STRUCTURE OF FLORAL ORGANS 163 
Fig. 120 will serve to furnish examples of some of the 
shapes which pollen grains assume; ¢ in the latter figure is 
perhaps as common a formas 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, 6, mark 
Fic. 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.1 
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 with some fresh 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 Oliver’s Natural History of Plants, Vol. II, pp. 95-104. 
