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 tne 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. 



