318 Proceedings Portland Society Natural History 



alone which the writer believes should be regarded as the 

 eastern deciduous climax forest. The region which this 

 type of forest occupies is distinct, both in its climate, and to 

 a large extent in its soils. It has a rainfall-evaporation 

 ratio 1 of from 80%-110%, according to Transeau (28), and 

 in general is unglaciated and underlain by rich basic Siluri- 

 an and Devonian rocks. From its area of greatest develop- 

 ment in the Ohio basin and the southern Appalachians it ex- 

 tends northward to southern Michigan, Ontario, and west- 

 ern New York, with a branch running along the Piedmont 

 region, east of the mountains, to the Hudson valley and 

 southwestern New England. Numerous individual species 

 have a similar range indicating that this region is a definite 

 floristic as well as climatic unit area. 



Immediately north of the deciduous forest region as thus 

 delimited lies that part of the Transition region, which is 

 more southern in its affinities and which may be termed the 

 Alleghanian-transition zone. Here the climax forest is a 

 mixture of sugar maple, beech, yellow birch, hemlock and 

 white pine, the more striking trees of the typical deciduous 

 forest being absent. The nature of such forests has been 

 adequately treated by Whitford (30), Gates (12), Bray 

 (2), and Nichols (21) and needs no discussion here. One 

 point in connection with this zone, however, should be em- 

 phasized. Owing to the presence of unfavorable physio- 

 graphic factors in many localities, the regional climax can- 

 not be attained and the succession of vegetation stops at 

 some physiographic climax. The white pine region of the 

 lumbermen, so characteristic of the sterile sand plains of 

 southern New England, is a good example of such a physio- 

 according to Transeau the determination of the rainfall-evapora- 

 tion ratio affords a method whereby data concerning temperature, rela- 

 tive humidity, wind velocity and rainfall, all of which affect plant dis- 

 tribution, may be combined in a single number. This ratio is obtained 

 by dividing the amount of rainfall at a given station by the depth of 

 evaporation at that station. The resulting ratios when plotted on a 

 map show climatic centers which correspond in general with centers of 

 plant distribution. 



