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TEXTBOOK OF BOTANY 



tube enters the 



Fig. 167. Dia- 

 gram of a pistil 

 with germinating 

 pollen grains and 

 pollen tubes of 

 various lengths. 

 The embryo sac 

 is in the 7-celled 

 stage, with a cen- 

 tral fusion nu- 

 cleus and an egg 

 (below). Fertil- 

 ization occurs 

 when a 

 from a 



tube unites with 

 the egg. After J. 

 T. B u c h h o 1 z. 

 Courtesy of 

 World Book Co. 



sperm 

 pollen 



embryo sac, often through the micropyle; the end of the 

 pollen tube swells and bursts, and the two sperms are 

 released. The tube nucleus then disintegrates. The em- 

 bryo sac now contains, in addition to the five transient 

 nuclei, an egg, a fusion nucleus, and two sperms. 



Fertilization. The next event is the fusion of one 

 sperm with the egg, resulting in the fertilized egg, or 

 zygote. The union is called fertilization. The other 

 sperm unites with the fusion nucleus, and the resulting 

 3-nucleate structure is the triple-fusion nucleus, also 

 called the endosperm nucleus. The union of the three 

 nuclei is termed triple fusion. 



The embryo and the endosperm. The fertilized egg 

 soon begins to grow, and manv cell divisions result in a 

 tissue known as the embryo ( Fig. 168 ) . The sugar used 

 in the growth of the embryo must have come from the 

 green parts of the plant on which the flowers were 

 borne. The embryo, when mature, is differentiated into 

 cotyledons, hypocotyl, and plumule, as noted in the 

 preceding chapter. It is a new plant. 



While the embryo is developing in the ovule, an- 

 other tissue, known as the endosperm, may develop 

 from the triple-fusion nucleus (Fig. 169). The cells 

 of the embryo and endosperm often contain consider- 

 able food, chiefly oils, starch, and protein. The condi- 

 tions in these cells are particularly suitable to the con- 

 version of sugar to these more complex foods. In seeds 

 of many plants the endosperm is very small or absent, 

 because it failed to grow. In such seeds the cotyledons 

 of the embryo are usually large and contain most of 

 the food. 



The embryo and endosperm of an angiosperm differ 

 little in their origin, but in their final outcome they are 

 quite unlike. The endosperm, when present, makes its 

 complete growth within the seed, and ordinarily no 

 other structures develop from it; the embryo, on the 

 other hand, is the young stage of a new generation of 

 plants. 



The seed and the fruit. The growth and hardening 



