8 



BULLETIN 169, U. S. DEPARTMENT 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 a normal 

 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.) 



