CHAPTER XVII. 



THE EMBRYO AND THE SEED. 



In norma] cases if no pollen-tube arrives at the micropyle of the ovule, 

 and the ovum is consequently not fertilised, the ovule develops 

 no further. But if fertilisation by a pollen-tube has been carried 

 out, changes follow not only in the ovum itself and the other contents 

 of the embryo-sac, but also in other parts of the ovule. The term 

 Seed is applied to the body which results from this further development 

 of the ovule, while the germ which originates from the fertilised ovum 

 is called the Embryo. 



After fertihsation the earliest changes appear in the embryo-sac 

 itself. The Synergidae shrivel, and a cell-wall is deposited round 

 the fertilised ovum, or Zygote, which remains attached at the micro- 

 pylar end of the embryo-sac. It soon elongates, its free end extending 

 into the cavity of the sac. Meanwhile its nucleus, which has resulted 

 from the fusion of the male and female nuclei, divides, showing the 

 double number of chromosomes characteristic of the diploid genera- 

 tion. The zygote then divides into two cells by a transverse wall, 

 and this may be followed by further divisions in planes parallel 

 to the first ; so that a simple filament, or Pro-embryo, is formed 

 (Fig. 217, i.). The very first division of the zygote stamps the polarity 

 of the pro-embryo, and from that point on defines its apex and base. 

 The basal end of the pro-embryo remains attached to the micropylar 

 end of the sac, the apical end projects into the cavity, and produces 

 the embryo. It is a significant fact that in the great majority of 

 Flowering Plants, though not in all, this filamentous stage, showing 

 polarity and consisting of a varying number of cells, appears first. 

 It has the practical result of carrying the embryo deep down into 

 the sac, but it may also have a phyletic meaning, as indicating 

 an ancestral filamentous construction. The pro-embryo then difTer- 



274 



