Habits and Habitats of Plants. 153 



3. Plants which grow submerged in stagnant or gently running 

 water have commonly long slender stems and leaves of mem- 

 branous texture, either simple and narrow, as in the pondweeds 

 {PotanuH/eton) and other monocotyledons, or torn into capillary 

 segments as is more frequent among dicotyledons, — e. ij., the 

 Ranunculi of the aquatilis section, MtjriophyUwn, Utriciilaria, 

 Hottonia, and Ceratophyllum. Chara among cryptogamous plants 

 has a similar habit. Submerged leaves are supported by the 

 water in which they float, their buoyancy being often increased 

 by air-cavities in their substance ; hence they do not need the 

 stiffness and strength of leaves which have to support their own 

 weight in air; and the narrow or finely divided form, _ while 

 exposing a large surface to the water for purposes of respiration 

 and nourishment, offers less resistance to currents, and so is less 

 liable to injury than a broad flat blade would be. The stems of 

 these plants throw out roots freely at the nodes, and thus pieces 

 broken off accidentally readily take root and grow in new 

 places to which they may be borne. Utricularia and Hottonia, 

 belonging to different orders, agree in habit in a further respect, 

 viz., that the flowering stem, which is much thicker and stiffer 

 than the submerged stems, rises erect out of the water, like a 

 mast on a raft, from the centre of a whorl of radiating horizontal 

 branches. Both of these plants have conspicuous flowers, ad- 

 apted to be fertilised by insects, and it is necessary, therefore, 

 that they should be borne aloft in the air, an object which this 

 arrangement secures. 



4. Plants, on the other hand, which float on the surface of 

 quiet waters have broad orbicular or oblong leaves, borne on 

 long flexible stalks, and sustained like a raft by the surface- 

 tension of the water. The flowers are commonly borne up above 

 the water. Familiar examples are found in the yellow and 

 white water-lilies, which belong to the order Nymphaeacese ; 

 others are Umnanthcmmn nijmphaoides (Gentianacese), and the 

 frog-bit (Hydrocharidacese). 



Other water plants, again, have leaves of two kinds, the sub- 

 merged ones being narrow and finely divided, and the floating 

 ones broad and orbicular, or oblong. Instances are Ranunculus 

 peltatus, and some of the Potamogetons, especially P. heterophyllus. 

 In the arrow-head {Satiittaria) the first submerged leaves are 

 narrow and strap-shaped (like those of Sparganium, Butomus, 

 (Hi/ceria, and other monocotyledons); later leaves are oblong 

 and float on the surface of the water ; and the fully developed 

 ones are acutely arrow-shaped and stand out of the water : a 

 complete gradation being found between all of these forms. 



Certain plants which float on the surface of stagnant water 

 have their vegetative organs reduced to a mere rounded or 

 branched frond with roots. Examples are the duckweeds ; some 



