Former Distribution of British Plants. 99 
flourishes throughout the Mediterranean region, and extends 
into Central Europe. This maple grows well in gardens 
in the South of England and seeds freely; though I have 
not heard of any case in which it has spread from self- 
sown seedlings. Mr. A. R. Wallace has undertaken some 
experiments in Dorset with a view to ascertain whether 
this plant can establish itself under natural conditions. 
Trapa natans is found in the Cromer Forest-bed, but 
does not seem to have re-entered this country after it had 
been driven out by the cold. It is an aquatic plant still 
living as near as the South of Sweden, and has large 
edible fruits known as water-chestnuts. Its absence in 
Britain seems to be unconnected with changed climatic 
conditions. 
Salix polaris occurs abundantly in Glacial deposits, 
both Early and Late; but it has now completely dis- 
appeared from Britain. It grows within the Arctic 
Regions, and on the highest mountains of Scandinavia. 
Picea excelsa was common in the East of England in 
Preglacial times. It is apparently another large-seeded 
plant that has been unable to re-establish itself here, now 
that Britain is separated from the Continent. There is 
nothing in the modern distribution of the spruce-fir to 
suggest that it is unsuited for our present climate, though 
this tree does not tend to spread from seedlings as do 
Pinus sylvestris and Pinus maritima. 
Najas graminea has only been found in the Interglacial 
deposit at West Wittering in Sussex. Its recent distri- 
bution is throughout the Tropics of the Old World, and 
also in the Mediterranean Region. In Britain it has been 
introduced at one spot, where it grows in a canal which 
receives waste hot water from a mill. 
Najas minor occurs in Preglacial deposits, and at 
West Wittering. It also belongs to warmer climates, 
