Problems in Aquatic Biology. 155 
relative degree of aeration of the water; poor aeration necessarily 
excluding forms with broad filaments, especially if these latter have 
thick walls (as in Cladophoraceae). Air-content is probably as 
important in the water as water-content is on land. 
In characterising our submerged aquatic formations it will be 
necessary, however, to take the other submerged plants into 
consideration as well; and it may be that in some cases the formation 
will be characterised entirely, or in part, by certain dominant 
Phanerogamic aquatics. On the latter point I do not care to 
pronounce an opinion as yet, but there is no doubt that certain 
Algae and Phanerogams frequently, if not invariably, go together. 
Thus Myriopliyllum appears to occur in waters frequented by 
Cladophoraceae, whilst waters with Ranunculus aquatilis seem to be 
rich in Conjugates (often together with Microspora). Other aquatic 
Phanerogams probably, like many Algae, are not so particular and 
occur in a number of different types of water; such forms will be 
of no value in characterising aquatic formations. 
We have next to consider the other types of vegetation, 
occurring in the water, viz., the floating and the marginal (semi- 
aquatic) forms. The advisability of separating the floating from 
the submerged vegetation of a piece of water as a distinct formation 
is open to discussion ; it is almost impossible to establish a sharp 
delimitation, and, if we resolve to regard the two as separate 
formations, we shall in many cases be confronted by the anomaly of 
an individual belonging to one formation in its young stages, and to 
another in later ones. The floating vegetation of a piece of water 
constitutes a biological group within a biological group, and is better 
regarded as an association or group of associations, comparable in 
some respects to the undergrowth and epiphytic vegetation of a 
piece of woodland. On the whole it is dependent on the same 
collection of factors as influence the submerged vegetation of the 
pond, and often at least its nature depends largely on the character 
of the remaining aquatic growth. I should propose as a preliminary 
to distinguish the following associations among the floating 
vegetation:— 
(«) Free-floating Cormophytes, such as Lemna, Azolla, Salvinia, 
Riccia, etc. 
(b) Attached Cormophytes with floating leaves, e.g., Nymphaea, 
Limnanthemum, Elisma, etc. 
(t) The Plankton or microscopic (thallophytic) forms, e.g., 
forms which are free-floating or actively motile throughout the 
vegetative stage of their life-history, 
