192 
B. F. LUTMAN 
of the two preceding days had been rainy. Later observations revealed the 
fact that such atmospheric conditions tend to lower the pressure in the 
leaves while that in the tubers and stems remains almost constant. The 
records made two days later are, therefore, to be taken as representing 
more nearly the relative pressures in the various parts of the plant during 
average July weather. At that time the pressures had all risen, but par- 
ticularly that of the leaves. Two hot, clear days had intervened. In the 
latter case, the stalks displayed a high osmotic pressure but it was not much 
above that of the leaves. The same observations can be made on the 
determinations of July 22 when tip-burn began to be noticed on some of the 
plants. 
A period of intense heat and extensive tip-burn injury then ensued. 
The sap in the leaves and stalks exhibited a much smaller depression when 
obtained at 5 A.M-. on July 25 than that at 5 P. M. on July 23, 10:45 P.M. 
July 25, 9:30 A.M. July 27, or 11 A.M. July 29. The great preponderance 
of the pressure in the juice from the stalks is the striking feature of the 
observations made during this intensely hot week; in one instance this 
pressure increased to 11.37 atmospheres. The tuber sap pressure seemed 
to be more stable, although in one case it rose to 7.16 atmospheres. An 
analysis of the juices revealed a high sugar content in the stalks at all times, 
2.146 percent of sucrose on July 25 being the maximum. The osmotic 
pressure in the tubers must also be largely maintained by sugars, both re- 
ducing sugars and sucrose being always present in large amounts. 
A heavy rain on the night of July 29 broke the hot spell of weather, and 
the response of the plants was almost immediate, as the observations made 
at 10 A.M. on July 30 indicate. The sap in the stalks, with a sucrose con- 
tent of 2.27 percent, did not change as rapidly, although the pressure was 
somewhat less than it had been the previous day. 
The high pressure in the juice from the stalks was at 5 A.M. July 31, 
but the pressures in the juices of the leaves and tubers were approximately 
the same as those obtained at 10 A.M. the preceding day. 
The next observation was made on August 7, when the sap from young 
plants was compared with that from fully grown specimens. The striking 
fact here presents itself that in the young plants all the organs exhibited 
nearly the same osmotic pressure and that the leaves take the lead by about 
a half-atmosphere. The juice from the old seed tuber, as might be expected, 
produced the smallest depression. The pressure of sap from the stalks of 
the old plants was almost an atmosphere greater than that from the leaves. 
The relatively high sucrose percentage in the stalks should be noted. 
A repetition of this series of observations was made at 2 130 P.M. August 
8. The stalk sap, in this case, recorded a slightly larger depression than the 
leaf sap, but the difference between the juices of the old leaves and those of 
the stems amounted to over 0.8 atmosphere. 
The trials made on August 17 were meant to determine the osmotic 
