48 Bristol. — On the A Iga- Flora of some Desiccated 
seven of them. This association is not universal, however, since each of the 
three algae has been found in soils from which both of the others were 
absent. It is noticeable that, of the soils examined, those that are rich in 
blue-green algae contain only few species of diatoms, and vice versa, though 
certain exceptions to this rule occur. It seems from the results of the 
cultures that diatoms occur most frequently in soils from old gardens, 
whereas the blue-green algae are more characteristic of arable soils ; the 
green algae, on the other hand, are distributed universally. 
It is a feature of the cultures that certain species of algae which are 
regarded as typical and commonly occurring soil-forms either are completely 
absent from them or else occur only rarely. It is possible that the drying 
of the soil-samples may be responsible for this, for in a number of these 
species no record has ever been 
made of their ability to form 
resting spores ; the degree of 
desiccation produced in the 
soils by their preliminary treat- 
ment considerably exceeded 
that likely to occur under 
natural conditions, and would 
render the survival of vegeta- 
tive filaments of the algae quite 
impossible. An interesting ex- 
ample of this is found in the 
species Nostoc commune , which 
has been obtained only in the 
cultures of three arable soils 
from Tisbury (Nos. 51, 52, and 
53). Up to the present time 
spores of this species have been 
unknown, and can therefore be produced only rarely, but in the forms 
found in these cultures spore-formation has been observed (Text-fig. 4), 
and it .is probably only owing to their production during the first drying 
of the soils that the species has survived in these particular samples. 
Similarly, Nostoc minutum , Desmaz, has not previously been observed 
to produce resting spores, yet in cultures 53, A, B, and C, of an arable soil 
from Tisbury, the species was not only obtained after a period of desiccation 
lasting for more than six weeks, but was also observed to produce spores in 
the culture (Text-fig. 6). 
On the other hand, certain species have appeared in the cultures which 
were entirely unexpected. The most interesting of these is perhaps 
Bumilleria exilis (Text-fig. 1 ), which was described by Klebs in 1896 from 
cultures of a loamy soil, and has not since been recorded from any other 
Text- fig. 1 . Bumilleria exilis , Klebs. a. and 
b., vegetative filaments showing variable number of 
chloroplasts, X 825 ; c. and d. 
in formation of zoogonidia 
, filaments showing stages 
x 1435 - 
