124 BOTANICAL GAZETTE [AUGUST 
TRANSEAU suggested that in the regions farther north the tem- 
perature of the soil may be of prime importance. Temperature 
readings in deeper layers in the bogs of southern Michigan than 
those which he discusses, and yet not too deep for the root sys- 
tems, resemble those of northern areas, and it is probable, in the 
light of his experiments, that they may be a controlling factor, 
even in those of southern Michigan. 
Yapp (21), in his excellent paper dealing with the relation of 
marsh flora to evaporation, has shown that great variation exists 
in the amount of water that is evaporated by instruments placed 
at different heights above the surface. Hence two plants may 
grow side by side and yet not be under identical conditions. He 
produces a large amount of data, especially concerning factors 
affecting the aerial parts of plants. He concludes that “any argu- 
ment drawn from mere proximity of position, without reference 
to the varying physiological problems of the different species, is 
entirely insufficient,’ and that “the arguments of authors, who 
insist that the so-called xerophytic structures of marsh plants 
can have no reference to present-day conditions, because both 
xerophytic and non-xerophytic Bp often grow side by side in 
nature, are entirely inconclusive.” 
Summary 
1. The bogs around Ann Arbor are not xerophytic habitats, 
as such, but contain xerophytic, hydrophytic, and even mesophy- 
tic areas. 
2. A study of the conditions in those areas which now are 
xerophytic indicates clearly that xerophytic conditions in bogs of 
southern Michigan will shortly disappear. 
3. The presence of definite groups of plants in each zone is due 
chiefly to soil conditions found in that zone; also to position of the 
water table and secondary changes dependent thereon, as aeration, 
temperature, composition of the peat, etc. 
4. The absence of certain plants from certain zones is due to 
decrease in the amount of light. Chamaedaphne apparently 1s 
not able to grow in this area in a light of 0.033. 
UNIVERSITY OF VERMONT 
BURLINGTON, VT. 
