INTRODUCTION 9 
make up for evaporation, or salt-water) should be 
supplied drop by drop. One of the best ways of 
cultivating seaweeds is by suspending them in 
baskets in the sea at proper depths from anchored 
buoys. (See Reinke, in Botan. Centralblatt, 1890 ; and 
Oltmanns, in Pringsheim’s Jahrbiicher, xxiii. 1891.) 
It has been commonly supposed that the composi- 
tion of sea-water, and particularly its degree of 
concentration, has a powerful influence on the dis- 
tribution of seaweeds. The North Sea, where the 
salinity reaches 35 per cent., is, for example, much 
richer in its marine flora than the Baltic—cven the 
western part, where the salinity is 1-7 per cent., and 
still more the eastern and northern parts, where the 
salinity declines to 0°15 per cent. Oltmanns! has 
shown, however, that the degree of salinity has much 
less influence than has been believed, but that rapid 
variations of this condition are hostile to the exist- 
ence of seaweeds. Where fresh water runs into the 
sea, it arrives in conditions varying with its abun- 
dance, with the currents it meets and forms, and 
with the direction of winds. There are thus set 
up differences in the density of the water, and 
these differences, acting on the cells of seaweeds, 
are of very detrimental effect. Oltmanns’ observa- 
tions at Warnemunde, near Rostock, are of great 
interest in this respect. A canal there connects the 
sea with a lake that receives almost all the fresh- 
water of Mecklenburg, and many species of seaweeds 
grow in this lake at places where the salinity is 
almost nil, while almost al] are absent from the canal, 
1 Sitzungsber. d. K. preuss. Akad. d. Wiss. (Berlin. 1891.) 
