2 30 THE PHYSIOLOGY OF PLANTS 



rounded by a thin cell-wall, and differ in this respect from 

 the cells of the egg apparatus. But we have stated that there 

 were four cells at each end, and we have so far only spoken 

 of three ; there remains, therefore, one nucleus at either end. 

 These will, however, be noticed to move towards the middle of 

 the embryo sac, and there to fuse into a new nucleus the 

 secondary nucleus of the embryo sac. 



Having reached this stage of development, the constantly 

 enlarging embryo sac is now receptive, and the pollen tube 

 makes its way through the nucellus down to the embryo sac. 

 In some cases the pollen tube has less resistance to overcome, 

 as the tissue of the nucellus lying above the embryo sac be- 

 comes disorganised (Zabiata:), while the wall of the embryo 

 sac, lying directly below the raicropyle, breaks down or is broken 

 down by the synergidpe. 



Before reaching the entrance to the micropyle, the develop- 

 ing pollen tube may find certain arrangements which make 

 its progress more easy. Thus we find ridges, or papillae, or 

 glandular excrescences, or specially loose tissues in the style 

 which lead to the ovules, and are therefore able to direct the 

 course of the growing pollen tube. Every ovule of the ovary 

 requires a special pollen tube for its fertilisation, and as there 

 are enormous numbers of small ovules in some ovaries (Orchi- 

 dacecc, Ericacew, Gesneriacecc), we can well perceive how it is 

 that sometimes bundles of innumerable pollen tubes will be 

 met with in the style, visible to the naked eye as a thread of 

 delicate silky hairs. 



During this period of active growth the pollen tube is thin- 

 walled, and its basal portion, which is next to the pollen grain, 

 is transparent, because all the contents are collected near the 

 apex. As soon as the pollen tube enters the micropyle, it 

 begins to swell up and becomes mucilaginous. This process 

 of swelling up and softening of tissues, as we have seen above, is 

 no uncommon occurrence in the case of the nucellus, or indeed 

 of the wall of the embryo sac. It enables the contents of the 

 pollen tube to pass more easily into the egg-cell lying at the 

 apex of the embryo sac. 



The transference of the fertilising contents of the pollen 

 tube must take place by a diffusion through the softened 



