ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 915 



the growth in length of positively or negatively geotropic organs, 

 . when in a vertical position of equilibrium. Two modes of investiga- 

 tion agreed in answering this question in the negative. Finally, the 

 author instituted a series of experiments on the effect in producing 

 curvature of gravitation and of centrifugal force, the conclusion 

 arrived at being that the curvature is increased by an increase of the 

 external force. 



Influence of Physical Conditions on the Forms of Water- 

 plants.* — Dr. W. Behrens illustrates by several examples the rule 

 that water-plants become modified in the forms of their leaves and 

 other parts according to the increase or diminution in the movement 

 of the water which they inhabit. He first points out that the con- 

 stant kidney-like outline of the leaves of the Frog's Bit, Hydrocharis 

 morsus-ranoe, is correlated with its invariable association with still 

 water ; the same is the case with its South American representative, 

 Trianea bogotensis. 



Potamogeton nutans is' a familiar instance of the converse, the 

 breadth of its leaf being as 1 : 1^ of the length in still water, while 

 in ditches in which a good current runs the leaf is narrow and lancet- 

 shaped ; in another species the breadth is as 1 : 3 of the length. 



Plants of the sub-genus BatracMum (Ranunculus) afford especially 

 good examples of the above-named principle, as many of the species 

 frequent all kinds of water, and most of them possess two kinds of 

 leaves, the broad floating and the deeply cut immersed ones. Of the 

 five species inhabiting Central Europe, B. aquatile {^Ranunculus aqua 

 tilis) is the most variable in form. It has two chief varieties ; the 

 one, B. aquatile var. heterophyllum, has two kinds of leaves, viz. flat 

 floating and immersed linear. It occurs chiefly in slow-flowing 

 ditches and streams, and in sluggish branches of small rivers. Where 

 the motion of the water is very slow, the floating leaves are peltate 

 and almost circular, the edge marked with but five slight incisions, 

 forming the sub-variety peltatum ; in more rapid streams these cha- 

 racters are modified, and the leaves are more deeply cut, more 

 strongly lobed and jagged. In somewhat faster streams the float- 

 ing leaves become quinquelobate (sub-variety quinquelohatum). In 

 tolerably quick-flowing ditches, the basal lobes of these leaves 

 are lost, and the three remaining are wedge-shaped, but are them- 

 selves deeply notched ; the leaves are smaller than in the pre- 

 ceding form (sub-variety tripartitum). Another form, found in 

 similar localities, shows a further advance in the same direction ; 

 the leaves, which are trilobate and deeply notched when young, 

 have the points which separate the notches much prolonged as they 

 grow older, and approximating closely to the hair-like tips of the im- 

 mersed leaves, though they still preserve a web of green parenchyma, 

 uniting the bases of the lobes (sub-variety laciniatum). In very 

 rapid water this series of modifications is concluded by the produc- 

 tion of the B. aquatile var. trichophyllum, with but one kind of leaves, 

 which are immersed and hair-like. A similar result is produced by 



* Jahresber. Naturw. Ges. Elberfeld, 1880. Kosmos, vii. (1880) pp. 466-71. 



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