392 JOURNAL OF FORESTRY 



Stands are consistent!}' sterile ; but the reason for such conditions is 

 not clear. 



Staminate trees, while they occupy a wider area than pistillate trees, 

 still fail to cover any considerable percentage of the total aspen-cov- 

 ered area, and it would be quite possible for pistillate trees to exist a 

 mile or more distant from staminate trees, thus making general fer- 

 tilization very difficult. In the spring of 191 7, in a total of approxi- 

 mately 2,500 acres of aspen under observation, only about 20 acres of 

 staminate trees were seen and less than one-half acre of pistillate 

 trees. 



The development of pollen was also observed this year, and this has 

 thrown considerable light upon the reason for the general sterility of 

 the fruiting capsules. A careful microscopic examination of thirty- 

 six nearly mature and mature anthers from several trees showed only 

 44 per cent of the pollen grains normal, round and full of protoplasm, 

 the greater number being small, nearly empty, and frequently wrinkled. 

 Pollen sterility is a condition that is always correlated with hybridism. 

 Five per cent of infertile pollen is considered by Forsaith^ to be suf- 

 ficient to indicate this condition. In this case, however, it is impossible 

 to ascribe the condition to this cause, as the species is strictly mono- 

 typic and associates in this locality with no other species in the genus 

 Populus. In addition to this sterility of the pollen, the staminate 

 flowers, like the developing fruits, appear to start out normally, but 

 just prior to maturity the catkins suddenly dry up and fall from the 

 trees, the anthers withering and failing to open. From only eight out 

 of twenty-five apparently mature staminate aments picked from the 

 trees could visible amounts of pollen be shaken, and this was a very 

 small amount which came from a few anthers near the base of the 

 aments which opened a little, although hardly normally. Nearer the 

 ends of the catkins the anthers had withered without dehiscence. It 

 is little wonder, therefore, that seed reproduction is lacking, since there 

 is so little chance of the seed becoming fertilized, and then the fur- 

 ther improbability of the seed reaching successful maturity. It may 

 also be mentioned in passing that the surface soil at the time of seed 

 dispersal is very dry and showers are infrequent, so that germination, 

 except in local wet places, would be extremely unlikely in the short 

 period of viability which these seeds are known to have. 



^ Forsaith, Carl C. "Pollen Sterility in Relation to the Geographical Distribu- 

 tion of Some Onagracese." Botanical Gazette, 62:466-488, 1917. 



