Lecture XXVI. 235 



of the carpels, where it adheres. These adhering pollen-grains may 

 usually be seen if mature stigmas are microscopically examined. 



Just before the dehiscence of the pollen-sacs the single nucleus 

 of the spherical pollen grain divides into two and a protoplasmic 

 partition develops dividing the grain unequally into a small len- 

 ticular cell containing one of these nuclei and leaving over the 

 greater part of the grain to be tenanted by the other. In this con- 

 dition the pollen-grain is transferred to a stigma by an insect. It 

 adheres among the papillae, absorbs nutriment and immediately the 

 larger of its two cells begins to form a tubular outgrowth. This 

 penetrates into the stigma and, passing down among those cells 

 which form the style, emerges in the upper end of the cavity of 

 the ovary. The tube continues its path along the inner surface of 

 the ovary and makes its way to the base of the stalk of the ovule. 

 Here it turns abruptly towards the micropyle and enters it. 

 Throughout its development this tubular growth absorbs food from 

 the cells among which it penetrates. In fact it behaves just like 

 a parasitic fungus. During its passage the lenticular cell detaches 

 itself from the wall of the pollen-grain, passes into the tube 

 and wanders down after the nucleus of the tubular cell. The pro- 

 toplasm of the lenticular cell becomes elongated and its nucleus 

 divides into two, which with the covering cytoplasm become two 

 sperm -cells. The two elongated sperm-cells pass by the pollen- 

 tube nucleus. By the continued growth of the tube they are 

 brought through the micropyle. The tube, having forced through 

 what remains of the nucellus applies itself to the embryo-sac. At 

 the point of contact the wall of the sac and tube dissolve and the 

 two sperms passing the synergidae enter the sac* One fuses with 

 the ovum and the other with the fusion-nucleus. 



The oosperm formed of the united sperm and ovum gets a cell- 

 wall and proceeds by nuclear and cell-division to form a short 

 column or row of cells, fixed in the upper end of the embryo-sac 

 by one end and projecting into the middle of the sac where it 

 terminates in a globular cell. The latter is called the embryonic 

 cell and the rest forms the suspensor. The embryonic cell sub- 

 divides and becomes a bilobed mass. The two lobes become the 

 two seed-leaves (cotyledons). In the groove between them the apex 

 of the stem of the embryo develops. The root develops at the 

 opposite pole where the embryonic mass abuts on the suspensor. 

 The conducting tract of the root develops from the former and its 

 cortex and root-cap from the latter. 



As the embryo differentiates the ovule becomes the seed. The 

 fusion-nucleus in the embryo-sac divides and its products continue 



