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 Pinns 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, 



