FERTILIZATION 



213 



watch crystal under the bell-glass. Examine from time to time to 

 note the appearance of the pollen tubes. Try several kinds of 

 pollen if possible, using syrups of various strengths. The follow- 

 ing kinds of pollen form tubes readily in syrups of the strengths 

 indicated. 



Tulip . 1 to 3 per cent. 



Narcissus 3 to 5 " 



Cytisus canariensis (called Genista by florists) 15 " 



Chinese primrose . . . . . 10 " 



Sweet pea l . ... . . 10 to 15 " 



Tropseolum 1 . .... 15 " 



9 



225. 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 adhere (Fig. 

 162). Beneath these superficial cells 

 and running down through the style 

 (if there is one) to the ovary is 

 spongy parenchyma. In some pistils 

 the 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. 163) there is a canal or passage, 

 along which the pollen tube travels 

 on its way to the ovule. 



FIG. 162. Stigma of Thorn- 

 Apple (Datura) with Pollen. 

 (Magnified.) 



1 The sweet-pea pollen and that of Tropseolum are easier to manage than 

 any other kinds of which the author has personal knowledge. If a concaved 

 slide is not available, the cover-glass may be propped up on bits of the thin- 

 nest broken cover-glasses. From presence of air or some other reason, the 

 formation of pollen tubes often proceeds most rapidly just inside the margin 

 of the cover-glass. 



