300 Yapp.— On Stratification in the Vegetation of a 
without exception both the highest and the lowest temperatures for any- 
given day occurred in this position (cf. also the temperature curves in Text- 
figs* 5 > 7 and 8). Comparing A and B, this difference of diurnal range was 
considerably greater in 1908, when B was only just below the general 
vegetation level, than in 1907, when it was situated about two feet below 
the top of the vegetation (see Text-figs. 4 and 6). Nocturnal radiation is no 
doubt largely responsible for the low night minima of B, especially on clear 
nights. 1 The high day maxima are not quite so easily explained. Perhaps 
they may be partly due to a greater loss of heat from A, by convection 
during the daytime, owing to the fact that air-currents have freer play 
in this position than in B. 
The mean diurnal range of temperature at C was in both years much 
less than at B, but while in 1907 it was also considerably less than at A, in 
1908 it was slightly greater. This apparent discrepancy is probably due to 
the greater height and density of the vegetation in the former year. 
It is interesting to note that the mean daily temperature in the positions 
A and B was nearly identical, while that of C was lower, and approached 
much more nearly the mean temperature of the soil. In fact, in 1908 the 
mean temperatures of C and D were practically the same, though the 
divergence was greater in 1907. 2 
From the foregoing it seems that the greatest diurnal range of tem- 
perature (when unscreened thermometers are used) is at or just below the 
general vegetation level. Further, it would appear that this daily range 
diminishes as the vertical depth below the superficies increases, 3 until 
it approximates to that in the free air above the vegetation. Below this 
point the range still further diminishes, and the temperature conditions 
become, in consequence, more uniform. But even in the lowest strata 
of the vegetation the diurnal range is still, of course, much greater than 
that of the soil itself, though the mean temperatures of the two may be 
similar. 4 
Though the evidence available certainly points in the direction of the 
above conclusions, it must be remembered that they are drawn from 
observations taken, in each year, at three levels only. 
But at all events it seems clear that not only are the upper strata 
1 Cf. Hann (’03), p. 41. 
2 In connexion with this divergence it is to be noted that, apart from the generally cooler 
summer of 1907, not only was the vegetation employed in this year taller and less open, but the soil 
was considerably wetter than in 1908. Further, the soil temperatures were not taken, in the latter 
year, at exactly the same spot as that where the instrument-stand was placed. This may have made 
some difference. 
3 Probably also the relative density of the vegetation is important. 
4 In considering these temperature relations, due allowance must be made for the fact that the 
thermometers in position A were exposed to direct sunshine. Those at B varied in this respect in 
the two years ; while C was scarcely, and D not at all, under the influence of direct insolation. Of 
course the same is true of the various plant organs which occupy similar positions. 
