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geotropic in somewhat varying degrees; they lie indifferently 
upon the surface of the ground; and thus arises the mode 
of growth characteristic of Empetrum. Wherever circumstances 
are favourable, its long, creeping shoot-systems will spread out 
on all sides; the growth becomes decidedly espalier-like.! But 
unfavourable circumstances of locality, caused by the substratum 
or by competing plant-growth; or disadvantageous circumstances 
of light and wind; or possibly yet other factors, very often 
interfere with the regular development of the espalier-like growth. 
Besides the long, creeping shoots, short, erect ones also 
occur; sometimes on the more central portions of the single 
individuals, where upward growth of the new shoots is com- 
pulsory on account of circumstances pertaining to locality; 
sometimes where, in many habitats, the external conditions 
are such that the formation of short and erect shoots is in 
excess of that of the long and creeping shoots. 
We may reasonably assume that the yearly addition to the 
growth of the shoots is much less in an Arctic climate than 
in southern regions. Naturally, in all places, it is the creeping 
shoots which elongate the most; their internodes are longer and 
their leaf-whorls more numerous than are those of the short, 
erect shoots. But while the length-increment on Danish heaths 
often amounts to about 10 em., and sometimes even 14 or 
15 cm., in Arctic regions the length of the long shoots is but 
3—4 cm. at most, and often only 1—2 cm. Only in a single 
specimen (from Disko) did I find the increase to have been 
either 5 or 7 cm. These few cases, in which the length-incre- 
ment was greater, are doubtless due to special conditions of 
the habitat—especially damp and shady soil—and also favourable 
conditions regarding shelter. Unfortunately it is only rarely 
that travellers give any information regarding this point; but 
N. Harrz (in the Arctic Herbarium of the Botanical Garden) 
! Warming's term for prostrate, outspread growth. 
