LAND-FORMING PLANTS. 71 



Alisma, Acorns, Sparganium, Typha, Polygonum amphibiicm, Corn- 

 arum paluslre, Nasturtium officinale, Ranunculus Lingua and 

 R. Flammula, Equisetum limosum and Isoetes. 



Other water-plants are of importance in this respect, chiefly, 

 because by their decay they fill up shallow ponds or edges of 

 rivers. They may be classed as Plankton or free floating forms, 

 of which Lemna, Hydrocharis, Riccia, Volvox and the Diatoms 

 are the most important. Diatoms appear to develop best in 

 muddy water, and have apparently two crops per annum, one in 

 spring and the other in autumn. Great masses of Lemna may be 

 seen in Possil Marsh, forming a regular beach of decaying organic 

 matter. Another group in which the leaves alone float is poorly 

 developed in this country. Nymphcea, Nuphar, Litnnanthemum, 

 and Potamogeton natans are almost the only representatives. As 

 land-formers they cannot be given a high place. 



Floating submerged plants, on the other hand, are fairly numerous. 

 There is an extraordinary similarity in the leaves of Ranunculus 

 aquatilis, Myriophyllum, Apium, and Ceratophyllum ; some Pota- 

 mogetons and Scirpus fluitans tend to the same type. Hottonia 

 and Uiricularia also show it in a less regular condition. This fine 

 division is undoubtedly brought about by the water-life, and the 

 result is to make a sort of tangle of leaves very like what one sees 

 in a grass meadow. 



Callitriche and Elodea are of a different structure. The first is 

 intermediate between the floating-leaved and the submerged type. 

 The second is transitional between the submerged-floating and 

 submerged and attached type. This last consists of certain rare 

 plants which are fixed on the floor and give a sort of submerged 

 aquatic meadow. Lobelia Dortmanna, Littorella, Limosella, Zostera, 

 Naias, Zannichellia, Ruppia, Lsoetes, Fontinalis, Chara, and many 

 alga? come in here. They simply fix (and assist in adding to) the 

 soil. 



I have mentioned the leading forms of these aquatic plants to 

 bring out a curious point. There are nearly three times as many 

 genera of Dicotyledons as compared with Monocotyledon genera 

 in the Flora, and yet more than half of these aquatic genera are 

 Monocotyledons, showing, as comes out also from geological data, 

 that the Dicotyledons are, on the whole, driving out the lowlier 

 forms, and forcing them to return (like whales) to a water life. 



