581 



age daily evaporation amounts for the several stations for the entire 

 period of 147 days: a, 3 cc. ; c, 4.27 cc. ; b, 4.5 cc. ; and d, 7.91 cc. 

 Or, taking the rate for d as 100%, then the rate for a was 38% ; 

 for c, 54% ; and for b, 57%-* Expressed in general terms, the 

 evaporation rates were inversely proportionate to the hydrophytism 

 of the station. This is due chiefly to the greater amount of moisture 

 in the air where the station is hydrophytic; and again, the greater 

 amount of atmospheric moisture was due, in many places, not merely 

 to the greater sources of supply (soil moisture or surface water) 

 but to the more difficult means of escape (because of the tall rank 

 vegetation evoked by hydrophytism). It will be noted that the aver- 

 age rate in the outer part of the reed swamp (b) slightly exceeded 

 that in the swamp meadow (c). This may be explained easily, how- 

 ever, by the fact that in the swamp meadow the vegetation remained 

 more dense and compact in late summer than in the outer part of 

 the reed swamp, thus retarding evaporation. 



Transeau ('08) has obtained in a mesophytic forest on Long 

 Island, N. Y., an average daily evaporation rate of 8.5 cm. This 

 was based upon readings taken during a period of less than one 

 month. More recently. Fuller ('11) has obtained for typical meso- 

 phytic forest, based upon readings extending over 155 days, the 

 average daily rate of 8.1 cc. While we are not justified by the data 

 at hand in attempting final comparisons, yet, so far as they go, these 

 data indicate that evaporation is slightly less rapid in the swamp 

 white oak - white ash forest than in climax mesophytic forest. If 

 this indication is sustained by further study, as it undoubtedly will 

 be, it will coincide quite closely with the fact that in the normal de- 

 velopment of mesophytic forest from hydrophytic formations Qiter- 

 ciis bicolor, Fraxinus aincricana, F. nigra, etc., are antecedent to trees 

 of the climax mesophytic type (Fagns grandifolia, Acer saccharum, 

 etc.). By way of comparison, it is interesting to note here the very 

 recent paper of M'Nutt and Fuller ('12), in which the oak-hickory 

 forest association is maintained (because of its intermediate evapora- 

 tion rate) "midway between the black oak dune association and the 



*In interpreting these data, however, allowance must be made for the fact 

 that in different associations the percentage of species which start out each year in 

 the delicate and hence more critical seedling stage, varies. For young seedlings, 

 dependent as they are upon their own photosynthetic activity for food, growth up 

 to about 25 cm. (the height at which these comparative readings were taken) is 

 accompanied undoubtedly by much more risk than is the growth of young shoots 

 from old, well-established perennial rhizomes, bulbs, tubers, etc. Hence the evap- 

 oration rate for an entire association can not show with precision the extent to 

 which each species, as such, is influenced during its most critical stages, viz., the 

 first seasonal growth of its aerial shoots. 



