OF A CAMBRIDGESHIRE WOODLAND. 347 
Experiments with other species gave very unsatisfactory results, so that 
these figures will not be taken into account, and the water-content figures are 
to be taken as referring to the holard in all cases. But: it may be noted here 
that this never fell so low as the amount of the echard obser ved, and 
consequently here the plants never get into a state of extreme drought. 
Another feature of the soil that has been studied to some extent is the 
temperature. 
The temperatures were taken with ordinary mercurial thermometers, 
those of the soils being taken as far as possible for the | layers in which the 
roots of the principal species occur. 
In the early parts of the year the soil temperature was considerably below 
that of the air, but from about Mareh onwards the soil showed a steady 1 
in temperature with only minor fluctuations. This rise is independent « ‘of 
temporary fluctuations of the air temperature, so much so that the soil may 
at times be warmer than the air: e. g., on May 23rd, 1910, during a sudden 
cooling of the air the temperature just clear of the ground vegetation fell to 
10:5? C., while the temperature of the marl in parts where it is fairly dry 
stood at 1:5? С. This drop for the air, however, was only temporary, and 
later it got well above the soil again. 
The temperature of the two soils varies somewhat : the marl being darker 
in colour would be expected to absorb more heat ; and this occurs in the 
early part of the year when the two are both equally wet, but later in 
the year the low water-content causes the loam to get considerably warmer 
than the mass of the calcareous clay—though dry parts of this approach the 
temperatures of the loam very closely. 
The average temperature of the soil in the wood seems practically 
independent of different degrees of shading that occur, though the more 
open parts show greater fluctuations and on the whole warm up more slowly 
in spring. 
As is well known the temperature of the surface of the soil is higher 
than that of the soil or the air ; it fluctuates in general with the changes of 
the air, but the variations are not quite so great as a rule. This warming up 
of the surface layer is a feature of considerable importance in connection with 
the sprouting of herbs in the spring, which have their roots and aerial organs 
at very different temperatures. This might be expected to cause considerable 
disturbance to the plant were it not for the increased relative humidity at 
the surface which reduces transpiration *. 
The other factors, light and evaporation, will be discussed later along with 
the details of the societies of the ground vegetation. 
* Cf. MacDougal, 1903. 
