106 



The Heath Formation 



lieatUand, mainly along the sides of tracks and paths. 

 Professor Weiss' observations^ have shown that ants carry- 

 away the seeds for the sake of the bright orange oily 

 caruncle (elaiosome) , which they bite and tear as they push 

 the seed along. It is probable in fact that Ulex ewropseus, 

 and also the broom {Cytisus scoparius), are typical myrme- 

 cochorous plants. Ants often make use of human tracks, 

 and it is likely that the invasion of heaths by the gorse 

 (and less commonly by the broom) is brought about 

 through their agency. 



Wet heath sub-association. In constantly wet places, 

 for instance, where owing to local hollows the ground- 

 water reaches the surface and a bog is formed, a 

 characteristic sub-association is found (Plate Via). In 

 such situations peat is frequently accumulated to a greater 

 depth than on the dry heaths, and owing to its acid 

 character the association contains many species of the 

 moor formation (Chapter XI), though in very different 

 proportions. 



1 New Phytologist, Vol. vii. (1908), p. 27, and Vol. vni. (1909), p. 81. 



