STRUCTURE OF FLORAL ORGANS 



163 



Fig. 120 will serve to furnish examples of some of the 

 shapes which pollen grains assume ; c in the latter figure is 

 perhaps as common a form as any. Each pollen grain con- 

 sists 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 opaque particles, and usually 

 containing grains of starch and little drops of oil. The 

 knobs on the outer coat, as shown in Fig. 120, h, mark 



a b c d e 



Fig. 120. 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.^ 



191. 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 XIX 



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

 syrup witli some f»esli pollen in a concave cell ground in a microscope 

 slide ; cover with a thin glass circle ; place under a bell-glass, with a 

 wet cloth or sponge to prevent evaporation of the syrup, and set 



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



