The cause of plagioh-opy in maritime shore planta 



25 



movements characteristic of the peduncles of Anemone nemorosa and A. sfellata were 

 referred to the same physiological category by Vöchting, as were also the movements 

 found by Wille (33) in some wintergreeu plants, which at a low temperature have 

 their leaves firmly pressed against the soil (for instance, Geum urbaimm). In dealing 

 with the phenomenon of psychrocliny Lidforss calls attention to the (physiologically) 

 very heterogeneous group of movements so named. He points out that the move- 

 ments which cause the species of the genus Anemone to becoine upright at a high 

 temperature and to drop at a low temperature are effected by thermouastic curva- 

 tures. Furthermore, the movements characteristic of the leaves of Geimi and other 

 wintergreen plants are no stimulus reactions but wholly physical processes. Lidforss 

 is therefore of the opinion that — even though these different phenomena may 

 have the same biological significance — the term psychrocliny should be reserved 

 for such movements as are released by gravity in response to variations in tempe- 

 rature. This is the case with the movements found by Lidforss in the above men- 

 tioned spring plants and by Vöchting in Mimulus. The term is used in this narrow 

 sense by Lidforss in a later publication dealing with psychrocliny (Lidforss 13). 

 The term photocliny is coined per analogiam by Lidforss (12), and is used to de- 

 note such movements as are released by gravity in response to variatit)ns in light, 

 or just the kind of movements described in the above as characteristic of shore 

 plants. 



And now something must be said as to previously recorded instances of plants 

 in which the nature of the geotropic response is affected by light, or governed by 

 photocliny, as we prefer to say in the following. Such a case of photocliny is pre- 

 sented by Stahl (36) with reference to the rhizomes of Adoxa and Circœa, which 

 normall}^ run horizontally in the soil. When illuminated, however, the rhizomes 

 bury themselves more or less vertically in the soil. This change in the direction 

 of growth is not brought about by negative heliotropism but by positive geotropism 

 induced by the light. The alteration in »disposition» of the plant is of course of 

 greatest service to the plant since it keeps the rhizome from growing out of the 

 soil. That lateral roots behave in a similar way as the rhizomes of Adoxa and 

 Circœa is illustrated by Jost (8, pag. 451). A similar change in the »disposition» 

 of the plants has been found by Czapek (4) and Oltmanns (19) in Lysimachia num- 

 mularia, Glechoma hederacea, and in the runners of Fragaria vesca and Rubus cœsius 

 which are plagiogeotropic in light but negative geotropic in darkness. Oltmanns 

 remarks concerning the behaviour of Lysimachia nnmmularia that »Jeder Lichtinten- 

 sität eine bestimmte Lage des Sprosses entspricht». We have found that the same 

 rule applies to the shore plants discussed above. 



4 



