The cause of plagiotfopy in maiititne shore jilants 11 



Warming points out that »neither the wind nor its direction can be responsible, 

 since individuals of one species growing on the same shore may have tlieir main 

 shoots pointing in ditïerent directions». He is of the opinion that temperature 

 plays a part in the plagiotropic orientation of the stems and branches and suggests 

 that the different degree of lieating of the plants during their development above 

 ground is the cause of the prostrate growth. Thermoti'opic movements would be 

 responsible for the prostrate growth in these cases. It is also suggested by War- 

 ming that hydrotropic movements may be of some impottance in some cases. Men- 

 tion is also made of another class of movements by Vöchting (29) and Lidforss 

 (12) called psychroclinic, which movements are released by changes in temperature. 

 Of these movements we shall have more to say later. 



A similar view — that the plagiotropy in these plants is due to thermotropism 

 — is held by Abromeit (1). Some interesting observations are made by Warming 

 (32, pag. 27) as to the occurrence of erect and prostrate individuals, growing side 

 by side, e. g. of Atriplex, Salicornia, and Sueda and denoting, in the opinion of 

 Warming, that the decisive cause of the prostrate growth is no general factor pre- 

 vailing at all seasons in a definite place. The following remarks are of great in- 

 terest (30, pag. 94): »Specimens of Taraxacum growing on the beach at Tisvilde 

 had all or in any event the outer leaves of the ground-rosette and the scapes ho- 

 rizontally expanded, while all these parts were greet on specimens growing in the grass 

 close by». On page 172 the following observation is made with regard to PotentiUa 

 anserina: »When it is growing on open ground the leaves are generali}'' more or 

 less horizontally expanded and hairy while in moist soil and particularly between 

 other taller plants the leaves rise upwards and become larger and fresh green 

 in colour ». 



Vöchting (28) gives an account of the behaviour of Taraxacum and Erodium 

 cicutarium that resembles the observation made by Warming. He says (pag. 177): 

 »An grell von der Sonne beschienenen Orten, besonders auf magerem, steinigem 

 Boden, legt die Pflanze ihre Blattrosette dem letzteren dicht an. Wo sie dagegen 

 im Gedränge mit höher aufstrebenden Mitbewerbern zu kämpfen hat, vor Allem 

 an schattigen Orten, richtet sie ihre, dann meist auch längeren Blätter lîiehr oder 

 weniger empor, manchmal so weit, dass sie der Verticalen nahe kommen.» While 

 Warming and others are of the opinion that temperature is the cause of the diffe- 

 rences in growth, the belief is expressed by Vöchting that the negative heliotropism 

 induced when these plants are exposed to intensive light is responsible for the 

 downward movements of the leaves and stems. 



That Corydalis, Lamium jpurpurewn and Veronica hederifolia are prostrate on 

 open ground, but erect in the shade, is an observation made by Lidporss (13). 

 Another field observation published by Lindman (14) becomes of interest in this 

 connection. In a paper dealing with plants suppressed by other plants an account 

 is given of the behaviour of Hypochœris maculata in some places in Great Britain. 

 On dry hillsides and in close, short-cut pastures the radical leaves of this species 



