Ill THE LESSON OF THE BRITISH FLORA 27 



than forty plants are the seeds or seedvessels still afloat. These 

 forty plants, excluding two or three littoral plants, are nearly all 

 plants of the borders and vicinity of rivers and ponds. (They are 

 indicated in the list given in Note 10 by the numbers vi. and xii., 

 the last being those where the flotation experiment was prolonged 

 to a year and over.) 



It would thus seem — I am now quoting mainly from my paper 

 in Science Gossip for May, 1895 — that there are gathered at the 

 margins of rivers and ponds, as well as at the sea-border, most of 

 the British plants that could be assisted in the distribution of their 

 seeds by the agency of water. This great sifting experiment has 

 been the work of the ages, and we here get a glimpse at Nature in 

 the act of selecting a station. But the curious character of the 

 sorting process becomes yet more apparent when we discover that 

 the buoyancy of the seeds or fruits of species of the same genus 

 may become a matter of station. 



We will first take the four British species of Stachys (arvensis, 

 betonica, sylvatica, and palustris). Of these the fruits of S. palus- 

 tris alone possess any buoyancy, being able to float for weeks. It 

 is the only species that finds its characteristic home at the water- 

 side ; and as observed by Sernander its reproductive shoots occur 

 in the Scandinavian fresh-water drift. 



Galium illustrates the same principle. Whilst in my experi- 

 ments the fruits of G. aparine and of another species growing in a 

 dry station displayed little or no floating power, those of G. palustre, 

 which alone grows at the water-side and in wet situations, have 

 great buoyancy. As my observations show, they float unharmed 

 through the winter in our ponds and rivers, and, according to 

 Sernander, are often found in the Baltic sea-drift. (See Note 12.) 



The achenes of Potentilla afford another example. Those of 

 P. tormentilla and of another species from dry situations have but 

 little floating power. On the other hand, those of P. comarum 

 float indefinitely. The last also came under my notice in the 

 floating drift of ponds in February ; and we learn from 

 Sernander that they occur in the fresh-water and salt-water drift 

 of Scandinavia. 



As a further instance, I will take the two British species of 

 Iris. The familiar river-side Iris pseudacorus has seeds that float 

 unharmed in our ponds and rivers from the autumn to the spring, 

 and often for a year or more. On the other hand, the seeds of 

 Iris foetidissima, which has its home in the shady wood, sink at 

 once even after drying for months. 



