148 BOTANICAL GAZETTE [AUGUST 
ments with cereals, legumes, and with plants from the various 
zones of the island have shown that most plants are unable to pro- 
vide for a balanced relation between the supply of physiological 
water which the bog substratum can furnish and the excess of water 
lost during transpiration even when the temperature or the evapo- 
rating power of the air are favorable for any length of time. The 
susceptibility of the plants to the presence of small traces of 
deleterious bacterial transformation products accumulating in 
the surface layers of the peat substratum has been demonstrated 
elsewhere. An intimate and controlling relation has been found 
to exist between soil bacteria and the plants growing in the central 
zone. This has shown itself by various physiological and chemical 
tests, and by the fact that the presence and fitness of bog plants 
in the central zone is due mainly to more efficient functional 
responses to physiological drought. The edaphic aridity prevail- 
ing in this zone decreases the absorption of water by the roots in 
wheat plants about 50-65 per cent, at a time when transpiration 
and the growth of the plants demand a greater physiological water 
content. The further quotation of definite examples must be post- 
poned. The ratio of the possible rate of water absorption to the 
rate of transpiration and growth becomes thus the real determin- 
ing factor in the bog habitat and in the selection and in the dis- 
tribution of plants. In all cases cultivated agricultural plants 
become flaccid and the roots appear gelatinous or as if burned 
black at the tips. The general dwarfing of roots (see illustrations 
in 7 and 8) offers very little efficiency to physiologically arid con- 
ditions; nor is the change in form characters of shoot and leaf 
induced by the consequent lack of coordination of functions an 
advantage or an adaptation. Resistance to desiccation and the 
capacity for conserving water are more direct and more efficient _ 
responses to the limiting condition which the plants meet. This 
fact is not necessarily to be taken as valid in accounting for all 
highly specialized and inheritable structures so frequently met 
with in plants occurring in these habitats. The alteration of 
shoots and leaves in response to the stimulation of external factors 
may or may not increase fitness to the conditions, but it is safe 
to assume that the capacity for physiological changes and responses 
