English Soils: an Important Factor in Soil Biology . 53 
frequently found. The spores germinated in the cultures either after 
a period of rest or almost immediately after formation. The contents of the 
spore were observed to become much increased in volume (Text-fig. 3, F, 
a and h) and the entire spore-wall to be gradually converted into mucilage. 
Within this mucilaginous investment the protoplast began to divide in all 
directions of space, producing a more or less spherical cluster of cells in 
which no trace of a filamentous arrangement could be detected. These cells 
continued to divide until a large irregular mass of closely clustered cells was 
produced which bore no specific characters at all. Heterocysts were usually 
Text-fig. 5 . Tribonema bombycinum , (Ag.) Derb. and Sol. a., typical vegetative filament 
with walls of end cells broken into H-pieces ; b. and c., somewhat irregular filaments probably pro- 
duced as a result of cultural conditions ; d ., breaking up of filament into aplanospores e., formation of 
aplanospores within cells of filament and liberation by splitting of mother-cell wall. x 825. 
completely absent except in quite young colonies, where a single heterocyst 
was often observed at the surface of the cell-cluster (Text-fig. 3 , F, d). 
The mucilaginous investment usually became firm on the outside and of 
a light brown colour, and remained permanent. In some cases in the 
cultures the alga remained in this condition for a considerable time, and 
sometimes the conditions were such that the cells at this stage became con- 
verted directly into resting spores without having developed into the adult 
stage at all, but this was probably quite abnormal and due entirely to the 
cultural conditions. In the majority of cases, sooner or later the juvenile 
form became gradually transformed into the adult stage with definite 
