xxn] TROPISMS AND SLEEP MOVEMENTS 281 



On the subject of heliotropism, we do not appear, in the case 

 of water plants, to possess much experimental evidence. The 

 work of one observer seems to suggest that the heliotropism 

 of stems is less intense in the case of submerged than of terres- 

 trial plants 1 . Positive heliotropism has been recorded for the 

 leaves of Aponogeton distachyus and A. fenestralis 2 ", the floating 

 leaves of Trapa natans*, on the other hand, are described as 

 transversely heliotropic and as owing their horizontal position 

 on the surface of the water to their response to light. It was 

 shown, in certain experiments, that, after a week in darkness, 

 the new leaves, which had unfolded, stood upright out of the 

 water. In this connexion it has been recalled that, among the 

 near relations of Trapa, there are land plants with transversely 

 heliotropic leaves. 



The leaves of the water form of Myriophyllu m proserpinacoides 

 exhibit * sleep ' movements when living submerged. The young 

 leaves, which, normally, are spreading, rise up at night and 

 cover the growing point, thus returning more or less to the 

 position they occupied in the bud. Sleep movements also occur 

 in Limnophila heterophylla*. The leaves of Myriophyllum and 

 Ceratophyllum excluding those of the apical bud are said 

 to have the peculiarity of bending downwards on darkening 5 . 



As regards geotropism, aquatic plants seem to be generally 

 comparable with land plants. In Aponogeton^ for instance, it has 

 been observed that the leaves are negatively, and the adventi- 

 tious roots positively, geotropic 2 . The present writer has, how- 

 ever, noticed in the case of the seedlings otNymphaea lutea, that 

 the short-lived primary root, after the earliest stages are past, 

 shows little response to gravity, sometimes pointing vertically 

 upwards. But this is probably merely a sign of its early de- 

 generation and decay. There are also instances of the stems of 

 water plants, in certain specialised cases, responding to gravity 

 in the reverse of the usual way. For instance, the lateral 



1 Hochreutiner, G. (1896). 2 Sergueeff, M. (1907). 



3 Frank, A. B. (1872). 4 Goebel, K. (1908). 



5 Mobius, M. (1895). 



