TO THEIR ENVIRONMENT. 65 



the submerged again being finely divided and the floating ones 

 entire ; while on mud the plant produces only entire leaves. 



III. — When we make a comprehensive survey of the various 

 genera and species of aquatic plants, we find that the greater 

 number of them have only their lower parts immersed in water 

 (Nymp/uea, Nuphar, various UmbellifercB, Alisma, Sparganium, 

 Typha, etc.), while comparatively few are totally submerged 

 {Ruppa, various Potamogetons, Myriophyllum, Anacharis), and 

 fewer still float freely without attachment. The first cannot live if 

 wholly immersed for any length of time, and the two last if they 

 are removed from the watery medium which surrounds them and 

 exposed to the air. 



The unattached plants usually grow in quiet pools and ditches, 

 and are therefore not subject to any great displacement. Some of 

 them {Lemna polyrhiza, L. gibba, L. minor) possess roots, and 

 others {Le?nna trisiilca, Cerafophyllum, Utricularia, Hottonia) none 

 at all. Those provided with roots, when left on moist earth, can 

 still exist, being capable of absorbing food from the ground by 

 these appendages in the same manner as a land-plant, but, under 

 the same adverse conditions, the less perfectly equipped rootless 

 plants perish. 



Some of the partially immersed plants (reeds, rushes, water- 

 lilies) may have their submerged parts bared by the retreating water 

 without injury, as they will continue to thrive just as well on the 

 wet earth, their leaves being adapted to live an aerial life. In 

 others {Potamogeton natans, P. heterophyllus, Ranunculus aquatilis) 

 which have both submerged and floating leaves, when thus stranded, 

 the submerged leaves alone decay, as their structure renders them 

 incapable of absorbing carbon-dioxide from the atmosphere itself, 

 but the floating leaves continue to thrive, and any new leaves 

 produced conform to the shape of the hitherto floating ones. 



The cause of these phenomena is to be looked for in the 

 structural differences of the parts. The cellulose, the substance 

 of which the cell-wall largely consists, becomes in the aerial por- 

 tions of plants corky, very elastic, and nearly impermeable to 

 water, or cuticularised, as it is termed. This modified cellulose is 

 highly important to the plant, as it prevents excessive loss of water 

 by evaporation through the cell-wall. In hydrophytes, the epider- 



International Journal of Microscopy and Natural Science. 

 Third Series. Vol. VI. f 



