The cause of plagiotropy in maritime slioie planta 



27 



peratur beruht. — Vielleicht wirken intensive Beleuchtung und niedrige Temperatur 

 in demselben Sinne». 



The biological significance of the photoclinic as well as of the psychroclinic 

 movements is evident. There are few locahties within the temperate region which 

 offer such adverse conditions of life as does the dry, burning sand in the coast 

 and dune regions exposed to sun and wind. It is of vital importance for the plants 

 of such localities — more particularly is this true of the shallow-rooted annuals — 

 to reduce transpiration to a minimum. The prostrate forms are therefore better 

 prepared than other plants to withstand the extreme conditions of life, and the same 

 is true of the prostrate alpine plants which are exposed to the same danger, viz. 

 desiccation. 



It has been sliown in the above that plagiotropy is a feature characteristic of 

 a nunibei' of plants on the beach and elsewhere, and that this character of plagio- 

 tropy is conditioned by photocliny. Now, it is of very great interest to find that 

 this type of vegetation is made up of two genetically different elements, viz. modi- 

 ficatory prostrate forms, and hereditary prostrate variations. Both forms are some- 

 times found within the same systematic species. A few examples suffice to illustrate 

 this condition. Atriplex latifolium, which is erect in ordinary light but plagiotropic 

 in intense light, possesses a hereditary prostrate form in which plagiotropy is a 

 fixed congenital feature. Tliis form may be provisionally called A. latifolium x^ro- 

 stratum. Atriplex patulum prostratiim is another hereditary form which is constantly 

 plagiotropic, thus differing from A. patulum which becomes plagiotropic only in 

 intense light. Chenopoditim album possesses likewise a hereditary prostrate form 

 which is commonly found on the west coast of Sweden. When they grow together 

 on the exposed beach some diflficulty is experienced in separating the different 

 types, and the cultivating of the forms becomes necessary. It was found in raising 

 a large number of selffertilized A. lat. prostratum and A. pat. prostratxim that these 

 two prostrate forms breed true to plagiotropy. 



The hereditary prostrate variations differ physiologically from the prostrate 

 modifications in being more sensitive to light; they respond to conditions of illu- 

 mination which leave the latter unaffected and in a vertical position. The prostrate 

 modifications do not begin to grow downwards until the light reaches a high degree 

 of intensity. (Prolonged exposure to light of less intensity has the same effect in 

 releasing the downward movement, but the position taken up by the branch cor- 

 responds closely to the degree of light intensity, as stated above.) We are thus able 

 to distinguish between different degrees of photocliny according to the intensity of 

 light necessary for the releasing of the downward movements. We find it most 

 pronounced in the hereditary prostrate forms, less marked in the modificatory forms. 

 The different degrees of photocliny found in some forms of the genus Atriplex is 

 illustrated in table XV. The figures at 0 hours indicate the magnitude of the angle 

 formed by the apices of the branches with the perpendicular. The light intensity 

 was 10—12. 



