GERMINATION OF POLLEN GRAINS 



141 



consists mainly of a single cell, and is covered by a moderately 

 thick outer wall and a thin inner one. Its contents are thickish 

 protoplasm, full of little opaque particles and usually containing 



bed 

 FIG. 153. Pollen grains 



a, pumpkin; b, enchanter's nightshade; c, Albuca ; d, pink; e, hibiscus. Very 

 greatly magnified. After Kerner 



grains of starch and small drops of oil. During the germination 

 of a pollen grain the outer coat bursts at some point, forced out- 

 ward by the pressure of a tube formed from the tough inner coat. 

 Sometimes, as in Fig. 153, b, there are knobs or other indications 

 of the places at which the outer coat 

 is most easily ruptured. After the tube 

 has pushed its way out it continues to 

 elongate rather rapidly. 



174. Microscopical structure of the 

 stigma and style. Under a moderate 

 power of the microscope the stigma is 

 seen to consist of cells set irregularly 

 over the surface, and secreting a moist 

 liquid to which the pollen grains ad- 

 here (Fig. 154). Beneath these super- FlG 154 stigma of thorn 

 ficial cells is spongy parenchyma, which apple (Datura), with 

 runs down through the style, if there is P llen 

 one, to the ovary. In some pistils the Magnified. After Faguet 

 pollen tube proceeds through the cell walls, which it softens by 

 means of a substance which it exudes for that purpose. In other 

 cases (Fig. 155) there is a canal or passage along which the pollen 

 tube travels on its way to the ovule. 



