Pirola minor occur in the Arctic regions. If the three other 

 species be mentioned in order of their relation to the Arctic 

 region, Pirola uniflora and secunda come first and Chimophila 

 umbellata comes last. On grouping these five species according 

 to the xerophilous structure of their leaves — they are all ever- 

 greens — then Chimophila is the most xerophilous and Pirola 

 uniflora the least. There need not necessarily be anything 

 peculiar in this. The reason why the purely Arctic form Pir. 

 grandiflora, as also Pirola minor and secunda (as regards 

 Pir. uniflora see below), do not present any especially xero- 

 philous characters is that these three forms being saprophytes 

 are subject to quite different conditions from those of the 

 autophytes and consequently they cannot in so high a degree 

 as the latter offer resistance to external factors such as wind 

 and weather. The view that these latter plants are more inde- 

 pendent has been put forward by Warming (Biol. opt. p. 36) 

 according to whom it was first given by Heinricher. 



The structure of Pirola nniflora is not xerophilous, but 

 it is reported to have evergreen leaves. There must be some 

 cause underlying this, as the structure of the leaves is exactly 

 that of a deciduous leaf. Possibly some saprophytic circum- 

 stance is connected with this. The species is not entirely 

 Arctic and lives in Finmark under conditions which do not 

 occur in Arctic regions (viz. as a woodland-plant). In spite of 

 its xerophilous structure Chimophila is not an Arctic plant. 

 Thus the Pirolacece cannot be said to make any special con- 

 tribution towards the interpretation of the influence of Arctic 

 climate upon plant-structure.^ 



^ A general account of the Ericaceae is given in a separate section at the 

 end of this paper. 



