THE WALLFLOWER 127 



When it reaches an ovule which is ready for fertilisa- 

 tion, the tube curves away from the placenta, and 

 grows along the funicle, until its end reaches the 

 micropyle of the ovule (Fig. 43, pt). We are at 

 present unable fully to explain the causes which 

 direct the growth of the pollen-tube with unerring 

 accuracy into the micropyle of the ovule. Recent 

 researches make it probable that the apparent attrac- 

 tion is due to some sugary substance secreted by the 

 ovule. The tube grows on through the narrow passage,, 

 and eventually its tip reaches the summit of the embryo- 

 sac, and meets the pointed ends of the synergidse. 



And now the growth of the pollen-tube has reached 

 its limit, and its destination is attained. The length 

 of the tube is enormous compared to the size of the 

 pollen-grain which produced it. In the Wallflower 

 the distance from the stigmatic surface to the lowest 

 ovule in the ovary is about 6 mm. (J in.) in a straight 

 line. The diameter of the pollen - grain is about 

 02 mm. 



The tube does not really grow straight, so that we 

 shall be within the mark in estimating its length on 

 the average at about three hundred times the diameter 

 of the pollen-grain before germination. There are other 

 plants in which the growth is ten times as great, or 

 even more. It is evident that this enormous growth 

 is only possible if food is taken up on the way from 

 the cells of the style and ovary. In fact, the 

 germinating pollen-grain lives at the expense of the 

 tissues of the pistil, just as a parasite lives at the 

 expense of its host. We must not, however, suppose 



