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 

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

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

 indicated. 



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. 



Fio. 162. — Stigma of Thorn- 

 Apple (Datwra) with PoUen. 

 (Magnified.) 



1 The sweet-pea poUen 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. 



