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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 Bhododendron 

 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 Iha-ursi, Andromeda, Lijonia calyculata, Vac- 

 cinium Vitis-idœa 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 Bhododendron; 

 among the less protected species this tissue occurs in Vac- 

 cinium Vitis-id(ea only. This species has about an equal degree 

 of protection as Vaccinium 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 



