54 Bristol. — On the Alga- Flora of some Desiccated 
filaments producing heterocysts and spores in the normal manner, and in 
this stage it was possible to identify the species more or less accurately. In 
Text-fig. 4 , E and F, are depicted the initial stages of this mode of germina- 
tion in two species, Nostoc sphcieroides , Kuetz., and N. muscorum , Kuetz. 
Among the Chlorophyceae the most interesting record of this kind is 
the formation by Tribonema bombycinum of a somewhat unusual type of 
aplanospore. The filaments of this species appear to be somewhat sensitive 
to cultural conditions, as can be seen in Text-fig. 5 , b and c , and in the 
cultures of one soil were observed to be converted into one- or two-celled 
fragments which rounded themselves off from the rest. In some cases these 
fragments appeared to be produced simply by the rounding off and breaking 
away of the cells from one another (Text-fig. 5,d) y and the wall of the 
original cell was incorporated with that of the spore ; but in the majority . of 
cases the aplanospores arose singly within the cells of the filament, producing 
new cell-walls of their own, and were set free by the splitting of the mother- 
cell wall (Text-fig. 5 y e) which was afterwards observed gradually to dis-. 
integrate. 
IV. Discussion of the Possible Significance of Algae and 
Moss Protonema in the Economy of the Soil. 
The results of the foregoing experiments, taken in conjunction with 
those of Esmarch, Robbins, and Petersen, indicate that there is in many 
soils a definite algal flora that is especially well developed in cultivated 
soils, even in those that would appear at first sight to offer no conditions 
suitable to the growth of algae, and that this algal flora may include blue- 
green algae, diatoms, and green algae. Esmarch observed that the blue- 
green algae found by him are not confined to the surface-layers of soil, but 
are to be found, more or less completely, in soils that have been cultivated, 
even to a depth of 10-25 cm., while a smaller number of species may be 
found as much as 40-50 cm. below the surface ; and he observed that the 
species in the lower layers are almost identical with those of the surface- 
layers of the soil. He attributes this extensive distribution of blue-green 
algae in cultivated soils chiefly to the cultivation of the ground, in that by 
ploughing, digging, and hoeing not only are the conditions of the soil made 
more favourable to the growth of algae, but also fresh layers of soil are con- 
stantly being brought to the surface, and the surface-algae buried more and 
more deeply. He considers that the spores contained in the new surface- 
layers germinate on being exposed to the light and form fresh vegetative 
filaments on the surface, which at a later date are again buried. As other 
factors working for the distribution of algae in the soil he considers that 
the percolation of water and the burrowing of worms and other small 
animals are also effective. 
