MacDoKgal : distuihutiox of razoumofskva robusta. 173 



by radiation during the time of exposure to the sun's rays, it 

 rises and expands. During the ascent some heat is converted 

 into the work necessary in expansion, causing a cooHng of one 

 degree Fahrenheit for every one hundred and eighty-eight feet 

 of elevation. The decrease in temperature lowers the dew point 

 or increases the relative humidity, a matter of very great im- 

 portance to germinating seeds and transpiring leaves. Razou- 

 uiqfskya is especially abundant, precisely at the places where 

 the effect of the ascending humid currents of air is greatest, 

 along the margins of hills and mesas and the rims of canons. 

 This is very noticeable along the Grand Canon of the Colorado 

 river, in the Coconino Forest reserve, where the air rising 

 more than a vertical kilometer from the riverbed pours across the 

 pine-covered mesa at a much lower temperature and very much 

 nearer the dew point than the body of air which it replaces. 

 In its rise it has lost heat at the normal adiabatic rate to the 

 amount of about twenty-five degrees F., and has undergone a 

 great variation with respect to the dew point. As a consequence 

 of the increased humidity favorable to germination, the pines 

 near the rim of the canon are most thickly infested with the 

 parasite over a belt one to four or five kilometers in width run- 

 ning parallel to the margin. One may walk through the forest 

 and note the decreasing abundance of Razoumofskya as the dis- 

 tance from the canon increases. 



In recapitulation of the facts adduced in this note it is to be 

 said that the berries of Razoumofskya are to be classed as sling 

 fruits, the only one from North America hitherto described, and 

 that this genus is the only one of the Loranthacece furnished 

 with means of seed-dissemination independent of gravity and 

 animals. The writer also believes that he is justified in an- 

 nouncing the discovery of the influence of vertical air-currents 

 upon the distribution of plants, and that this factor must be taken 

 into account in the consideration of the boundaries of zones in 

 mountainous regions or those with irregular topography. 



Explanation of Plates. 



Plate XV. D. Staminate plants of Razozimofskya. B. Pistillate 

 plants with mature berries. The distortion of the branch of the host 

 is plainly shown. 



Plate XVI. A. Pimis ponderosa dying from the effects of the para- 

 site, photograph of a specimen growing on the extreme edge of the rim of 

 the Grand Canon of the Colorado, June, 189S. C. Specimen of Pinus 

 fofiderosa showing drooping of branches attacked by Razoumofskya. 



