4i 
Some Aspects of the Alga. 
Algae, and contains excellent discussions of the factors which affect 
the distribution of the different groups and species. Though the 
ecology of algae is yet in its infancy, there are a good many obser¬ 
vations on record, and Professor Oltmanns makes the most of these. 
It is pointed out that it is impossible for algae to gain a hold on 
a sandy or muddy sea-bottom liable to constant shifting by the tide, 
though certain forms can do so where the substratum is compara¬ 
tively at rest. Single rocks or any resistant substratum such as a pier 
or pile in such a shifting area always furnishes a place of attachment 
and gives rise to an algal oasis in the desert. 
A good account is given of the zonation of algae on a coast 
and it is shewn to depend partly on mechanical adaptation and 
partly on light, but also on the different facilities for the supply of 
food, and particularly oxygen, available in relation to spray, waves, 
etc. Another interesting section deals with the composition of the 
medium, particularly in relation to salt-content. The diminution of 
salt in the Eastern Baltic (from the 3’5%, which is the normal 
oceanic salinity, to *74% and even to -15%) is shewn to cause a 
considerable decrease in the number of species of marine algse, 
many of which cannot adapt themselves to such conditions. The 
extraordinary vegetation of Warnemunde is described. Here 
Phragniites communis and Fucus vesiculosus flourish side by side. 
Potamogeton pectinatus bears Ectocarpus, Polysiphonia violacea grows 
on Myriophyllum, while Spirogyra and Chara occur between. This 
is in a salt-concentration of *5%. Though the brown and red genera 
mentioned can tolerate such a small amount of salt, most of their 
congeners cannot. Ulva, Enteromorplia and Cladophora on the other 
hand, are the most indifferent to such changes, as is well known. 
Not only can they flourish in almost fresh water, but they can also 
tolerate high salt concentrations up to 10% and 13%, though they 
can only flourish in salt solutions of somewhat less strength. They 
may be called euryhaline as opposed to stenohaline species such as 
most of the Floridese and Pha^ophyceas. The euryhaline habit 
depends directly upon the power of these algas to take in salts from 
outside and thus temporarily raise the osmotic strength of the cell- 
sap. Again some forms can bring about these osmotic alterations 
quite rapidly, while others cannot. All these factors clearly control 
the possible habitats of the various forms in relation to salt-content 
of the medium. The possibility of permanent migration from salt to 
fresh water and vice versa must also depend upon the acquirement 
of similar powers. The salt-relation has considerable taxonomic 
importance. Mostof the unicellular green forms are exclusively fresh- 
