88 
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 Warne (Biol. opt. p. 36) 
according to whom it was first given by Heinricaer. 
The structure of Pirola uniflora 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 Pirolaceæ cannot be said to make any special con- 
tribution towards the interpretation of the influence of Arctic 
climate upon plant-structure. * 
1 A general account of the Ericaceæ is given in a separate section at the 
end of this paper. 
