236 
Journal of Agricultural Research 
Vol. XT, No. s 
methods the relative density of the crude chlorophyll. Extracts were 
made in the dark with cold 90 per cent methyl alcohol. The results 
indicated in Table XXIV show that the chlorophyll content is about 
twice as great in the stimulated zone as outside. The results are quite 
uniform for the different plants and also for the Agaricus and Calvatia 
rings. Similar measurements of the chlorophyll content in oat fields 
showed the amount in the stimulated zone to be twice as great as normal. 
The cause of the increase in vigor and chlorophyll content seems to be 
amply explained on the basis of the available supply of nitrates and 
salts of ammonia. 
CAUSE OF THE DEATH OF THE VEGETATION 
The death of the vegetation noted by many investigators has been 
attributed to a number of different causes. The fungus filaments were 
thought to absorb all nutrient material from the soil and thus cause the 
death of the grasses, etc. This view was held by Wollaston (1807), 
Van Tieghem (1884, p. 1044-1045), Olivier (1891), and Beille (1904, 
p. 381). 
Berkeley (i860, p. 41) attributed the death of the grasses to the effect 
of the death of the spawn of the fungus, while Jorden (1862) attributed 
the death of grasses to the entangling action of the filaments and their 
strong “effluvia.” 
The cause of the death of the grasses was attributed to parasitism on 
the part of the fungus by Ballion (1906) and Bayliss (1911), the latter 
stating that the roots were directly attacked by the fungus filaments 
and killed by toxic excretion. 
The impenetrability of the soil to moisture, or the dry condition of 
the soil, was believed by Ritzema Bos (1901) and Molliard (1910) to be 
the chief cause of death of the grass. The latter attributed part of the 
harmful effect to the high content of ammoniacal salts. 
In the investigations of the writers the death of the vegetation noted 
in rings formed by Agaricus tabularis was always accomplished by lack 
of available soil moisture. No harmful effect could be noted on the 
vegetation as long as the soil contained available water. During years 
that were uniformly wet throughout the growing season the vegetation 
was not noticeably damaged, while during dry years or periods of drouth 
the plant cover would be partly or entirely destroyed just above the 
dense mycelium. As stated elsewhere, the penetration of water into 
the mass of soil filled with mycelium is very slow if the soil is once dried. 
The stimulation of growth hastens the drying out of this soil mass; and, 
when once dry, it remains dry even through heavy and continued rain. 
During ordinary years, when the moisture supply was sufficient on the 
natural sod, drouth was severe over the mycelium-impregnated soil. 
If parasitism or toxic secretion were the cause of the death of grasses 
in this area, the vegetation would be attacked while still growing and 
