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its origin back to the Pteridosperms, and is even dimly fore- 

 shadowed in the spores of the ferns. The three-furrowed grain 

 and its many-furrowed and many-pored derivativesare by com- 

 parison quite modern, appearing nowhere below the higher 

 dicotyledons. 



Pollen grains, like the plants themselves, are greatl.v' modi- 

 fied by adaptation to their environment, but their environment 

 is very much simpler than that of the plants. In fact all their 

 environmental factors resolve themselves into just one, their 

 mode of pollination. So, while grains which are decorated with 

 spines and ridges and covered with a layer of sticky oil may be 

 just right to be carried ofif by insects, they would never do if 

 they are to be carried by wind. Such must freely separate from 

 each other and must not stick to anything until they reach the 

 stigma which is designed to catch them. And so it is that ane- 

 mophilous grains must abandon their golden yellow layer of 

 sticky oil and their beautiful sculptured patterns. 



So far is this simplification carried that it is often difficult to 

 even guess from their pollen grains to what entomophilous group 

 of plants an anemophilous plant is related. For example the 

 pollen grains of willows which are primarily insect pollinated 

 have a rather thick outer coat bearing a beautiful reticulate 

 pattern and covered with a thin layer of sticky oil, and are 

 provided with the customary three furrows. The pollen grains 

 of the poplars, on the other hand, which everybody agrees are 

 closely related to the willows but which are entirely wind pol- 

 linated, are almost perfectly smooth, with no oil, with an ex- 

 tremely thin — almost fragmentary — outer coat, and no trace 

 of furrows. Another example is found in the tansies and sage 

 brushes. These two genera are very closely related. In fact the 

 sage brushes may be regarded as a group of wind pollinated 

 derivatives of the insect pollinated tansies. The pollen grains of 

 the tansies are provided with a thick spine-covered outer coat 

 like those of most Compositae and covered with sticky oil, 

 while those of the sage brushes have a much thinner outer coat, 

 scarcely any oil and with only occasionally the merest traces of 

 spines. Still another example is found in the ragweed pollen 

 grains. They are characterized by a rather thick exine which is 

 provided with short conical spines and covered with a thin layer 

 of oil and have three small slit-like furrows. One would naturally 



