114 



conclude from appearances that such a grain must be ento- 

 mophilous. It does have some entomophilous characters, but, if 

 compared with the pollen grains of their truly entomophilous 

 relatives, these characters are seen to be very much suppressed. 

 For example the pollen grain of the related sunflower has a 

 much thicker exine which is provided with long stiletto-like 

 spines, covered with an abundant layer of oil and provided with 

 three broad and freely functional furrows. The spines and oil 

 of the ragweed grains must be regarded as only vestiges, harping 

 back to their entomophilous ancestors. In the pollen of the 

 closely related cocklebur which appears to be older in anemoph- 

 ily these characters are more completely suppressed; the 

 grains are almost smooth, with only a trace of oil and with 

 furrows reduced to small pits which could be of no possible use 

 in adjusting the grain to changes in volume. Such a function is 

 rendered unnecessary by the thinness of the exine. 



Such, then, is the result of anemophily. In most anemophi- 

 lous plants both the reduction of their pollen grains and the 

 simplification of their floral structures have been carried so far 

 that we have scarcely any clue to their relationships. The grass 

 family affords an example of this. Their pollen grains present a 

 single pore, showing that these plants probably belong to the 

 basic monocotyledonous stock but further than this their pollen 

 grains tell us nothing. Other examples are the birch family, the 

 walnut family and others of the wind-pollinated Amentiferae. 

 Their pollen grains tell us that they belong to the basic dicotyle- 



EXPLANATION OF PLATE II 



Beginning at the top and following down to the bottom the grains are 



named and their diameters given in microns. 



Top, left — Ginkgo, Ginko biloha L., 28.5 long, ventral view. 



Top, center — Pepper, Piper nigrum L., 13 long, ventral view, above dry, below 

 moist. 



Top, right — Willow, Solix purpurea L., 20.5, polar view. 



Second, left — Poplar, Populus tremtdoides'M'ichx., 28.5. 



Second, right — Tansy, Tanacetum vulgare L., 26, polar view, optical sec- 

 tion. 



Third, left — Mugwort, Artemesia vulgaris L., 28. 5, polar view, optical sec- 

 tion. 



Third, right — Ragweed, Ambrosia elatior L., 17, side view 



Bottom, left — Sunflower, Helianthus annuus L., 29, side view. 



Bottom, right — Cocklebur, Xanthium canadense Mill., 27, side view. 



