DISPERSAL BY WATER. 807 



The Bladderworts (JJtricularia), Aldrovandias (Aldrovandia vesiculosa, see 

 vol. i. p. 151), and the Water Violet (Hottonia palustris), which desert the cold 

 upper strata of water as winter sets in, and sink down to the relatively warm 

 depths below, develop special wandering biads for this purpose; these are not 

 enveloped in scale-leaves like those of the Frogbit; they are in reality merely much 

 abbreviated shoots whose leaves are so crowded and folded so closely together, that 

 the whole shoot looks like a rounded green ball. These balls at first remain con- 

 nected to the piece of the floating stem which gave them origin. This attachment 

 is lost towards the end of autumn, and the little buds sink down to the bottom 

 of the pond and necessarily get distributed in various directions. Next summer, 

 when the balls leave their winter quarters and are again carried to the upper strata 

 of water, they expand into foliage-bearing plants. It has been already stated 

 (vol. i. pp. 76 and 658) that the Water Soldier (Stratiotes aloides), which is closely- 

 related to the Frogbit, undergoes similar changes during the year, and we need 

 here only draw attention to the fact that it sinks down to its winter quarters 

 at the bottom of the pond as an open rosette, and not in the form of buds, and rises 

 again the ensuing spring when the weather is more favourable. 



The Pond weeds Potamogeton crispus, obtusifolius, pusillus, and trichoides 

 behave differently from the marsh and water plants hitherto described. Here, 

 as autumn approaches, buds are developed which become detached from the old 

 decaying stems (fig. 136, vol i. p. 551), and sink down to the bottom of the pond; 

 but in the following summer they remain sticking in the mud at the spot where 

 they fell, and do not rise again to the surface. They send out roots and develop 

 much-branched leafy stalks, and these rapidly grow up to the surface of the watei'. 

 These Pondweeds, firmly rooted to the bottom of the pond, multiply not only 

 by these free-swimming offshoots, but also by stolons which creep far and wide 

 through the mud; but of course the plants are distributed to much greater distances 

 by the sprouts or buds which are developed in the autumn on the upper intemodes, 

 and which then become detached and float in the water, than would be possible by 

 the creeping stolons alone. 



A very remarkable distribution of offshoots is to be observed in the marine 

 Cymodocea Antarctica, which is very common on the coast of Australia, south of 

 the Tropics. This plant has an erect stem, thickly covered with dull-green foliage- 

 leaves, arranged in two rows. The lower leaves fall off prematurely, and the bare 

 scarred stem then cames only a bunch of ribbon-shaped leaves at its summit. 

 Towards the clcse of the winter the end of the stem above these leaves is seen to 

 become peculiarly modified. Its intei-nodes become much contracted, and at the 

 lowest node is developed a scale-leaf with four lobes, which surrounds the leaves 

 developed from the upper nodes, like a cup. Buds arise in the axils of one or two 

 of these leaves, while the leaves themselves die and decay. The parenchyma of the 

 four-lobed, cup-shaped scale-leaf also decays, and only its stiff' veins remain, so that 

 instead of the cup, there are now only comb-like scales. After this alteration 

 has taken place, the tissue of the stem below the pectinate scales breaks across. 



