CHAP. XL HYDROCHARID-FORMATION OR PLEUSTON 165 



Hydropterideae, with Azolla and Salvinia, both of which are 

 swimming plants. 



Spermophyta, which can be ranged into the subjoined groups : 



Submerged : Ceratophyllum, Utricularia, Aldrovanda, Lemna trisulca, 

 Stratiotes aloides. 



Floating by leaves or shoots : Hydrocharis, Hydromystria stolonifera 

 (Trianea bogotensis), Lemna minor, L. polyrhiza, L. gibba, Wolffia 

 arrhiza, Pistia, and Eichhornia crassipes. 



Transitional to the rooted limnaea-types : Hottonia palustris, Jussieuea 

 repens, and others. 



Many spermophytes, such as Lemna, Pistia, and Pontederia crassipes, 

 can choke the water with their enormous numbers. 



ADAPTATIONS 



The submerged species, as in the case of plankton-organisms, must 

 be approximately of the same specific gravity as water ; normally floating 

 species are kept at the surface of water by air-containing cavities in 

 their leaves and stems. This finds outward expression in thickness of 

 the shoots and great convexity of the lower face of the floating organ 

 of Lemna gibba and Hydromystria. Special floating-devices are shown 

 by Eichhornia crassipes, Neptunia and Jussieuea repens. 1 



As in some cases the assimilatory organs projecting into the air are 

 necessarily adapted to transpire, this formation shows a certain transition 

 to land-plants, just as, on the other hand, through its submerged species it 

 is allied to plankton. 



The morphology of the shoot varies. In the majority of submerged 

 Spermophyta the shoots have very long internodes and very thin stems ; 

 the usually sessile or shortly stalked leaves are often divided into filiform 

 segments, as in Utricularia, Ceratophyllum, and Hottonia. But in floating 

 species the shoots mostly have short internodes and are condensed ; while 

 the leaf-blades frequently assume the shape of typical floating-leaves, 

 being very broad, peltate-cordate or ovate-cordate, as in Riccia natans, 

 Salvinia, Hydrocharis, Hydromystria, Lemna polyrhiza and other species, 

 and Azolla ; somewhat differently shaped are the shoots and leaves of Pistia. 

 One of the offices of the floating leaf is to ensure equilibrium to the 

 plant in water ; accordingly, floating leaves or analogous balancing- 

 organs are developed early in the seedlings of Salvinia, Lemna, and some 

 others. 1 



That this broad distinction between submerged and floating leaves 

 represents a true adaptation to environment is clearly shown by Salvinia, 

 and by aquatic plants such as Ranunculus (Batrachium), Trapa, and 

 Cabomba, that are fixed by roots ; for all these have both submerged and 

 floating leaves which differ in form. 



In free-floating submerged plants nutriment is absorbed over the whole 

 surface, and in Vascular plants the root is accordingly either absent, as in 

 Aldrovanda, Wolffia, Lemna trisulca, Ceratophyllum, and Utricularia vulgaris, 

 or very reduced ; the most important part played by the root in such plants 

 as Lemna and Hydrocharis is indubitably to secure the plant in a definite 



1 Gobel, 1891. 



