English Soils: an Important Factor in Soil Biology . 39 
It is conspicuous that of twenty-one different species obtained by 
Robbins from the Colorado soils only one belonged to the diatoms and one 
to the green algae ; the rest all belonged to the blue-green algae. 
Petersen, on the other hand, found in cultivated Danish soils no less 
than twenty different species of diatoms and eighteen different species of 
green algae, a result very similar to that obtained in the present work, hence 
it appears from Robbins’s record that the Colorado soils may be peculiar in 
this respect. 
A comparison of the results obtained with the different culture-solutions 
used in the present work showed that there was very little difference be- 
tween the cultures. Germination of the green algae and diatoms took place 
rather more quickly in the more dilute mineral-salt solution and in rain- 
water, while the development of the blue-green algae was on the whole more 
vigorous in the stronger mineral-salt solution ; rain-water appeared as a rule 
to favour especially the development of the green algae. In every case, 
however, the final results were identical, cultures of the same soil giving 
always the same record of species present, whichever culture-medium might 
be used. 
III. Results of Cultural Experiments. 
A great deal of difficulty was experienced in identifying the algae 
found in the cultures for various reasons. In the first place, the preliminary 
treatment of the soils was such as to preclude the possibility of the presence 
of any algae except in a resting condition in the initial stages of the 
cultures. The length of time taken for the germination of these resting 
forms varied in individual species, and for some months the cultures con- 
tained largely developmental stages which it was impossible to identify with 
any degree of certainty. Again, the somewhat abnormal conditions of 
excessive moisture under which the algae were growing tended to produce 
forms which in some cases were rather different from those of typical 
species already described, and it was necessary to decide whether such 
variations were the result of these conditions or whether they might perhaps 
characterize new species or varieties. Further, it was necessary in many 
cases to follow out the complete life-history of the alga before coming to 
any decision, and for this purpose it was sometimes necessary to prepare 
sub-cultures in order to separate the alga in question from the other species 
in the culture. This was especially the case for the species Chlorococcum 
humicola , (Naeg.) Rabenh., the complete isolation of which was effected by 
picking out individual cells under the microscope with fine glass capillary 
tubes and placing them in hanging-drop cultures in a damp chamber. -It 
was found that the algae rarely lived very long in these conditions, but 
sufficient evidence was obtained to establish the course of the life-history of 
the species. 
In identifying the diatoms various methods were attempted, but the 
