482 FUNDAMENTALS OF FRUIT PRODUCTION 



immediately, ''As soon as the end of the pollen tube enters the embryo- 

 sac it opens, discharging the two male gametes and other contents. One 

 of the male nuclei enters the egg-cell and applies itself to the nucleus of 

 the egg, while the other passes into the cavity of the sac. ... It is pre- 

 sumably the first male nucleus which escapes from the pollen tube that 

 unites with the nucleus of the egg, but positive proof on this point is 

 wanting. ... As fusion progresses, the nuclei become quite alike in 

 shape, size and structure. Their membranes gradually disappear at the 

 place of contact, their cavities become one, and the resulting fusion 

 nucleus, which is in the resting condition, can scarcely be distinguished 

 from the nucleus of an unfecundated egg. The nucleoli finally unite 

 also." The fertilized egg cell becomes the embryo cell, the antecedent 

 of the embryo. 



Secondary Fertilization. — Attention has been called to the presence 

 of two nuclei, the so-called polar nuclei, near the center of the mature 

 embryo sac. These are shown clearly in Fig. 4 of Plate I. Usually these 

 two fuse with the second sperm nucleus and the nucleus resulting from 

 this triple fusion divides repeatedly giving rise to many daughter nuclei, 

 shown in Fig. 5 of Plate I. Soon these daughter nuclei are separated by 

 the formation of cell walls, the resulting tissue being the antecedent of the 

 seed endosperm. 



Sometimes the second sperm nucleus fuses with but one of the polar 

 nuclei^^^ and sometimes it degenerates in the cytoplasm of the embryo 

 sac. In the former case, the endosperm is of the same parentage as the 

 embryo beside which it develops; in the latter case it is built from 

 maternal tissue alone. In plants with albuminous seeds, this results in 

 the condition known as xenia. 



Development of the Embryo and Endosperm. — Following the process 

 of fertilization the embryo cell "divides by a transverse wall into two 

 cells, one directed towards the micropyle, the other towards the base of 

 the embryo sac. The upper of these two cells stretches, and is repeatedly 

 segmented; thus a string of cells is formed, known as the suspensor, bear- 

 ing at its lower extremity the embryo-cell, which gives rise to the greater 

 portion of the young plant. "^'^ This stage is shown in Fig. 1, Plate III. 



Coordinate with the development of the embryo is that of the 

 endosperm. To be exact, in most developing seeds the growth of the endo- 

 sperm is at first more rapid than that of the embryo. In many exal- 

 buminous seeds there is a period of very rapid growth of the endosperm 

 during which the young embryo either grows very slowly or persists in 

 a practically resting stage. This is followed by a period of rapid embryo 

 development, which occurs largely at the expense of the materials accu- 

 mulated in the endosperm. The initiation of this period of rapid growth 

 in the slow growing or resting embryos is apparently one of the "sticking 

 points" in the process of seed formation and in many species it is very 



