282 PHYSICAL FACTORS [ch. 



branches of Potamogeton pectinatus^ when swelling up to form 

 tubers, become positively geotropic. They bend towards the 

 soil and bury themselves in it to pass the winter. This has an 

 important result, because, being lighter than water, these winter- 

 buds would otherwise be liable to rise to the surface when set 

 free by the decomposition of the parent plant^. Again, there are 

 many cases of fruiting peduncles bending downwards and thus 

 allowing the ovary to ripen under water; a similar curvature 

 occurs not infrequently in terrestrial plants. Positive geotro- 

 pism of the fruit stalk is characteristic of the Pontederiaceae^ 

 (Fig. 155, p. 240). Limnobium Boscii is a similar case; here it 

 has been shown that the geotropic curvature is independent of 

 fertilization^. 



Hochreutiner^, who has paid special attention to the response 

 of water plants to certain physical stimuli, has made some obser- 

 vations on 'rheotropism^,' or reaction to current. He noticed 

 that, in the case of Zannichellia palustris^ where the water was 

 still, the stem-branches rose erect, as would be expected of a 

 negatively geotropic organ, but that, where there was a current, 

 the axes adopted its direction. Hochreutiner observed this in 

 the case of a current of such slight force that he was convinced 

 that no mechanical compulsion was exerted, but that the stems 

 responded to the stimulus by their own activity and might 

 thus be called positively rheotropic. Roots, on the other hand, 

 seem to show a tendency to grow against the current. It is 

 suggested that this sensibility would be useful to the plant, 

 since it would lead to the roots and stems taking up a position 

 in which they would be unlikely to be damaged by the pulling 

 force of the current. Further experimental work on rheotropism 

 is obviously needed, however, before the subject lends itself 

 to generalisation. The question is complicated by the fact that 

 a rapid current alters the conditions of life of the plant very 

 materially. Differences between the morphology of the same 



1 Hochreutiner, G. (1896). 2 Muller, F. (1883). 



3 Montesantos, N. (1913). ^ Hochreutiner, G. (1896). 



^ This term was suggested by Jonsson, B. (1883). 



