THE WALLFLOWER 129 



on observations recorded on other plants. Eeference 

 may be made to figures given at pjx 185-187 (Figs. 

 86, 87, 8 8), of fertilisation in the Lily, a plant in which 

 all the details have been fully worked out. 



All three nuclei travel down the pollen-tube with the 

 protoplasm. The vegetative nucleus, however, i.e. that 

 which was the product of the first division (see p. 115), 

 becomes disorganised and disappears, having nothing 

 further to do with the process of fertilisation. The 

 two generative nuclei alone remain, and when the 

 pollen-tube has penetrated the micropyle of the ovule 

 and reached the embryo-sac, these nuclei are to be 

 found in the protoplasm at the extreme end of the 

 tube. The cellulose wall of the pollen-tube is very 

 soft, and allows the passage of the generative nuclei. 

 Sometimes one, sometimes both, leave the tube. One 

 of the two generative nuclei passes on, usually between 

 the synergidse, and reaches the protoplasm of the 

 ovum itself. It must be remembered that these cells 

 have no cell-walls, and so no resistance is offered to 

 the passage of the nucleus. Ultimately the generat- 

 ive nucleus reaches the nucleus of the ovum. The 

 two nuclei then fuse together into one, and fertilisation 

 is effected. It is probable that in all cases some 

 protoplasm passes over from the pollen-tube with the 

 nucleus, and unites with a certain part of the pro- 

 toplasm of the ovum. What, however, we at present 

 know to be constant in fertilisation is the union of the 

 nuclei of the sexual cells. This statement holds good 

 for plants generally, and not for flowering plants alone. 1 



1 For the function of the second generative nucleus, see below, pj. 

 134 and 186. 



