The cause of plagiotropy in maritime shore plants ^ 



There are few localities that show such a great variety in structure and the 

 morphological features of their vegetation as does the beach. A number of modi- 

 fications of the vegetative organs of littoral plants, often represented as «adaptations», 

 have been recorded, including wax, hair-coating, succulence etc. Many of these 

 modifications represent definite reactions to definite habitat conditions, and are there- 

 fore better termed reaction structures. Other peculiarities of structure in littoral 

 plants are doubtless hereditary and fixed once for all. The distinction between 

 these two kinds of variations, sometimes called, respectively, facultative and obligate 

 variations, is most important. 



Among morphological features cliaracteristic of maritime shore vegetation the 

 prostrate or trailing habit is one to which attention has repeatedly been called by 

 writers on ecology. Systematists have likewise recorded the fact, and prostrate 

 shore varieties of a number of plants belonging to Atriplex, Jasione, Polygonum, 

 Trifolium and other genera are found in floristic handbooks. The plagiolropic ori- 

 entation of stems and branches in littoral plants which is almost everywhere notice- 

 able is perhaps most pronounced on tropical strands. Pes-caprœ, Euphorbia-asso- 

 ciations and similar kinds of shore vegetation formed by plants creeping above the 

 surface of the soil are widely distributed on tropical sea shores. Although not re- 

 stricted to the shore only, prostrate plants must be considered typical of such lo- 

 calities. This character of plagiotropy is sometimes impressed on almost all the 

 separate constituents of the vegetation, giving the appearance of a distinct growth- 

 form. This is true not only of plants possessing well-developed stems and branches 

 but also of csespitose forms and rosette-plants which on the exposed beach have 

 their basal leaves horizontally expanded and firmly pressed to the ground. 



It will be seen from the following that various attempts have been made 

 to explain the prostrate and creeping habit of shore plants, althougli few, if any, 

 serious experiments have been performed to clear up the matter. The ecological 

 interpretation of the phenomenon will be given in the following chapters, and conside- 

 ration will be paid to that important point in all questions of variation, viz. the 

 necessity of distinguishing between hereditary characters and habitat responses. It 



* Some of the results of this study were published in Botaniska Notiser. 1917 (see Tu- 

 EESSON 26). 



