II2 BOTANICAL GAZETTE [AUGUST 
beginning of the next reading, which in this case is the next hour. 
A record of the foliar transpiring power at hourly intervals at 
Chicago gives results that are similar to those obtained for plants 
of the same species in southern Arizona. 
The series of 1916 show conclusively that the older leaves are 
the first to wilt. In an examination of series Ia the older set of 
leaves is almost completely dry at the time of the permanent wilting 
of the plant. On July 29 the edges of the leaf are dry, but at the 
same time there is a different form of response in the younger leaves, 
in that the apparent recovery occasioned at the time of permanent 
wilting does not present itself. The same situation is true for the 
series Ib, where the older leaves wilt on July 31. That the older 
leaves are the first to wilt has previously been determined by a 
number of investigations (15, 33). BAKKE and LIvINGSTON 
have presented evidence that there is considerable variation in the 
index of foliar transpiring power of young and old leaves. The 
fact that the younger leaves wilt later than the older leaves is not 
necessarily connected with the environment. This is true whether 
the evaporation is low or whether it is high. The production of 
the absciss-layer may at least be indirectly formed as a result. 
PRINGSHEIN (33) previously has shown that young leaves retain 
their freshness for a longer time than older ones. This he ascribes 
to a greater osmotic pressure. During the march of wilting it is 
also noticed that the foliar transpiring power index of the older 
leaves is always higher, at least than that of the leaves of the 
tip. The older leaves then give a higher foliar transpiring power 
throughout. 
There is also in evidence during the march of wilting not only a 
low index of foliar transpiring power, but also a gradual increase of 
the force in opposition to the passage of water. When for a short 
time there is an evident break or a serious rupture, there is a 
decrease in the resistance, but an equilibrium with the atmosphere 
is soon reached. The assumption that there is a temporary open- 
ing of the stomata may be made at this point. Employing the 
porometer upon Helianthus plants placed in an environment of 
high evaporating power and one of low evaporating power, the 
author failed to find that the stomata are concerned. 
