212 FOUNDATIONS OF BOTANY 
the grains assume; ¢ 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 6, mark 
c d e 
Fic. 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.! 
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 
1 See Kerner and Oliver’s Natural History of Plants, Vol. Il, pp. 95-104. 
