POLLEN 



159 



Drimtjs is a prominent exception. The tetragonal arrangement results 

 from successive divisions; this arrangement is frequent in monocotyle- 

 dons, lower dicotyledons, and gymnosperms. 



Pollen grains are commonly shed individually or in loose clusters but 

 may remain in tetrads — Tijpha, Droseraceae, Winteraceae, Lactorida- 



Fig. 62. Microsporogenesis in Lobelia cardinalis. A, second meiotic division within 

 the mother-cell wall; B, microspores nearly formed, partition walls developing; C, 

 mature microspore; D, spore enlarged, first cell division completed; E, vegetative 

 and reproductive nuclei mature; F, mature poUen grain with sperm nuclei. (After 

 G. O. Cooper.) 



ceae, Juncaceae, several sympetalous families. They are held together 

 sometimes by the persistent mother-cell wall, sometimes by close ap- 

 pression or by adhesive surfaces. The grains may remain in tetrads even 

 in pollinia. Rarely, they remain in pairs called "dyads" — Scheuchzeria. 

 They may form pollinia or massulae, the compacted contents of indi- 

 vidual sporangia, anther sacs, or fused anther sacs — Asclepiadaceae, 



