39 
part of December there was a rapid drying back of the streams running 
to the farm from the mountains, and the desiccation of the ground became 
rather abruptly evident a month later. Through January, surface culti- 
vation kept all but the most porous ground in good condition, but after 
this time it was practically useless so far as the soil was concerned. 
By the middle of March the soil, where it was not sandy and ready to 
crumble, was as hard as if baked, and under the thoroughly cultivated 
surface it was full of fissures as much as a centimeter in breadth. The 
hardness was shown by the behavior of main roots nearly 1 centimeter in 
diameter, to whose disposition to grow in a straight line the tree owes its 
firmness. ‘These, upon entering the cracks, turned almost at right angles 
and started to follow them. 
In such a soil it is obvious that, in a short time, growth will be 
suspended. On March 21, I was unable to find any roots apparently 
normally active. The cessation of growth had been accomplished, or 
initiated, by the shortening of the growing region until the hardened 
hypodermis had advanced to within the root cap, obliterating the white 
absorbing surface. The disappearance of the absorbing region in the 
small branch roots, with short caps, was at first less complete, but by 
April 11 that portion remaining unlignified at the tip even of these was 
more or less flaccid, even in the early morning. 
The turgor in the cortex of these roots equals nearly 0.4 normal potassium 
nitrate. Approaching the meristem it is higher, probably 0.5 normal. In the 
cap and epidermis 1 was unable to determine it. It will be noticed that the in- 
crease in turgor caused by desiccation and cessation of growth is more than half 
What it is when the cessation of growth and immersion in 0.5 normal potassium 
nitrate act together. This shows what proportion of the increase in the latter 
is directly due to the absorption of the salt.* 
Of course, the roots of a tree do not all suffer alike, because different 
strata of the soil do not become equally dry. I tested the amount of 
moisture in the soil on April 11, at depths of 20, 60, and 100 centimeters, 
determining the weight lost by drying at a temperature of 40.3° C., the 
dew point being 25.5° C. The loss was— 
At depth of— Per cent. 
20 centimeters..............:...j5gk eee 16.6 
60 centimeters.................. cee eee 21 
100 centimeters.............c..< ee eee ss 0 
“For the influence of rather concentrated solutions on the turgor of immersed 
roots, see Stange, in Bot. Zeit, 1892. For the influence which the mechanieal 
prevention of growth exerts upon the turgor, see especially Pfeffer, Ueber Druck- 
und Arbeitsleistung, 1893, For a general treatment of the dependence of the 
turgor upon the rate of growth, see my paper, Ueber den Hinfluss von Licht 
and Temperatur auf den Turgor, loe. cit. 1896. 
“The difference in available water is much greater than these figures would 
indicate, for at 20 centimeters in depth the soil is the hard clay already mentioned, 
while at 100 centimeters it is a sandy loam, crumbling readily; at 60 centimeters 
it is intermediate in character. 
