136 



described first by G. Kraus, but since then very little noticed. ^ 

 In all the evergreen forms these contents are doubtless con- 

 siderably contracted for a shorter or longer period during the 

 winter, either in the palisade-cells only, or also in a part of 

 the cells of the mesophyll. With regard to the individual cases 

 in which this condition has been demonstrated, reference should 

 be made to the particular section. 



The Stem. 



In the cortex of the stem of Ledum, Andromeda, Cassiope 

 hypnoides, Phyllodoce and Loiseleuria occur transparent aqueous- 

 tissue cells, among which are scattered a few thicker-walled 

 cells with contents, or else lamellæ of the above cells occur; 

 the tissue in question occurs also below the central vascular 

 bundle in the leaf of Ledum and Andromeda. The protection 

 of the stem against the danger of excessive transpiration does 

 not on the whole present any features, other than those men- 

 tioned above, which can be serviceable for classifying the indi- 

 vidual forms. The fact that the cortex has often no decided 

 xerophytic character is doubtless occasioned either by the com- 

 pact and low growth of the plant or by the circumstance that 

 the old parts of the cortex, though more or less separated, 

 yet adhere for a long time to the stem. The differences 

 with regard to the formation of the annual rings which have 

 been pointed out above, as also the often very slight distinc- 

 tion between the different elements of the wood — both these, 

 and especially the latter, are no doubt systematic characters. 

 In the southern specimens of certain species that extend south- 

 wards, we find the elements of the wood as slightly differentiated 

 as those in the northern specimens — stereom is especially 

 ill-developed, — a circumstance which must bring us to abandon 



^ Cf. Ritter л'. Guttemsekc: Anatom, phys. Untersuchungen über das 

 immergrüne Laubblatt der Mediterranflora. Engl. bot. Jahrb. Bd. 38, 

 1907, p. 410. 



