300 SCHREINER AND REED: EXCRETIONS BY ROOTS 
A very apt illustration of the way in which toxic excreta may 
act in bringing about migration is the case of the “ Fairy Ring” 
fungi. The curiously regular growth of the ring in a continually 
widening circle may be due to the production of toxic excreta by 
the growing fungus. The young mycelium grows best on the 
outside of the ring because it is less affected by the excreta left in 
the soil within the ring. The common explanation for the devel- 
opment of fairy rings is based upon the assumption that the soil 
within the ring is so depleted of nutrients that it is unable to sup- 
port the growth. An investigation of the chemistry of fairy rings 
by Gilbert (’75) and Lawes (’83) showed sucha slight difference be- 
tween the soil inside and that outside of the ring that this difference 
cannot explain the entire absence of fungi within the ring. “The 
total nitrogen outside of the rings (average of five rings) was 281 
per cent. ; inside of the rings it was .247 per cent., a difference of 
.034 per cent. The average content of carbon outside the rings 
was 3.30 per cent., inside the rings 2.78 per cent., a difference of 
.52 per cent. With our present knowledge of the ability of plants 
to absorb and utilize nutrients, these slight differences are utterly 
inadequate to explain the entire absence of fungi within the ring. 
The existence of toxic excreta in the soil would, however, explain it. 
It is of interest to note that such an explanation of fairy rings was 
suggested by Way (’47). This investigator admitted ‘ that by far 
the most scientific and intelligible solution of the question is that 
which was based upon DeCandolle’s theory of the excretions of 
plants.” But on account of objections which appeared insuperable 
to him he was unable to accept it as a satisfactory explanation. 
Another illustration of the possible effect of root excreta in 
producing associations may be found in the ‘oak openings.” 
These characteristic grassy tracts existed in the natural oak forests 
of Ohio, Indiana and southern Michigan before they were modi- 
fied by man. From some hitherto unexplained cause the forest 
was apparently unable to advance into these small prairies. In 
assuming that the grasses produced some substances which were 
unfavorable to the roots of trees, we have a factor which possibly 
has importance in the maintenance of such natural societies. 
It is quite probable jthat the excretion of small amounts of 
deleterious substances is a general phenomenon among all plants. 
