178 BOTANICAL GAZETTE [MARCH 
shade which aspens and birches might not be able to endure. 
Finally the pines and oaks in turn may be succeeded by such trees 
as the beech, the sugar maple, and the hemlock, since these trees 
are able to develop in a considerable amount of shade. The latter 
trees may continue indefinitely, unless climatic or topographic 
changes intervene, since, unlike most species of trees, their seedlings 
are able to develop in shade as dense as that which is cast by the 
parent trees. While the influence of increasing shade, as here set 
forth, is undoubted, the extent of its influence is not known; pari 
passu with the increase of shade, and partly on account of it, there 
goes on the accumulation of humus. On uplands in our climate 
each of these factors tends to bring about the development of a 
mesophytic forest, but as yet it is impossible to determine which 
as the more potent influence. Increasing shade favors the 
mesophytic trend of upland successions in yet another way than 
through its direct influence and through its effect upon humus 
accumulation; the cutting off of light results in increased atmos- 
pheric humidity and hence in decreased evaporation. Some recent 
observations by FULLER (29), as yet unpublished, show that the 
pioneer plant formations of the Indiana sand dunes are character- 
ized by high evaporation, and that this evaporation progressively 
decreases until the minimum is reached in the climatic forest. 
In contrast to ordinary uplands is the influence of light upon the 
development of vegetation in lakes. At the outset there are many 
lakes which are too deep to have a conspicuous vegetation of aime 
plants on the bottom. Through the accumulation of inorganic 
detritus and of humus, the latter arising from the decay of green 
plants living in the upper waters and from the decay of other organ- 
isms at all levels, there gradually is made possible the development 
of a plant formation on the bottom, composed of plants which 
require only a miminum amount of light. In succeeding years the 
shallowing of the lake makes possible a greater and greater develop- 
ment of green herbage, unless the development of a rich floating 
vegetation again cuts off the light. It is obvious that the influence 
of light and shade on succession is not so explicitly related to life as 
is that of humus; humus can arise only from organisms, but shade 
may be cast by many other things than trees. The rapid develop- 
