THE FLOWER 
microspores or pollen grains fill two cavities, one on either side 
of the connective. The microspores or pollen grains at first show 
m Typha latifolia (A), 
Fic. 197.—Various forms of pollen grains. Pollen fro : 
Zea mays (B), Ambrosia elatior (C), Lilium philadelphicum (D), Pinus (E), Ranunculus 
bulbosus (F), Carpinus caroliniana (G), Althea rosea (H), Ocnothera biennis (I). All 
highly magnified. Drawing by Hogstad. some 
only a thin, clear, cellulose layer, but from this, by a differentia- 
tion of the exterior film, the exospore layer or extne becomes cut off. 
