CONJUGATION IN FLOWERING PLANTS 



109 



frequently bent sharply round as shown in Fig. 55. Opposite 

 to the spot where the funiculus is attached to the ovule an 

 aperture is left in the integuments known as the micropyle (w). 

 It is through this micropyle that the tip of the pollen tube 

 usually forces its way in search of the female gamete (<?), which 

 lies at the end of the embryo sac close to it. The wall of the 

 pollen tube and that of the embryo sac are absorbed where 

 they come in contact with one 

 another, and thus a passage is 

 opened for the male gamete, 

 which passes down the pollen 

 tube into the embryo sac and 

 there conjugates with the ovum. 

 The zygote thus formed develops 

 within the embryo sac into an 

 embryo sporophyte l ; the integu- 

 ments of the ovule become har- 

 dened to form the seed coat ; 

 reserve food material, such as 

 starch or oil, is stored up 

 either in the embryo itself or 

 in the endosperm 2 around it, and 

 the ovule and its contents 

 separate from the parent sporo- 

 phyte as the ripe seed (Fig. 

 56, A). 



When the seed ripens the 

 development of the contained 

 embryo is suspended for an 

 indefinite period, to be resumed 

 again if and when the seed finds 

 a suitable situation in which to germinate. The embryo may then 

 continue its development into the adult sporophyte (Fig. 56, B). 



It is interesting to notice that no less than three distinct 

 generations take part in the formation of the seed. The seed 

 coat belongs to the parent sporophyte, while the contents of the 



1 Vide pp. 48 oO (Fig. 14). 



2 The second generative nucleus from the pollen tube conjugates with the nucleus 

 in the middle of the embryo sac (Fig. 55 A'), and the " endosperm " arises by repeated 

 division of the nucleus resulting from this conjugation. The whole process is 

 somewhat complex and it is not essential to our present purpose to describe it in 

 detail. 



FIG. 55. Diagram of the unfer- 

 tilized Ovule of a Flowering 

 Plant (Angiosperm) in longi- 

 tudinal Section. (From Vines' 

 " Botany.") 



ai, outer integument ; at, antipodal cells ; 

 e, ovum ; E, embryo sac ; f, funiculus ; 

 ii, inner integument ; k, central or 

 definitive nucleus of embryo sac ; K, 

 nucellus ; m, micropyle ; s, synergidae. 



