18 BULLETIN 169, U. S. DEPARTMENT OF AGRICULTURE. 
to come up, but caused them to die a few days later. With the 
weeds, nearly all that came up were quite certain to survive. The 
extent of the injury to weeds was shown chiefly by the small number 
of weeds which appeared on the acid plats as compared with 'the 
checks. The failure of seriously injured weed seedlings to appear 
above ground, as did most of the injured pines, may be due in part to 
a larger amount of stored food material in the pine seed and in part 
to a greater depth of soil over many of the weed seeds. It is barely 
possible that many still dormant weed seeds were killed at the time 
of the application of the acid. Some of the weed seeds in late-sown 
plats commence germination at or before the time of acid application, 
and are therefore probably killed at the time of application. The 
frequent occurrence of healthy Equisetum in beds where the acid 
killed the pines may be due entirely to the presence of old rootstocks 
and not to superior tolerance of acid. It has been suggested that 
the survival of grass where acid prevented 'the appearance of dicoty- 
ledons may be due to the branching habit of the grass roots, which 
makes injury to the tip of the primary radicle of less importance 
than with the plants which depend largely on a main taproot. 
Despite the qualifications in the preceding paragraph it seems 
quite certain that a great many germinating weed seeds which were 
dormant at the time of the application of the acid and were deeper 
in the soil, and therefore exposed to lower concentrations of acid 
than the pines, were killed in much the same way as the pines by 
amounts of acid which would not injure the pines. The experiments 
indicate not only a distinctly greater tolerance for sulphuric acid in 
the pines than in the angiosperms most commonly represented in the 
beds, but within the angiosperms a somewhat smaller difference in 
tolerance between the grasses and dicotyledonous species was ob- 
served. Tests in water culture would be necessary to establish the 
differences in resistance of the various species observed in these 
experiments and to give the differences a quantitative ^alue. 
Treatments several days or weeks before sowing also had consider- 
able effect on the number of weeds found in the seed beds during the 
first few weeks after the germination of the pines. The use of 0.3 
ounce of acid 14 days before sowing, with sufficiently frequent 
watering after sowing to prevent injury to yellow pine, prevented 
the appearance of any dicotyledons for at least 43 days after treat- 
ment and allowed only a few grass seedlings near the edge of the 
plat and a couple of Equisetum plants. Mollugo, grass, and Portulaca 
seedlings were common in all the check plats in this series, and Ama- 
ranthus and Euphorbia were present, while Equisetum was at least 
no more common in the checks than in the acid plats. 
