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peripheral than that of the two former. The palisade-cells in 
Vaccinium Oxycoccos and Andromeda are long and occur in 
layers which are fewer and of greater thickness than in Ledum, 
And they bear further testimony to the fact that in the Arctic 
regions the palisade-tissue is independent of light and of drought. 
The want of protection exhibited in Vacc. Vitis-idæa and Andro- 
meda appears to have resulted in the cells of the mesophyll 
becoming fairly thick-walled. The development of cuticle on the 
inner walls of the guard-cells and adjacent parts is common to 
all the forms upon which hairs are not developed. 
All the forms with furrowed and acicular leaves have in 
their mere outline, and also in the position of their stomata, 
a special advantage over the broad-leaved forms. Loiseleuria 
is least protected in regard to its outline. But nevertheless 
the leaves of this form may be said to be well protected by a 
thick epidermis, by mucilaginous walls, and by dense hairiness. 
The palisade-cells are of about medium height. It will excite 
no surprise to learn that Loiseleuria is common in Arctic 
regions. 
Phyllodoce somewhat resembles Loiseleuria in structure, 
but is possibly more xerophytic than the latter. The palisade- 
cells are here also of medium size as are those in Ledum. 
Phyllodoce is hardly so decidedly Arctic as is Loiseleuria. 
Cassiope hypnoides and Cassiope tetragona, especially the 
latter, must be regarded as those which shew most adaptation. 
I shall not here repeat their characteristics, but it will suffice 
to point out that they are decidedly Arctic (Cass. hypn. occurs 
in the Southern Alpine regions), and that the palisade-tissue 
consists of cells which are, if anything, somewhat small. As 
is the case with Ledum and with Rhododendron among the 
broad-leaved species, so also here, the protectional devices 
against the danger of excessive transpiration are highly deve- 
loped. These two forms have probably originally been developed 
under Arctic (Alpine) conditions. I am not able to decide what 
