204: BOTANICAL GAZETTE [SEPTEMBER 
ing months. The high maximum occurring at midsummer would 
probably prove the excluding factor for all mesophytic plants, 
even if not combined with such other factors as the deficiency of 
soil water at the same time. Such a graph seems to depict rather 
well a habitat of atmospheric extremes making large demands 
upon all available water, and naturally and necessarily resulting in 
a xerophytic plant association, with a very limited undergrowth 
and an almost entire absence of herbaceous plants and seedlings. 
Perhaps nowhere could an association be found so entirely de- 
pendent upon vegetative reproduction for its maintenance, for 
almost without exception any increase in vegetation is the result 
of subterranean branches. 
The graph for the pine dunes is decidedly lower and more 
regular in its contour than that of the association which it succeeds. 
Its four nearly equal maxima would indicate that within its limits 
there was throughout the summer season a continuous stress rather 
than a series of violent extremes. On the whole, it shows a water- 
demand of little more than half of that occurring in the cottonwood 
dune. Its greatest divergence is plainly due to the evergreen 
character of its vegetation, and is seen in its low range in May 
and the first part of June, and again in October, when it falls below 
that of the oak dunes and is even less than that of the beech- 
maple forest. This would give good reasons for expecting to find 
within this association truly mesophytic plants, whose activities 
are limited to the early spring. 
The graph from the oak dune stations shows two surprisingly 
high points; one during May, that may be partially explained by 
the absence of foliage; and the other near the end of June, which 
seems to coincide with maxima in the other associations. On the 
whole, it is more moderate during the months of summer than that 
of the pine dune, but the difference is not so great as to make it 
surprising that its undergrowth differs but little from that found 
in the pine dune association. 
The graph from the beech-maple forest stations is one of mod- 
erate height and great regularity. It is but fair to say that weekly 
readings would probably have introduced some minor irregular- 
ities, but without changing its general course or influencing the 
