igtt] COWLES—VEGETATIVE CYCLES 175 
is likely to favor their increasing development and hence an increas- 
ing supply of available nitrogen. A single instance will suffice 
to show the possible importance of soil organisms in succession. 
The beech, which is a characteristic member of the culminating 
forest of the eastern United States, has roots which are enveloped 
by saprophytic fungi; it is believed that these fungi represent the 
absorptive system of the tree, and it is likely also that they are 
able to make nitrogen available, since so many similar fungi are 
now known to possess this power. In any event, the beech is 
known to depend upon the fungus, being unable to flourish without 
it. Obviously, then, the beech cannot appear in a successional - 
Series until its associated saprophytic fungus finds conditions 
Tequisite for its development in the soil. It is likely, too, that 
other saprophytic organisms are detrimental to various green 
plants, thus becoming a factor in their elimination. There is 
Opened up here a great field of investigation, and all that can be 
Stated now with definiteness is that it is likely to be demonstrated 
that the accumulation of humus is of profound significance in the 
development of successive saprophytic organisms, and probably 
on this account in the succession of the higher plants. 
¢. Toxicity—Still another humus factor that seems likely to 
be of large significance, but whose exploitation is so recent that 
Wwe cannot yet appraise it, is soil toxicity. It has been known fora 
long time that the roots of plants give off various excretions, but 
It is only through the recent careful work of LivincsTon and his 
associates (22), and later of SCHREINER and REED (25, 26), that we 
ave come to know much concerning their nature and influence. 
In the case of wheat it has been ascertained that the roots give off 
‘ettain substances which are deleterious and perhaps actually 
toxic, especially to wheat. Such results should not occasion 
‘urprise, since it is well known that many bacteria excrete sub- 
stances which retard or even prevent the further growth of their 
Own kind. 
One of the greatest puzzles to the student of plant dynamics 
has been afforded by the successional series in bogs, since in Spite 
of the wet soil there are many. plants that obviously are xerophytic. 
There is universal agreement that there is something in bog soils 
