156 



ELEMENTS OP BOTANY. 



a bell glass, with a wet cloth or sponge, to prevent evaporation of the 

 syrup, and set aside in a warm place, or merely put some pollen in 

 syrup in a 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 following 

 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 ilorists) 15 



Chinese primrose ... . 10 



Sweet pea ' . .... 10 to 15 



Tropseolumi .... . 15 



Fig. 140 —Stigma of Thorn 

 Apple (Datura) with Pollen 

 (magnified). 



Fig. 141. — Part of Stigma of Thorn 

 Apple. Vertical section (magni- 

 fied), showing pollen tubes making 

 their way toward the ovary. 



191. Microscopical Structure of the Stigma and Style. — 

 Under a moderate power of the microscope the stigma is seen 

 to consist of cells arranged rather loosely over the surface, 

 and secreting a moist liquid to which the pollen-grains adhere 

 (rig. 140). Beneath these superficial cells and running down 



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 thinnest 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. 



