212 FOUNDATIONS OF BOTANY 



the grains assume ; c in the latter figure is perhaps as 

 common a form as any. Each pollen grain 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 grains of starch and little drops of oil. The 

 knobs on the outer coat, as shown in Fig. 161 5, mark 



a b c d e 



FIG. 161. Pollen Grains. (Very greatly magnified.) 

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



the spots at which the inner coat of the grain is finally 

 to burst through the outer one, pushing its way out in 

 the form of a slender, thin-walled tube. 1 



224, The Formation of Pollen Tubes. This can be 

 studied in pollen grains which have lodged on the stigma 

 and there been subjected to the action of its moist surface. 

 It is, however, easier to cause the artificial production of 

 the tubes. 



EXPERIMENT XXXVIII 



Production of Pollen Tubes. Place a few drops of suitably diluted 

 syrup with some fresh pollen in a concave cell ground in a micro- 

 scope slide ; cover with thin glass circle ; place under 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 



i See Kerner and Oliver's Natural History of Plants, Vol. II, pp. 95-104. 



