174 BOTANICAL GAZETTE [MARCH 
soil moisture in depressions; hence it is probable that the changed 
water relation due to humus accumulation is the dominating factor 
in determining the mesophytic trend, both in hydrophytic and in 
-xetophytic habitats. Although bare sand supports a xerophytic 
flora, the accumulation of a thin humus layer is sufficient for forest 
development, and the Michigan dunes show that the most meso- 
phytic of our forests can grow on a sand dune, if there is present 
a humus layer a few centimeters in thickness. On rock uplands, 
lichens commonly are the first humus accumulators; not only do 
they contribute humus by their own decay, but they give shelter 
and anchorage to plants of higher order, whosé humus-accumulat- 
ing capacity is greater. As long as the vegetation is open, and the 
humus exposed to the sun and wind, accumulation is slow, because 
of oxidation. But when the vegetation cover is more fully devel- 
oped, the humus is more and more protected and hence accumulates 
more rapidly. 
The relation of swamp successions to humus accumulation 1s 
particularly close. For each level both below and above the water 
table, there is a characteristic plant formation. In the deeper 
ponds only submersed aquatics can develop, but after a time their 
humus débris accumulates to such an extent that plants with long 
stems or leaf-stalks (such as the pondweeds and water lilies) are 
able to develop. They in turn build up the humus and prepare the 
way for their own elimination and for the development of such 
plants as the bulrush, which grows in shallow water. The latter 
again prepare the way by further humus accumulation for the first 
land plants, and they again for others. In all this well known 
successional series, the dominating factor clearly is a decreasing 
water content due to the accumulation of humus. 
- Soil organisms.—Another important influence associated 
with humus accumulation is the increase of soil organisms. These 
may play a part scarcely second to water, but as yet we know all 
too little of their activities to be certain of their precise place in 
the scale of importance. We know, however, that nitrogen is one 
of the essential plant constituents, and that it is made available 
chiefly by certain bacteria and fungi. Since these forms live on 
decaying organic matter, it seems likely that humus accumulation 
