126 STRUCTURAL BOTANY 



All that an insect directly does for a flower is to 

 perform the act of pollination, i.e. to bring pollen on 

 to the stigma. If all goes well, this act has its proper 

 effect, i.e. the fertilisation of the ovules. 



Hence it is usual to speak of bees and other insects 

 as fertilising flowers, though they are really only 

 concerned in the preliminary process. 



Now we will assume that the pollen has reached 

 the stigma, whether by the agency of some insect or 

 directly from the anther, and will see what next 

 happens. 



The surface of the stigma, as we have already learnt, 

 is coated with long unicellular hairs or papillae, between 

 which the pollen-grains are caught. These cells secrete 

 a slightly sticky fluid, containing sugar in solution, 

 which the pollen-grains absorb. In consequence of this 

 they begin to swell, as can be seen under the microscope, 

 and the thin parts of their walls become bulged out. 

 Under the influence of the food which it obtains from 

 the stigmatic fluid, the pollen-grain begins to germinate. 

 At one of the places where its outer coat is thin the 

 protoplasm grows, and forms a protrusion covered by 

 the thin cellulose membrane ; this protrusion increases 

 in length and becomes the pollen-tube. The greater 

 part of the contents of the pollen-grain, including all 

 the three nuclei, pass into the tube, which continues 

 its growth, makes its way between the stigmatic cells, 

 and penetrates into the tissue of the style below 

 (cf. Fig. 81, p. 181, from the Lily). It grows on among 

 the cells of the style, and at last reaches the cavity of 

 the ovary. Here the pollen-tube follows the placenta. 



