82 



The Plant World. 



dune plants develop strikingly extensive root systems, this being 

 particularly true of Abronia. The roots of such plants must serve 

 not only as absorptive organs, but they must effectively anchor 

 their possessors to an ever-shifting substratum. On examination 

 it will be found that many of the dunes consist largely of a plexus 

 of roots that by continued growth and extensive ramification 

 have become effective sand-binding agents. 



In passing over the sand-dune zone one is impressed with 

 the fact that it comprises dunes of differing ages, some being of 

 comparatively recent formation, while others are evidently 

 many years old. On examination the observer will note that the 

 more recent dunes are peopled with but few species, and these 

 consist of such plants as possess ample root systems of great 

 vigor and hardiness. These pioneers prepare the way for new 

 migrants, plants that are quite unable to take possession of the 

 jiand without the assistance of shelter and such forms as Abronia, 

 Franseria, CEnothera cheiranthifolia, and other typical sand-dune 

 species. 



Not uncommonly one may find the older dunes supporting 

 a flora containing among the hardier types such mesophytes as 

 Eschscholtzia, Diplacus, and Mimulus, plants that in them- 

 selves are quite unfitted to such a habitat, and are best able to 

 maintain themselves only after having undergone such modifi- 

 cation of structure and organs as to enable them to mature seed 

 in spite of new and trying conditions. Of course, such meso- 

 phytes as are least adaptable, and such as do not receive suffi- 

 cient protection during their incipiency succumb; leaving behind 

 those that have been fortunate in shelter, and those best suited 

 to meet the requirements of such an environment. In this way 

 among others, we may account for the presence of many plants 

 that combine with the typical possessors of the sand hills to 

 produce a sand-dune society. 



