FLOWERS 273 
The non-spiny-walled pollen grains differ not only in micro- 
scopic appearance, but also in size. Clove pollen grains are 
the smallest, while Spanish saffron pollen grains are the largest. 
NON-SPINY-WALLED POLLEN GRAINS 
In cloves the pollen grains show a six-sided, angled cavity 
and an outer wall which terminates in three slightly pointed, 
narrowly notched portions, separated by nearly straight walls. 
In santonica the pollen grains have smooth, unequally 
thickened walls, which are strongly constricted at three points, 
the outline resembling three half-circles placed together. 
In elder flowers the pollen grains appear circular or three- 
parted. The wall is of nearly uniform thickness, even at the 
constricted part of the grain. 
In century minor the pollen grains show three pronounced 
restrictions. The wall at these points is very thin. In pichi 
flowers the pollen grains are either circular or three-sided and 
three-pointed. Inside of each point there is a nearly white pore. 
In some of the grains the pollen tube has grown out of one of 
the pores. 
In cyani flowers the pollen grains are longer than broad and 
the cell contents appear to be divided into two end portions 
and an elevated middle portion 
SPINY-WALLED POLLEN GRAINS 
In anthemis the pollen grains have unequally thickened 
walls constricted in three places. The spines are short, broad 
at the base, and sharp-pointed. 
In arnica flowers the pollen grains show three light-colored 
pores and numerous short spines. 
In calendula flowers the pollen grains show one or more | 
pores, typically three pores. These pores appear as white spots, 
and the wall immediately over the pore is smooth and thinner 
than the remaining part of the wall; the spines are very numerous. 
In cassia flower pollen grains the outer wall is extended into 
a number of rounded projections which are frequently arranged 
in sets of fours. 
In American saffron flowers the pollen grains show one, two, 
or three light-colored pores; the spines are short and broad. 
