162 MORPHOLOGY OF THE ANGIOSPERMS 



Eupomatiaceae, Calycanthaceae, Lauraceae, and a few other families in 

 Other orders (Piperaceae, Saururaceae, Chloranthaceae ) . Pollen grains 

 of the monocolpate type occur also in the Cycadales, Bennettitales, and 

 pteridosperms. The tricolpate grain, with three meridional furrows, is 

 apparently the basic type in the dicotyledons. It is not known in other 

 seed plants. 



Position, form, and number of furrows and other structural features 

 of the exine are important diagnostic characters in the identification of 

 pollen grains. Larger numbers of furrows and their position on the 

 grain are related in part to the arrangement of the spores in the mother 

 cell. The sohtary furrow in a distal position — distal as related to posi- 

 tion of the spore in the tetrad — seems to represent a primitive form, from 

 which have been derived, along one line, the proximal monocolpate, 

 dicolpate, polycolpate, and acolpate (without a furrow) types; along 

 another line, primitive tricolpate has led to polycolpate and acolpate. In 

 the monocolpate line, specialization tends to eliminate the furrow^ 

 both under reduction of the flower, with establishment of anemophily, 

 as in the Gramineae, where the furrow is reduced to a small pore; and 

 under extreme zygomorphy, with elaboration of corolla and androecium 

 in entomophily, as in the Cannaceae and Musaceae. The end products 

 of specialization in the pollen grain seem to be the acolpate and poly- 

 colpate types. These have probably been derived from both monocolpate 

 and tricolpate types. Some doubt arises as to the homology of the mono- 

 colpate grain with distal furrow with the similar grain with proximal 

 furrow — Annonaceae. Evidence from pollen morphology of apparent 

 relationship among major taxa can be considered important only to- 

 gether with that from other fields. 



The fundamental characters of the pollen grain are probably number 

 and position of furrows, form and position of apertures, and pattern of 

 sculpturing. Shape and size are probably of little or no basic importance, 

 though very large and very small size characterize some taxa. Most 

 large families show a considerable range in pollen characters, but pollen 

 type in some is remarkably uniform, as in the Gramineae, where the 

 grains are spherical or ellipsoid, with one furrow. 



The pollen of anemophilous plants is usually small, rounded, smooth, 

 rather thin-walled, "dry" (nonadhesive), and with shallow furrows or 

 none at all; it often becomes angular when dry. The pollen of plants 

 distributed by insects and birds is large, sculptured, and often coated 

 with an adhesive waxy or oily substance. Beetle-pollinated plants have 

 simple, thick-walled Tpo\\en~Eupo7natia. Characteristic smooth, thin- 

 walled pollen is present in the wind-pollinated Fopulus, Gramineae, 

 Cyperaceae, Platanus, Plantago, Betulaceae, Fagaceae, Juglandaceae, 

 Ambrosieae. The Compositae, as a whole, have perhaps the most elab- 



