POLLINATION AND FERTILISATION 217 



ovule requires one pollen grain to form a tube to bring its 



generative nucleus to the oosphere. Why the pollen tubes 



enter the micropyle is not fully understood at present, but there 



must be some substance which attracts them. 



The pollen tube is guided to the oosphere by 



the egg-apparatus (p. 201), the tip of the tube 



is broken off, and the generative nucleus and 



some of the protoplasm is set at liberty. The 



liberated generative nucleus fuses with, and 



fertilises, the oosphere. 



In the Lily, the large size of the pollen grain 

 and tube enables all the stages in the produc- 

 tion of a pollen tube and in the process of FKJ ^ _ pT oj 

 fertilisation to be followed. The vegetative 'kn 'tube ;' N, 

 nucleus passes into the pollen tube first, the grammat la 

 generative nucleus following on. The vegetative 

 nucleus is used up during the growth of the pollen tube, and 

 the generative nucleus or cell divides into two. When the 

 micropyle is reached, the tube passes in between the egg- 

 apparatus, and the leading generative nucleus passes out at the 

 end of the tube along with some of the protoplasm. The nucleus 

 travels on until it reaches the nucleus of the oosphere, when 

 it fuses with it, the protoplasm uniting with the protoplasm of 

 the oosphere. After fertilisation, the oosphere becomes the 

 oospore or egg-spore, and it surrounds itself with a firm cell- 

 wall. 



Development of the Embryo. The development of the 

 embryo can be studied in a little wayside weed, the Shepherd's 

 Purse. This plant is self-fertilised and produces a very large 

 number of seeds, and as a rule, all stages can be obtained on 

 one plant, from the oospore up to the mature embryo. 



The oospore divides into two cells. The one nearest the 

 micropyle is called the upper cell. The upper cell produces a 

 row of cells called the suspensor ; the lower cell by division gives 

 rise to nearly all the embryo. The suspensor supports the 

 embryo and fixes it to the wall of the ovule. The embryonic or 

 lower cell divides into eight cells. The four which are the greatest 

 distance from the suspensor form the cotyledons and plumule, 

 the four nearest the suspensor form the radicle. The tip of the 

 radicle and the root-cap are formed by the upper cell of the 



