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more definite aqueous tissue. The palisade-cells are somewhat 
longer than in Rhododendron, particularly in the decidedly 
Arctic form L. decumbens. Here we have (in Rhododendron 
and Ledum), without going deeper into the matter, two distinct 
kinds of palisade-tissue; which shews that in the Arctic 
moorland plants the height of the palisade-cells is practically 
immaterial, so that with regard to this feature the plants may 
retain unmodified their natural condition. As regards Ledum, 
its southern distribution may possibly be connected with the 
height of its palisade-cells. Both the species are well fur- 
nished with protective devices against excessive transpiration, 
a fact which is of ordinary occurrence in the Arctic moorland 
plants. 
The next five forms which come under consideration, 
Arctostaphylos Uva-ursi, Andromeda, Lyonia calyculata, Vac- 
cinium Vitis-idea and Oxycoccos can scarcely be said to have 
their chief distribution in the Arctic regions. Among them, 
Lyonia calyculata and Arctostaphylos Uva-ursi extend, perhaps, 
farthest north. In connection with this, these two species, 
which also most resemble each other in points of leaf-anatomy, 
are best protected against the danger of excessive transpiration 
— Lyonia by its hairs, and Arctostaphylos by the thick outer 
walls of its epidermis. These two species have moreover the 
small-celled palisade-tissue characteristic of Rhododendron; 
among the less protected species this tissue occurs in Vac- 
cinium Vitis-ideea only. This species has about an equal degree 
of protection as Vacciniwm Oxycoccos and Andromeda, which two 
have wax on the lower surface of their leaves. With regard 
to protection against ‘the danger of excessive transpiration, all 
the five above-mentioned species rank low compared with Rho- 
dodendron and Ledum; the reason for this may perhaps be 
found either in their special nature, or, in their former or 
present position relative to the Arctic regions. As regards 
the three latter, the Arctic part of their distribution is more 
