Dispersal of Pond Floras

It is one of the essential postulates of Dr J. C. Willis's theory of Age and Area, that plant distribution is an extremely slow process and one greatly retarded even by small barriers'. Dr Willis quotes in his book one concrete case of this, in the shape of an early investigation carried out jointly by him and Mr Burkill, of the flora of the pollard willow-tops in Cambridgeshire2. These authors found that of 600 species capable of growing in the willow-heads and found in the neighbourhood, only 80 species actually occurred in the willowtops. Thus a barrier of only eight feet elevation was enough to prevent the spread to the willow-tops of about 87 per cent. of the species growing round them. This instance stands singularly alone, and it was by searching for a similar example that the present case was discovered.