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the theory that the short period of growth in the Arctic regions 
is the cause of this slight differentiation. But in spite of 
this it does not follow that the short period of growth is of 
no importance whatever in this connection; it is undoubtedly 
instrumental in lessening the difference between the different 
elements of the wood; but as yet there hardly exists any accurate 
data on this point. In some cases other circumstances, such 
as slight need of stereom (in the case of prostrate stems), are 
doubtless of importance. 
Lastly if, with reference to what I have indicated in the 
introductory lines, | am to point out the species which are 
most in conformity with the Arctic climate, then among the 
evergreen species it must be Rhododendron lapponicum, and 
to a certain extent Ledum and Cassiope tetragona (Cassiope 
hypnoides also in a measure) and among the deciduous species 
Vaccinium uliginosum. 
It is only the structure of the leaves which is of importance 
in this connection, the stem and the root not shewing any 
special points of interest in this respect. Among the evergreen 
species, the greatest progress has doubtless been made by 
Rhododendron lapponicum and Cassiope tetragona — both true 
Arctic species which have most probably been long-established 
in the region in question, -- and only such evergreen species 
as have, each in its own way, assumed the degree of protec- 
tion attained by these two, will be able to continue to exist in 
the Arctic climate. Among the deciduous species such decided 
differences are not to be found as occur among the ever- 
greens. 
