IV] GRASSLAND ASSOCIATIONS 125 



the former one. When, for example, a sheet of open water 

 becomes filled up with silt and peat, the aquatic formation has 

 succumbed and a fen or a peat moor has taken its place. There 

 are, then, not only intermediate associations in any single plant 

 formation, but also passage associations leading from one 

 formation to another. The limestone heath is such a passage 

 association. Geologists have long termed certain strata between 

 two geological formations transitional or passage beds; and it 

 is to be expected from the nature of the case, that similar tran- 

 sitional tracts of vegetation should connect certain allied plant 

 formations. Doubtless some difference of opinion may arise 

 as to which of two plant formations a particular passage 

 association should be referred ; but such a matter is not really 

 one of fundamental importance. 



Some of the bare or almost bare limestone rocks at altitudes 

 approaching 1500 feet (457 m.) also furnish an interesting 

 mixture of lime-loving and humus-loving plants. For example, 

 the following mixture of lime-loving and humus-loving species 

 was noted at Thirkelow rocks, south of Buxton : — 



Asplenium viride Galium saxatile 



Poterium Sanguisorba Deschampsia flexuosa 



Sedum acre Fe.stuca ovina 



Vaccinium Myrtilliis Nardus stricta 



Calluna vulgaris Luzula erecta 

 Thymus Serpyllum (agg.) 



In the case of the heather and the bilberry, it was found 

 that, whilst some of the roots of the plants were closely 

 appressed to the bare limestone rock, other roots of the same 

 plant were embedded in black humus formed chiefly of decaying 

 lichens and mosses. It is a matter for experiment whether 

 seedlings of these plants will germinate and arrive at maturity 

 if grown in a calcareous soil destitute of humus. 



The following list of species are illustrative of the limestone 

 heaths of north Derbyshire : — 



