English Soils : an Important Factor in Soil Biology. 41 
mentioned the forms met with from these soils are small in comparison 
with other forms and varieties of the same species occurring in aquatic 
habitats. These facts are also true for the diatoms described by Petersen 
from the Danish soils. He suggests that since the species belong to the 
pennate diatoms they are further adapted to their mode of life by their 
power of locomotion, so that they are enabled to move in times of drought 
to the moister layers of the soil. 
In a recent lefter to Professor G. S. West, Dr. A. Mann, of the U.S. 
Dept, of Agriculture, has suggested that the exceedingly small size of these 
diatoms, especially of Navicula contenta , var. biceps , is due to long starvation 
under unfavourable biological conditions, and that they are merely forms of 
some of the larger species which have undergone not only reduction in size 
but also suppression in the sharpness of their markings ; he suggests that 
Navicula contenta is either a dwarf variety of, or closely related to, 
N avicula gibba, Ehrenb., or some such variable form. These suggestions, 
however, do not seem likely, since in the cultures under present investiga- 
tion, where a plentiful supply of water and of mineral salts was always 
available, no change in the form or size of the diatoms was observable even 
at the end of three and a half years, but the species remained true to type ; 
and besides this the soil can scarcely be regarded as a medium in which 
diatoms are subjected to prolonged starvation, especially in a country like 
England, where there is a fairly adequate rainfall, and where, even in the 
absence of rain, there is usually a copious dew. The typical form of 
Navicula contenta , var. biceps , has also been recorded by Petersen from 
Danish soils, while specimens of the same variety growing on the leaves of 
trees at Watten Waven, Dominica, W. Indies , 1 were found on comparison 
to show characters identical with those of the soil form. The ability of this 
diatom to grow in so apparently unpropitious a habitat as the leaves of 
trees is possibly the result of the extreme moisture of the atmosphere in 
Dominica, and the diatoms are probably able to obtain all the nourishment 
they require from this source. 
It seems most correct, then, to regard the soil-diatoms as independent 
species or varieties which by their small size have been enabled to establish 
themselves in a habitat which would be unable to support life and growth 
in the larger species. 
In the soils examined in this work blue-green algae are less universally 
present than are diatoms or green algae, and the species found appear to be 
more local in occurrence. There can, however, be traced in a number of 
soils an association between the three species Phormidium tenue y Ph . 
autumnale , and Plectonema Battersii, at least two of the three species having 
been found together in no less than sixteen soils, while all three occur in 
1 West, W. and G. S. : A Further Contrib. to Freskw. Algae of W. Indies. Journ. Linn. 
Soc., Bot., xxxiv, 1899, p. 291. 
