26 



Göte Turesson 



5. The biological significance of modificatory and hereditary 



forms of prostrate plants. 



It has been repeatedly emphasised in this paper -that the prostrate mode of 

 growth is particularly common on seashores. This fact does not surprise us any 

 longer. Tiie statement that »the light hurts the eyes» cited by Waeming as to the 

 light conditions on the Danish dunes applies to most of our sandy shores that are 

 exposed to the bright sun. Much light is reflected from the white sand. Warming 

 (i. c.) obtained by means of a Steenstrup photometer in one instance for the direct 

 sunlight tiie value 44, and for the reflected light the value 38. The great amount 

 of light in such localities is also mentioned and discussed by Erikson (5), Häyrén 

 (7) and others. Similar light conditions were found by Cowles (3) to prevail in 

 the sand dune region of Lake Michigan. But great intensity of light is a predo- 

 minant factor in other localities as well, and photoclinic plants are by no means 

 rare in these localities. This is especially true of rosette-plants, which are parti- 

 cularly characteristic of sunny meadows and of rocky expanses covered by low 

 vegetation. It is a rule rather than an exception to find the leaves of these plants 

 horizontally expanded and firml}^ pressed against the hot, dry ground [Taraxacum, 

 Leontodon, Hypocliceris, Crepis, Plantago etc.). When placed in less intense light 

 the leaves of these plants immediately turn upwards (see plates). 



It is of further interest to find that many rosette-plants with hibernating leaves 

 undoubtedly show psychrocliny. Lidfokss (12) calls attention to the fact that the 

 leaves of these plants [Bellis perennis, Geranium, Oenothera, Saxifraga etc.) lie prone 

 on the ground and are pressed against it during winter. Psychrocliny is undoubtedly 

 the main factor in these downward movements, although thermonastic curvatures 

 may play some part. — Psychrocliny will probably be found to be of very frequent 

 occurrence in regions where wintergreen plants are common. Illustrative examples 

 of psychrocliny are met with in the Puget Sound region on the west coast of North 

 America, where wintergreen plants are abundant. The classical example of Mimulus 

 luteus is found almost everywhere in that region. Another instance of very pro- 

 nounced psychrocliny is shown by Claytonia sibirica, which during fall and winter 

 lies prostrate and radiates in all directions, while the leaves and flower-stalks stand 

 almost vertical during spring and summer. A number of similar examples of psy- 

 chrocliny could be mentioned from this region. 



Prostrate plants are further characteristic of alpine vegetation. Psychrocliny 

 is probably one of the causes of the creeping habit in alpine plants. It is known, 

 however, that the intensity of direct sunlight increases with altitude and is very 

 great on high mountain-tops. Photoclinic movements are, therefore, likely to be 

 found to be of some importance in the plagiotropic orientation of the above-ground 

 parts of alpine plants. The same opinion is expressed by Vöchting in "the following 

 remark (29, pag. 50): »Mit gutem Grunde darf man annehmen, dase das Kriechen 

 mancher Alpenpflanzen tbeilweise oder gänzlich auf dem Einflüsse niedriger Tem- 



