stigmatic surface of 

 the pistil which is covered 

 with unicellular hairs, and 

 the pollen tubes rapidly 

 penetrate the tissue of the 

 stigma and style. Once 

 inside the ovary, the pollen 

 tube follows the placenta, 

 growing within the tissue 

 or on the surface; and, 

 upon reaching the base of 

 an ovule, travels up the 

 funiculus toward the mi- 

 cropyle. The path of the 

 pollen tube through the 

 micropyle and into the 

 nucellar tissue is variable, the tendency being for the tube to pene- 

 trate for a distance between the integuments, or between the inner 

 integument and the nucellus, before entering the latter. This 

 results in the tube having a more or less branched or haustorial 

 appearance. 



Following the penetration of the gametophyte by the pollen 

 tube, double fertilization is effected. One microgamete fuses with 

 the megagamete to form the zygote, while the second microgamete 

 undergoes a triple fusion with the polar nuclei to form the primary 

 endosperm nucleus. Variation in the details of the triple fusion 

 has been reported, but most frequently the polar nuclei fuse before 

 the union with the second microgamete. In other instances, the 

 fusion of the two polar nuclei and the second microgamete seems 



Fig. zx6. A, diagram of transection of young 

 four-locked ovary showing placentae and develop- 

 ment of young ovules ; B, C, and D, successive stages 

 in development of ovule. (B and C, after Gore.) 



