52 



scant twenty to forty miles. These represent the western-most 

 extensions of species, all of which are much more abundant 

 farther eastward or southward. White oak, black jack oak, 

 laurel oak and pin oak are very rare within that area. Yellow 

 oak, low yellow oak and swamp white oak are relatively abundant 

 in a very few localities. 



The ecological relations controlling these distributional 

 phenomena are quite well known. The major ecological factors 

 indicate a very decided advancement toward an increased xero- 

 phytism extending westward and northward from the south- 

 eastern corner of the state. 

 Lincoln, Nebraska, 

 December, 1919. 



SHORTER NOTES 



The Paper Mulberry (Broussonetia) an "Artillery 

 Plant." — A number of the Nettle Family (Urticaceae) are 

 known be eject the pollen forcibly, one of these plants {Pilea 

 serpylUfolia Wedd., or P. miiscosa Lindl.) being often cultivated 

 under the name "artillery plant" because of the curious explosive 

 opening of the staminate flowers. Particularly when the plants 

 are placed in sunlight, after having been sprinkled, the pollen is 

 forcibly thrown out in a smoky cloud, reminding one of the 

 bursting of miniature shells or bombs. 



I was much interested last spring to 'find that the Paper Mul- 

 berry {Broussonetia papyrifera Vent.) has the same habit of 

 throwing its pollen as has the Pilea. It is interesting to recall, 

 also, that these plants are in closely related families, the. Tlforaceae 

 and Urticaceae having much in common. The 21st of May, 1919, 

 in Philadelphia was a warm showery day, the frequent thunder- 

 storms alternating with bright hot sunshine, and the paper 

 mulberries, dripping after a shower, presented a curious spectacle 

 in the bright sunlight. There was a continuous succession of 

 puffs of smoky pollen from various parts of the tree, apparently 

 all the flowers in a catkin exploding at once and filling the air 

 with yellow "smoke" to a radius of about an inch in all directions, 

 after which the pollen drifted lazily away on the gentle breeze. 



