20 
Ridley.—The Distribution of Plants. 
Brasenia peltata , Pursh. This little water-lily was apparently com¬ 
mon in Pliocene times in Russia, Germany, and Switzerland, but has 
entirely disappeared from Europe. It now persists in Manchuria, Kha- 
siya, Japan, Australia, Angola, and North America. It is obvious that 
at one time it was very widely diffused over the world, but, like so many 
of the earlier plants, it has died out except in the palaearctic regions and 
in a few other isolated spots. The drying up or, what is more common, 
the silting up of lakes may account for the disappearance and isolation 
of these aquatics. 
Naias. These water-weeds seem to be very easily dispersed, largely, 
I believe, by water-fowl, and some of them have a remarkably wide 
distribution ; one may compare them in the matter of distribution and 
dispersal with the Characeae. 
Naias marina, Linn., a brackish water and marine plant, practically 
occurs all over the world except tropical Africa and Malaya, but chiefly 
in temperate regions. It occurs in the Cromer forest bed. 
N. minor , AIL, which occurs with it in the Cromer forest bed, is con¬ 
fined to Europe and temperate and tropical Asia as far east as Manchuria. 
N. gr amine a, Del., occurs earliest in Britain in the interglacial 
period. It is widely spread over tropical and subtropical Asia. Its 
occurrence in a hot-water canal at Manchester shows how easily these 
plants get about the world. 
N. flexilis, Rostk., known from interglacial deposits in Sweden and 
Germany and confined to northern Europe and North America. 
Here we see that for four species of Naias , all of approximately the same 
geological antiquity and with very suitable dispersal mechanisms, we have 
very different areas of distribution. 
The marine species, which can adapt itself apparently to sea, brackish , 
and fresh water, and can stand considerable variations of temperature, 
is the most widely distributed. The continuity of the sea largely ensures 
this, and is the reason why maritime plants, as will be shown, are more 
widely distributed than freshwater plants of streams and rivers which are 
not continuous. 
N. minor , All., a freshwater species, has not reached America and 
does not seem to stand heat well. 
N. flexilis , Rostk., is palaearctic and nearctic only, being a cold-climate 
species. 
N. graminea, Del., is a warm-water species and cannot stand cold, so 
its area is limited. 
It is quite clear that the area of distribution of Naias depends not on 
the age of the species at all, but on adaptability to climate and environment, 
the species with the largest continuous area, the sea, being the most widely 
distributed. 
