176 POLLEN-TUBE. [CH. XIII 



a parasitic growth. In this way the pollen-tube travels 

 down the style, emerges into the cavity of the ovary, and 

 finally grows down the micropyle. By this time the tissue 

 of the nucellus has been so much encroached on by the 

 growth of the embryo-sac that the pollen-tube at the inner 

 end of the micropyle is ck>se to the egg-cell. The act of 

 fertilisation, the transference of something from the 

 pollen-grain to the egg-cell is not yet completed, but it at 

 last seems to be a possibility. 



The pollen-grain although it looks like a single cell is 

 in reality a compound structure. By appropriate treat- 

 ment two nuclei are revealed within the pollen-grain, 

 indicating the presence of two cells, which however in the 

 majority of the Phanerogams are not separated from each 

 other by cell-walls. Of the two nucleated protoplasts 

 contained within the wall of the pollen-grain, one is 

 called the generative, the other the vegetative cell. The 

 functions of these cells are indicated by their names, the 

 generative cell is essentially the reproductive part of the 

 grain, while it is the vegetative cell which germinates and 

 produces the pollen-tube. The generative nucleus divides 

 into two nuclei which travel down the pollen-tube and 

 finally escape, through the wall of the tube, into the 

 embryo-sac. One nucleus usually fuses with the secondary 

 nucleus of the embryo-sac (a process with which we are 

 not further concerned) while the other nucleus unites 

 with the egg-cell and fertilises it. 



Embryo. 



The development of the embryo from the egg-cell may 

 be studied in the Shepherd's Purse (Capsella bursa- 



