xvii] LUXURIANCE AND PERENNIATION 215 



branches of Utricularia vu/garis may be six feet long 1 . The shoot 

 system, as a whole, sometimes attains a remarkable development. 

 The present writer examined, for instance, a plant of Polygonum 

 amphibium growing at Roslyn Pits, Ely, on June 30, 1913, 

 which showed at the surface of the water only one flowering 

 branch with seven foliage leaves. The plant was pulled up with 

 a boat-hook and inevitably somewhat mutilated in the process, 

 but, notwithstanding the breakages, the various axes forming the 

 shoot system were found to measure altogether approximately 

 forty-two feet. Besides the two visible leafy shoots, eight of the 

 branches terminated in leaf buds, which looked as though they 

 would probably have reached the surface in the course of that 

 season. The longest internode in the horizontal part of the stem 

 measured as much as sixteen inches. 



The great development often reached by individual water 

 plants is no doubt an expression of the same tendency as that 

 which leads them so generally to perenniation. Annuals are 

 quite rare among hydrophytes ; only a few examples are known, 

 such as Naias minor, Naias flexilis and certain species of 

 Elatine*. There is of course no dry season to be spanned, and 

 many aquatics can continue their vegetation all the year round, 

 in some cases paying little regard to the passage from summer 

 to winter. Zannichellia palustris, for instance, may be found in 

 flower in November, while Aponogeton distachyus^ cultivated out- 

 of-doors in England, flowers sometimes in December and Janu- 

 ary. The strength of the tendency to perenniation may be illus- 

 trated by the fact that the following plants have at different 

 times passed successfully through one or more winters in so 

 unsympathetic a location as a rain-water tub in the present 

 writer's garden Hydrocharis Morsus-ranae y Stratiotes aloides^ 

 Spirodela polyrrhiza, Lemna trisulca, Myriophyllum sp., Qenan- 

 the Phellandrium v&r.fluviatiliS) Ceratophyllum y Hippuris,3.nd two 

 species of Potamogeton. That the perennial habit is directly 

 related to the environment, seems to be indicated by the fact 

 that, in the case of Callitriche 2 '^ the land forms are annual while 

 1 Burrell, W. H. and Clarke, W. G. (1911). 2 Schenck, H. (1885). 



