English Soils : an I mportant Factor in Soil Biology. 6 1 
formed in long chains ; they were oval, 4*5 to 7 /x broad by 6 to 7-5 fx long, 
with smooth colourless walls. 
The second form (b), though by its mode of growth and general 
characters obviously belonging to the same species, differed a little in its 
dimensions. The vegetative cells were mostly nearly spherical, 4-5 to 5-5 [x 
in diameter ; the heterocysts were somewhat reduced in size, being only 
4-5 to 6 jx in diameter, and two or sometimes three were frequently found in 
series. The spores-Avere oval, and might have their long axis placed either 
longitudinally or transversely ; the walls were smooth and colourless, the 
breadth 7 to 8 fx, and the length 10 to it [x. It is impossible to say from 
the evidence obtained which form of spore is to be regarded as more typical 
of the species ; form (a) was observed in the cultures of two samples, while 
form (b) was found only in one, but it is quite possible that (b) represents 
a later stage of development than (a), though, on the other hand, the small 
size of the heterocysts in (b) rather indicates that it may be a somewhat 
abnormal form due to cultural conditions. 
9. Nos toe sphaeroides , Kuetz. 
A blue-green alga appeared in two of the cultures which agreed more 
nearly with this species than with any other, and was probably only a cultural 
form of it. The young colonies were globular, consisting of filaments very 
closely entwined together and with a close gelatinous envelope. The 
vegetative cells were shortly barrel-shaped or nearly spherical and 5 to 7 [x 
in diameter. The ends of the filaments were somewhat attenuated, and the 
terminal cells longer in proportion to their diameter. Heterocysts were not 
produced in the cultures, but spores were formed in long chains ; they were 
subspherical or spherical, 10 to x 3 /tx in diameter, and frequently embedded 
in a wide and somewhat diffuse mucous sheath. At maturity the spore- 
wall acquired a golden-brown colour, but was never observed to become 
rough, as has been described for this species. It is possible, however, that 
this variation may be the effect of cultural conditions, since it has been 
observed that in the alga Trochiscia aspera , (Reinsch) Hansg., there is 
a great tendency in cultures for the cell-wall to lose its characteristic 
external ornamentation. This species has not previously been recorded 
from the British Islands. 
10. Nostoc minutum , Desmaz. (Text- fig. 6 ). 
This interesting little species was found in the cultures of only one 
sample of soil, but its presence there is all the more noteworthy since it was 
observed, for the first time on record, to produce spores. The colonies were 
extremely small and rather irregular in shape, consisting of closely coiled 
filaments embedded in a definite colourless mucous envelope. The 
vegetative cells were barrel-shaped, about 2-5 [x broad, and the end cell of 
the filament was conical. The heterocysts were subspherical, and some- 
what variable in size; the majority were 3-5 to 4*5 /x in diameter, but 
