961 



Frnllania tamarisci. Lichens, on the other hånd, are unimportant; 

 Cladonia rangiferina occurs of course in most heaths, but nearly 

 always as a very subordinate component. Still less conspicuous 

 and in very small numbers we have found other C/af/on m-species 

 and Cetraria islandica. 



The structure and biology of the true ericaceous shrubs is well 

 known from the investigations of Warming and other botanists. 

 We need only add that Erica cinerea stands doser to CaUiina as 

 regards shoot-structure than its relative Erica tetralix, The ripening 

 of fruit is successfully carried out in Calluna, and probablj'^ also 

 in Erica; it seems to be uncertain in the case of Empetriim, and 

 is rarely successful in the Vaccinia and also Corniis siiecica (see pp. 

 915—916). 



The phanerogams of the heath are all perennials (with the ex- 

 ception of the Rhinantheæ), but they differ as regards structure of 

 the shoots. It is noteworthy that the development of wandering 

 shoots occurs in very few of the species. Thus Potentilla erecta, 

 Viola siloestris, Hypericum piilchriim, Jiiniperus, Carex hinervis, Fes- 

 tiica ovina, Anthoxanthiim and Liiziila miiltiflora are spot-bound. 

 Subterranean wandering shoots occur in Corniis siiecica and the 

 two species of Vacciniiim. The remaining species have decumbent 

 shoots which occasionally become rooted, viz. Polygala serpijllacea, 

 Erica, Calluna, Galiiim saxatile, Thymiis and Empetrum; the order 

 in which they are given indicates approximately an increase in the 

 frequency with which adventitious roots are developed, Empetriim 

 having the greatest tendency in this direction. I regard this mode 

 of wandering as intermediate between spot-bound species and those 

 with specialised aerial wandering shoots, but as a means of migration 

 it is little or no benefit to the species. 



As regards flowering we find in the heath that the species 

 are generally adapted to insect-poUination. Those with wind-pol- 

 lination and the simpler modes of insect-pollination (i. e. open 

 and generally yellow flowers) are in a decided minority. Hence 

 it is probable that the poor fructification of some of the species 

 mentioned is correlated with the scarcity of insects on the islands, 

 although doubtless many of those so-called insect-flowers are self- 

 pollinating. 



Reference has already been made to the occurrence of a vege- 

 tation in which the subordinate elements of the heath, the herba- 

 ceous piants and grasses, were predominant; it was also suggested 



