136 REPRODUCTION IN PLANTS AND ANIMALS. 



elliptical assume a spherical form. A few hours contact is 

 all that is necessary to produce this change in some species, 

 whilst many days are required for others. The swelling in- 

 creases, until at. last the thin and highly extensible intine, or 

 inner coat of the pollen grain, is pushed through one of the 

 pores or ostioles in the surface of extine or outer coat, the 



Fig. 28. 



Longitudinal section, of the pistil of Helianthum denticulatum, showing the 

 ovules in the interior of the ovary attached to the placenta, by means of the 

 funiculus, or vegetable umbilicus ; b, style ; c, stigma ; a, pollen granules, the 

 tubes of -which have descended the style and entered the micropyle of the 

 ovules. 



