xxii] PETIOLE LENGTH 283 



species, when grown in still or moving water, are possibly due, 

 in some cases, to the better aeration of water which is in motion. 

 Such differences are markedly exhibited by Myriophyllum^^ 

 which in still, small pools may have leaves whose segments are 

 very tender and almost hair-like, while in strongly flowing 

 water they are shorter and firmer. 



One of the most interesting problems connected with the 

 tropisms of water plants, is the question of the influences which 

 regulate the length of the petiole in the case of floating leaves. 

 It is a matter of common observation that, in plants such as 

 the Waterlilies, the length of the petiole varies with the depth 

 of the water. The accommodation begins at the youngest stages, 

 for, if the seeds of Castalia alba^ are planted at different levels 

 in the mud, the length of the first internode, the acicular first 

 leaf, and the petiole of the second leaf, adapt themselves most 

 remarkably to their circumstances, elongating until they are 

 long enough to raise the leaves well into the water (Fig. 13, 

 p. 28). 



In free-floating plants, such as the Frogbit (Hydrocharis 

 Morsus-ranae)^ this power of accommodation to depth is also in 

 evidence, though it is naturally less conspicuous. The Frogbit 

 has gained notoriety in the present connexion, since it was the 

 subject of an oft-quoted series of experiments by Frank ^. Its 

 petioles are normally 6 to 8 cms. long, but when grown in 

 shallow water they may not exceed i cm. If the plant is 

 attached to the bottom of a deep glass vessel, on the other hand, 

 very long petioles may be produced, a length of nearly 14 cms. 

 being recorded in one case. Frank obtained a sensational con- 

 trast in petiole length, by growing a plant in a deep jar until its 

 youngest leaf had succeeded in reaching the surface by elon- 

 gating its petiole to 1 1 cms. It was then transferred to a shallow 

 vessel in which the terminal bud was only just covered. The 

 next leaf produced a petiole 1-5 cms. long, i.e. less than 14 per 

 cent, of the length of the preceding leaf-stalk. 



1 Schenck, H. (1885). 2 Massart, J. (19 10). 



3 Frank, A. B. (1872). 



