1914] CURRENT LITERATURE 85 
Soil moisture measurement.—The water content of the soil has long been 
recognized as the most important edaphic factor in limiting the occurrence and 
permanence of plant associations, but it has always been difficult to measure 
such a factor in terms that could be related to plant production. CRuMP,! 
in his studies of the vegetation of peat soils, has devised a method of expressing 
the relative humidity of these soils in such a manner that a definite index of 
water as an ecological factor is obtained. This index he has termed the 
“coefficient of humidity,” and it seems, for the habitats studied, to be a con- 
stant whose value may be determined for any given plant association. In 
obtaining this constant, the amount of water present in any soil is expressed in 
terms of percentage of the dry weight at 15° C., and the humus-content being 
determined in the usual way by combustion, this ratio of the water-content 
is obtained in terms of the humus-content as follows: 
water-content 
= coeffici f humidit 
humus-content Sees ee 
This coefficient is shown to vary directly with the amount of water available for 
the use of the vegetation of a habitat, and the investigator believes it to be a 
true integration of the relative humidity of the soil . different areas. He 
correction which permits it to be used for sub-peats containing large amounts 
of sand. 
Applying this unit of measurement to certain moor plant associations, he 
finds that the mean coefficients of humidity for the Eriophorum moor, the 
Calluna moor, and the Molinia moor of the Southern Pennines to be respectively 
6, 3.3, and 2; and thus he is able to institute a direct comparison between the 
water conditions of these associations and others in the same formation. 
t would seem that as the result of these investigations the ecologist has 
been given a most important method of expressing soil moisture, far in advance 
of anything before available, and it is to be hoped that it will be found to be 
applicable to a great variety of soils—Gxro. D. FULLER. 
Chromosomes in Allium.—In the nuclei of Allium Cepa BoNNEVIE? 
has described a large chromatin knot from which the chromatin | threads radiate. 
In the presynaptic stages in the pollen mother cells th ea 
From a comparison with the origin and behavior of similar sayen threads in 
Crump, W. B., The coefficient of humidity: a new method of expressing the soil 
moisture. New Phytol, 12:125-147. 1913. 
* Crump, W. B., Notes on water content and the wilting point. Jour. Ecol. 1:96- 
100. 1913. 
* Bonnevie, K., Chromosomenstudien. III. Chromatinreifung in Allium Cepa 
($). Arch. f, Zellforschung 6:190-253. pls. 10-13. IQtt. 
