THE NEW FIELD-BOTANY 361 



tarium, Plate LXVIL), Bird's-Foot Trefoil (Lotus corni- 

 culatus) and Centaury (Erythrcea Centaurium). Later 

 still, the Maram Grass is ousted and the Sand Sedge and 

 Sheep's Fescue Grass become dominant. Other plants 

 take up stations, among them Stonecrops, Mouse-Ear 

 Hawkweed (Hieracium Pilosella, Plate XXIX.), Whitlow 

 Grass (Draba verna) and Sandwort (Arenaria serpylli- 

 folia). The shifting dune is transformed by its plant- 

 colonists into a fixed dune, and ultimately the formation 

 is closed, becoming dominated by pasture grasses or 

 transformed into a heath in which Ling (Calluna vul- 

 garis, Plate LXIII.) is usually dominant. 



From this outline it will be seen that the dune- 

 formation is at first " open," and continues to be so for 

 some time. It is occupied by a succession of associa- 

 tions until it becomes fixed and closed, the final and 

 permanent association consisting of comparatively few 

 species. It is apparent that there is a struggle for 

 existence among associations as well as among individual 

 species. 



Mention of this succession of associations reminds 

 the author of a pond that he has had under observa- 

 tion for many years. A dozen or so years ago this 

 pond was fairly clear and deep and a haunt of Water- 

 lilies, and other aquatics. The Marsh Horsetail (Equi- 

 setum limosum) appeared on its margins and increased 

 with great rapidity; a mass of decaying leaves and 

 other detritus became entangled with the Horsetail 

 and the pond is gradually being silted up, the Horsetail 

 extending all over the area of the water and assisting in 

 the process. The pond is now little more than a marsh, 

 and marsh plants have followed the Horsetail wherever 



46 



