NATUEE AND ORIGIN OF FUESH-WATER ORGANISMS 357 
Although an overwhelming majority of salt-water species is 
characteristic in the case of animals, this is not so in the case of plants. 
It is difficult to be very precise in a matter which involves the 
counting up of an immense number of aquatic species, but there 
seems some evidence for believing that the number of fresh-water 
forms is actually in excess of those which inhabit the sea. Be this as 
it may, we can safely state that there is no such striking disproportion 
as certainly exists in the animal kingdom, and that the waters of the 
globe, both salt and fresh, are inhabited by very many forms of 
vegetable life. 
Of the higher plants, a large proportion of the Phanerogams are 
purely terrestrial ; but, with the exception of the Gymnosperm^e 
(among which, however, swamp-plants occur), most of the larger 
groups contain species of aquatic habitat. The marine flora includes 
comparatively few Phanerogams, which belong to the families 
Hydrocharitaceae and Potamogetonaceae, the so-called sea-grass 
{Zostera jnariiia) being a very common and widely distributed example. 
There are no marine Dicotylae. The Phanerogams of fresh water, on 
the contrary, belong to the most diverse orders of Angiosperms, and 
far exceed in number of species those of salt water. Of importance 
amongst the Dicotylae are the Nymphaeaceas, all fresh-water forms ; 
certain Ranunculaceas {Batrachiitm) ; Ceratophyllaceae ; Halorhagi- 
daceae {J^Iyriophyllum) ; and Utriculariaceae. Of Monocotylae we may 
mention the following families :— Alismaceas ; Potamogetonaceae (with 
Potamogeton 7iatans) ; Naiadaceae ; and Lemnaceae. 
It is amongst the Cryptogams, however, that we find the number 
of aquatic forms really great. Nevertheless, the Pteridophyta and 
Bryophyta are of little importance, for both of these groups are 
entirely unrepresented in the sea, although a few examples are known 
from fresh water. Of Pteridophyta, various Salviniaceae {Salvmia 
and Azolla\ Marsiliaceae, and Isoetaceae {Isoetes lacustris) occur in 
fresh water, and of Bryophyta a rather larger assemblage, among 
which we may mention Riccia Jiuitans^ Fontinalis antipyret'ica^ 
Hypnurn^ and Spliagnum. 
The Thallophyta, then, constitutes the great proportion of both 
salt- and fresh- water plants, but the classes differ markedly in their 
distribution between the two media. The Characeae, wdth the well- 
known genera Cha?n and Nitella, are exclusively fresh-w^ater forms. 
Both the Phaeophycea? and Rhodophyceae, on the other hand, are very 
widely distributed, and are represented by many species in the sea, 
while in fresh water there occur only a few isolated examples. Among 
the most important of these Algae we may indicate the genera 
Laminaria^ Fucus^ Sargassuin., and CJiondrus from the ocean, and 
Batrachospej'viiim from fresh water. 
