1 3 2 
W. B. Crump. 
than before and has attained its wilting value, but it is still approxi¬ 
mately the same for both. 
Some wilting experiments on moorland plants' carried out in 
1910 support this conclusion. On comparing the results obtained 
in the most extreme soils examined the differences lie within the 
experimental errors. 
Wilting of Eriophorum angustifolium. 
Air-dry Peat. 
Water remaining 
Water. 
(Echard). 
Hum us. 
Humus. 
Average of four cases 
55-2% 
7P1% 
0-77 
ft ft ft 
43-7 
50-7 
0-86 
Wilting of Calluna vulgaris. 
Air-dry 
Soil. 
Water remaining 
Water.. 
(Echard). 
Humus. 
Humus. 
Peat: average of three 
cases 25‘9% 
57-1% 
0-45 
Loam, or sand, ditto 
3-0 
7-6 
0-36 
Comparing these with the average coefficients of their natural 
habitats the inference appears to be that at the wilting-point the 
coefficients have about one-sixth their normal value. So five-sixths 
of the total water-content, irrespective of its amount, may be set 
down as physiological or available water. 
V.— The Coefficient of Soil Humidity. 
Now that the coefficient of humidity holds out the promise of 
being a standard index of the soil moisture, it becomes necessary to 
submit it to as wide a range of tests as possible to ascertain 
whether its values are sufficiently constant and distinctive to 
characterise both allied and diverse habitats—plant societies within 
an association as well as diverse formations. 
Seasonal Variation. Many habitats show a sufficiently marked 
seasonal variation in the soil moisture to be reflected in the co¬ 
efficient, as is shown in subsequent examples. But unless the 
seasonal variation is itself the subject of enquiry it seems better to 
limit the observations to the period of growth, which coincides 
roughly with the dry (i.e„ dry soil) season, between April and 
September. In no case should samples be taken within a few days 
of heavy rain and with this proviso, the results are reasonably 
comparable throughout the season. The ideal time to take samples 
1 Crump, W. B. “The Wilting of Moorland Plants.” Report of the 
British Association, Portsmouth Meeting, 1911. 
