214 BOTANICAL GAZETTE [September 



and sunshine records were considerably below the average. These 

 weather conditions have resulted in very moderate and equable 

 evaporation rates throughout all the associations and particularly 

 in the more mesophytic (fig. 6). A further study of these diagrams 

 in comparison with the data of the Chicago Weather Bureau would 

 prove even more conclusively the assertion that the evaporating 

 of the air represents a rather accurate summation of all the atmos- 

 pheric factors that may be related to the water content of the 

 aerial parts of plants. The differences in the records for the 

 various years also prove the necessity of continued observation and 

 extended records before definite conclusions are reached. 



The records of 191 1 and 19 12 serve to give greater emphasis 

 to some of the facts recorded and to the tentative conclusions arrived 

 at during 19 10 and published in the preliminary report 2 of these 

 investigations. Thus, the evaporation rate of the cottonwood dune 

 is farthest removed from the other associations, and by it there is 

 exhibited not only a very great evaporating power, but also exces- 

 sive and rapid variation, an increase or decrease of 50-100 per cent 

 between the rates for consecutive weeks being not uncommon. 

 This cannot but denote extremely rigorous conditions for the 

 development of seedlings of ordinary herbaceous plants. The 

 high midsummer maximum, apart from the question of soil mois- 

 ture, would likewise prove an efficient factor in excluding all meso- 

 phytic plants. 



Such graphs seem to depict rather well a habitat of atmospheric 

 extremes, making large demands upon all available water, and 

 naturally and necessarily resulting in a xerophytic plant association 

 with a very limited undergrowth and an almost entire absence of 

 herbaceous plants and seedlings. Perhaps nowhere could an 

 association be found more entirely dependent upon vegetative 

 reproduction for its maintenance, since almost without exception 

 any increase in vegetation is the result of subterranean branches. 



The records for this association, when considered in relation 

 to the character of its vegetation and compared with those of the 

 other associations, is believed to emphasize the fact that, although 

 these data are for the lower stratum of the vegetation only, they are 



2 Box. G \z. 52 : 193-208. 191 r. 



