The cause of plagiotropy in maritime shore plants 



29 



to its very prominent photoclinic character the prostrate race escapes the fatahty 

 which would inevitably meet its erect, less photoclinic congener, viz. that of being 

 cut down and deprived of fructification. 



It appears from what has been said above that in regard to the character of 

 plagiotropy a parallelism exists in the genus Alriplex between the prostrate modi- 

 fications (induced by environmental influences) and the hereditary prostrate varia- 

 tions. A similar parallelism between »climatic modifications» and »climatic variations» 

 has been discussed by Nilsson-Ehle (18). Although it is much to be desired, no 

 crosses have been procured between the two parallel forms here discussed, owing 

 to technical difficulties. The exact nature of the genetical differences between the 

 two forms is therefore largely a matter of speculation. However, the »height» of 

 Atriplex patulum (for instance) is probably a compound character and depends on 

 the existence of one -or several factors which suppress the stimulus that would 

 otherwise compel the plant to show photocliny in ordinary light. An example of 

 such a structural compound cliaracter is furnished by the crosses of Sweet Pea 

 varieties mentioned by Bateson (2). The prostrate race of Atriplex patulum would 

 then have come into existence by the dropping out of these height-determining 

 factors. 



The fact that erect and prostrate Atriplex-îorms belonging to the same » syste- 

 matic Speeles» are often found growing side by side on the beach, — a circum- 

 stance noticed and discussed by Warming (30, 32), and believed by him to prove 

 that the decisive cause of plagiotropy was no general factor prevailing at all seasons 

 in a definite place — is explained by the existence of physiologically and geneti- 

 cally widely different races within the »species» of this genus. 



It has already been pointed out that the importance of the wind has been 

 over-estimated in previous attempts made to explain the creeping and dwarf habit 

 of maritime and alpine plants. The literature on this and related topics has been 

 collected and discussed by Frödin (6). However, upon closer inquiry the factors of 

 photocliny and psychrocliny conditioned by light and temperature variations respecti- 

 vely will undoubtedly be found to account for a number of phenomena held by 

 some workers to be wholly due to the action of the wind. The existence of here- 

 ditary prostrate races even among woody plants (as in the case of Juniperus com- 

 munis, for instance) is, further, a fact of great importance which should accordingly 

 receive due attention. 



