94 



THE AMEBIC AN NATURALIST [Vol. XL VI 



Another common dioecious plant is Rhus typhina L. 

 (Rhus hirta (L.) Sudw.). 19 The staminate flowers are in 

 large white panicles. The thyrsoid, pistillate flower- 

 clusters are dark green; but as they are terminal and 

 borne well above the foliage they are visible at a long 

 distance, i. e., they have conspicuousness of position. Two 

 large groups of this shrub, or small tree, one of which 

 was pistillate and the other staminate, growing in an 

 open woodland only a short distance apart, were selected 

 for observation. During two collecting trips in July, 

 1909, I secured on the staminate blossoms 77 visitors, but 

 only 6 on the pistillate. Of the visitors to the staminate 

 flowers 63 were bees, 2 wasps, 2 flies, 8 beetles and 2 

 Hemiptera. Of the visitors to the pistillate flowers 4 

 were bees and 2 wasps. Even a brief inspection is suffi- 

 cient to show that the staminate flowers are more attract- 

 ive to insects than the pistillate. 



The staminate inflorescence of Salix discolor and Rhus 

 typhina is, then, undoubtedly more conspicuous and 

 attractive to insects than the pistillate ; but is this larger 

 company of visitors due wholly to its brighter coloration? 

 Evidently not. Great numbers of honey-bees and many 

 species of Andrena frequent the staminate aments of 

 Salix to procure pollen for brood-rearing. At least five 

 species of Andrena are oligotropic visitors of this genus. 

 Examination of the polleniferous scopa of the bees taken 

 on the staminate flowers of Rhus typhina showed that 

 they all contained pollen except nine specimens of Proso- 

 pis (P. modest a Say and P. zisice Bob.), a primitive 

 genus, the species of which possess only feebly developed 

 brushes on the posterior legs, which are not used for 

 carrying pollen. Of the eight species of beetles taken on 

 the staminate flowers microscopic examination showed 

 an abundance of pollen on the mouth-parts of four. The 

 other four beetles were of small size, and it was not 

 definitely determined whether they or the other insects 

 mentioned above were feeding on pollen or not. But this 



18 The flowers of Rhus Ujphina are given as polygamous in most plant 

 manuals, but they are certainly dioecious, as is also stated by Muller. 



