326 BOTANICAL GAZETTE [May 
and one which is particularly apropos at this time, is the study of 
the character and the development of drought resistance in bog plants. 
This should naturally be the first consideration. Too little experi- 
mental information is available in the whole field of plant responses, 
whether as adjustments or adaptations, and our knowledge of plant 
problems connected with resistance in plants to sterile soils or to 
unfavorable climate is particularly limited. The modifications of 
structure which can be attributed to the action of these and other 
environmental factors are well known, and may be referred to struc- 
tures serving to reduce the evaporation of water from leaves. The 
necessity for such protection in bog plants is the greater, not on account 
of the fact that the vegetation is directly exposed to the drying effect 
of wind, to lower humidity, and to stronger light, but because roots 
absorb water with difficulty when it contains any considerable 
percentage of toxic ingredients. Unless bog plants differ from other 
plants in some phase of root function, the amount of transpiration 
must be kept low by structural modifications, that is, in order to 
compensate a reduced absorbing activity of the roots, the escape of 
water from the shoots must be correspondingly checked. It is 
unnecessary here to cite in detail the structural modifications in bog 
plants that are effective in diminishing transpiration: the general 
reduction in size of the leaves; the thick-walled epidermis and hypo- 
dermal tissues, reinforced by cuticle, wax, and hairs; the mucilaginous 
and resinous bodies found in roots and leaves; and the prevailing 
grasslike form are well-known characteristics. But much of the 
local bog vegetation exhibits little or no.xerophytic structure. In 
some places the outermost growth which immediately borders the 
open water is, indeed, for the most part hygrophile. It is dominated, 
in part, by Typha latifolia, forming sometimes a nearly pure ass0- 
ciation. Sagittaria latifolia, Pontederia cordata, Peltandra virginica, 
Decodon verticillatus, Polygonum hydropiperoides, and others are 
ordinarily abundant with Typha. The xerophytic quality is least 
marked in this vegetation. Differences such as these must arise 
strictly from the edaphic causes, which seem to set a natural and 
inevitable limit to what may be accomplished through invasion. 
There is but one important point of difference between the environ- 
ment of the bordering vegetation and that of the bog plants occurring 
