RELATION BETWEEN EVAPORATION AND PLANT SUCCESSION 1 77 
quisite to succession. A change in dominant species in an area is 
fundamental to succession. 
8. The change in the rate of evaporation from the chamaephytic 
layer is produced by the development in density of the invading vege- 
tation. Being coincident with and not antecedent to it, the change in 
evaporation is a result and not a cause of succession. 
9. While it is necessary for certain species to develop under existing 
conditions to bring about succession, other species, of narrower physi- 
ological limitations, can not develop until conditions are brought within 
their range. Such species are secondary species, unable to cause suc- 
cession. 
10. Even though evaporation conditions are within suitable limits, 
succession will not take place unless the disseminuls of the dominant 
species of a higher genetic association arrive and develop. 
11. The average evaporation from the chamaephytic layer of the 
average aspen association for 40 days during the summer of 191 5, at 
Douglas Lake, Michigan, was 4.9 cc. per day; for the normal density 
of the beech-maple forest, 4.4 cc. per day; while the highest average 
rate for the season obtained from open ground was 14.7 cc. per day. 
For a single week the highest rate was 21.6 cc. per day. 
For 47 days during the summer of 19 16, the average evaporation 
from the chamaephytic layer of a densely developed Thuja bog was 
4.8 cc. per day. A rate of 26.6 cc. per day was recorded from an at- 
mometer in open ground at the crest of the low bluff a short distance 
from the laboratory. 
List of Plants Mentioned, with Authorities 
(Using " Gray's Manual," 7th Edition. Where different, the names 
used in Britton and Brown "Illustrated Flora," 2d edition, are given 
in parentheses.) 
Acer pennsylvanicum L. 
Acer rubrum L. 
Acer saccharum Marsh. 
Actaea alba (L.) Mill. 
Agrostis alba L. 
Agrostis hiemalis (Walt.) B.S.P. 
Aralia nudicaulis L. 
Aster laevis L. 
Aster macrophyllus L. 
Betula alba papyrifera (Marsh.) Spach. 
{B. papyrifera Marsh.). 
Betula lutea Michx. f. 
Chamaedaphne calyculata (L.) Moench. 
Clintonia borealis (Ait.) Raf. 
Convolvulus spithamaeus L. 
Cornus canadensis L. {Chamaepericly- 
menum canadense Asch. & Graebn.). 
Danthonia spicata (L.) Beauv. 
Diervilla lonicera Mill. {D. diervilla 
MacM.). 
Epilobium angustifolium L. (Chamaener- 
ion angustifolium Scop.). 
