40 Bristol . — On the Alga- Flora of some Desiccated 
dimensions of the species present were so small that only one method 
proved really satisfactory. In this the material to be examined was placed 
in water on a glass slide and broken up into very small pieces with a pair 
of needles ; the water was then allowed to evaporate slowly in the air, the 
slide being covered with a glass lid to prevent the access of dust. The 
material thus became dried on to the slide and could be subjected to further 
treatment with little risk of loss. After 24 hours’ drying, the slide was 
placed in a boiling-tube containing strong nitric acid and heated to remove 
all organic matter ; after careful washing in water it was transferred through 
95 per cent, and absolute alcohol to xylol, and the film of diatoms still 
remaining attached to the slide was then mounted in Canada balsam. 
Owing to the minute size of most of these soil-diatoms the markings on the 
walls were extremely difficult to make out, even under a magnification of 
1 , 435 , h en ce later preparations were made using dammar lac as a mounting 
medium in order to secure a better definition of the walls. 
The accompanying Tables I, II, and III give full particulars of the 
experimental details and serve for a comparison of the results obtained from 
the different cultures. An examination of these results leads to several 
interesting conclusions in relation to the distribution of algae in the soils 
examined ; and in this connexion it may be mentioned that though taxo- 
nomically it belongs to a different group of plants, the protonema of mosses 
has been included with the soil-algae in considering the possible economic 
significance of the microscopic green plants of the soil, since its form and 
mode of growth render it physiologically equivalent to them. 
It is easily seen that in a large majority of the soil-samples there is 
a central group of algae comprising most or all of fhe following species : 
Hantzschia amphioxys , Trochiscia aspera , Chlorococcnnt humicola , Bumilleria 
exilis , and less often IJlothrix sub tills, var. variabilis, together with moss 
protonema. These species thus appear to form the basis of an extensive 
ecological plant-formation in which, by the inclusion of other typically 
terrestrial but less widely spread species, smaller plant-associations can be 
recognized. In certain of the soils associations consisting very largely of 
diatoms are present, the species most generally found being Navicula 
mutica formae, N. Atomus, N. content a, var. biceps , N. Balfouriana , N. Bre- 
bissonii , var. diminuta , N. Pupula , and to a lesser degree N. borealis. It is 
conspicuous that with the exception of Navicula borealis , Hantzschia 
amphioxys , and Nitzschia obtusa , var. scalpelliformis , all the diatoms found in 
these soils are of very minute size, and it is no doubt this characteristic 
which enables them to withstand the conditions of drought to which the 
organisms of the soil are liable to be subjected ; since, as has been pointed 
out by Hedlund, 1 small organisms seem to be better able to resist 
desiccation than are larger ones. Even in the cases of the larger species 
1 Hedluncl, T. : Till fragan om vaxternas frosthardighet. Botanisker Notiser, Lund, 1913. 
