380 TEXTBOOK OF BOTANY 



embryo sac with an egg nucleus and a fusion nucleus; (5) pollination; 

 (6) the germination of the pollen grain and growth of the pollen tube 

 from the stigma to the ovule; ( 7 ) the fusion of one sperm with the egg — 

 fertilization — and the fusion of the other sperm with the fusion nucleus 

 — triple fusion; (8) the development of the embryo from the fertilized 

 egg; (9) the development of the endospenn from the triple-fusion 

 nucleus in seeds of some plants; (10) the formation of seed coats from 

 the ovule coats; and (11) the development of the fruit from the ovulary 

 and other parts of the flower. The fertilized egg and embryo are the early 

 stages of a new generation. 



Deviations from the usual story of sexual reproduction. An examina- 

 tion of a seed of lima bean or garden pea reveals the absence of an 

 endosperm. Seeds of such plants as onion, grapefruit, and pine often 

 have more than one embryo, a condition known as poli/embryony. 

 Fruits of certain varieties of sunflower, orange, grapefruit, and grape are 

 seedless, a condition referred to as parthenocarpij . What deviations from 

 the usual story of sexual reproduction would account for these phe- 

 nomena? 



Plants such as the common dandelion have seeds containing viable 

 embryos which developed from unfertilized eggs: a process termed 

 parthenogenesis. 



In the seeds of some plants, such as corn cockle, coffee, water lily, 

 spinach, and pepper, the nucellus grows and becomes much enlarged, 

 and foods accumulate in it just as thev do in the embryo and endosperm. 

 When mature, this tissue resembles the endosperm in appearance and is 

 known as the perisperm. Sometimes it is merely a thin compressed 

 membrane within the seed coats. 



The failure of pollination and fertilization often results in the abscis- 

 sion of the pistil and no fruit develops. The early fall of immature fruits 

 such as those of apple, peach, pear, and plum may of course be caused 

 also by the invasion of parasites and by other unfavorable conditions. In 

 spite of numerous exceptions, pollination and fertilization are generally 

 necessary for the development of fruits and seeds in the great majority 

 of flowering plants. 



One of the most striking deviations occurs in species of Trillium. As 

 already noted, the embryo sac of trilliums ordinarily contains but four 

 nuclei; one of these becomes an egg and two others form the fusion 

 nucleus. No fertilization occurs. An abortive embrvo develops from the 

 unfertilized egg, and an endosperm develops from the fusion nucleus. 



