8 
BULLETIN 169, U. S. DEPAKTMENT OF AGRICULTURE. 
time. Although injury to root apices commonly took place before 
the seedlings appeared above the soil, most injured seedlings came 
up, and when the soil around the short root was kept moist the 
growth of the stem and leaves continued for some time at abnormal 
rate. All of the development of the aerial parts of the seedlings 
shown in Plate I, figures 2, 3, and 4, was made after the extension of 
the root had been stopped by acid. 
Injured seedlings ordinarily lived till the surface of the upper 
part of the root became brown and presumably impervious, as in 
the older parts of the root in healthy seedlings 
after two or three weeks. In the worst in- 
jured seedlings this root browning seemed to 
take place somewhat earlier than in healthy 
plants. The decrease in diameter which is no- 
ticed in the older parts of normal roots at the 
time of browning was seldom observed in acid- 
injured roots. Because the injured seedlings 
were not able to develop new root tissue, ab- 
sorption ultimately became impossible and 
death from drought ensued. The seedlings 
shown in Plate I, figures 2 and 3, have prac- 
tically reached this condition, though both 
still appeared to be growing normally when 
they were dug up. Plate I, figure 4, shows a 
seedling injured at the same time as that in 
Plate I, figure 3, which has recovered by recom- 
mencing root growth. 
Where the roots of injured seedlings were 
very short, the plants died very soon, either 
because the soil was allowed to dry out to be- 
low the level reached by the short root or be- 
cause the short root did not afford sufficient 
mechanical support for the top-heavy stem, 
and the seedling fell over or was washed out 
in watering. In the cases where injury was 
earliest, so that the radicle had scarcely emerged from the seed coat 
by the time its tip was killed, the seedlings failed to appear above 
ground at all. 
In a good many cases seedlings which had extended their roots a 
centimeter or more before injury ultimately recovered, either be- 
cause of a resumption of terminal root growth, as shown in Plate I, 
figure 4, or by laterals starting just back of the apex, as in text figure 
1. In such cases the parts of the seedlings above ground at no time 
showed any effect of the acid, and the only way in which the existence 
of injury could be detected was by examining the roots. Renewal 
Fig. 2.—Pinus ponderosa in- 
jured by copper sulphate 
Root growth has been re 
sumed by a number of later- 
als. (Natural size.) 
