Marsh , and its Relations to Evaporation , &c. 309 
a modified form of one previously used by Livingston at the Arizona desert 
laboratory, 
5. In the field, both evaporation and temperature readings were taken 
at different levels in and above the vegetation. For this purpose, two types 
of marsh vegetation of differing heights and densities were selected. 
6. In 1907, the vegetation employed was about five feet high. The 
average evaporation at three different levels was according to the ratios 
A : B : C = 100 : 32-8 : 6-6. 
Position A was above the vegetation, B about the middle of it, and C 
in the lower strata (see Text-fig. 4). 
7. In 1908 the vegetation selected was only about two feet in height. 
The corresponding ratios were 
A : B : C = 100 : 56-2 : 14-7. 
Here position A was clear of the vegetation, B only just below the upper 
level of the vegetation, and C at the bottom (Text-fig. 6). 
8. Thus the air in the vegetation is on the whole very much more 
humid than that outside it. Further, the higher and denser the vegetation, 
the greater the differences in atmospheric humidity between the upper and 
lower strata. 
9. The temperature results show that the highest layers of the vegeta- 
tion possess a greater diurnal range of temperature than either the free air 
above or the lower layers of the vegetation. Regularly, especially in clear 
weather, both the highest day, and the lowest night temperatures were 
recorded in this position. 
The transpiring organs which occupy the lower strata of the vegeta- 
tion are thus not only in more humid air, but in general under considerably 
more uniform conditions than those which reach what is called in this paper 
the ‘ general vegetation level \ 
The general results of the experiments are given in Tables I to V, and 
in the form of curves in Text-figs. 5, 7 and 8. 
10. It is shown that the great majority of the shoots of the different 
species attain to very much the same height, thus giving rise to a fairly 
uniform ‘ general vegetation or shoot-level ’ . Moreover, the shoots of 
isolated individual plants, and also of most kinds of vegetation, show 
a gregarious habit, which results in a greater or less degree of uniformity 
of external contour. When the external conditions are of great severity, 
the contours thus produced are exceedingly regular (e. g. desert shrubs, 
wind-swept trees on sea-coasts, &c.). But there is also evidence that even 
in the case of the vegetation dealt with in this paper, the atmospheric con- 
ditions may be, at least at times, unfavourably severe, as regards the upper 
strata of the vegetation. This is suggested by the early death of the pro- 
jecting, exposed ends of the grass and sedge leaves (cf. Hansen’s experi- 
