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 
1 Cf. Rirrer v. GUTTENBERG: Anatom. phys. Untersuchungen über das 
immergrüne Laubblatt der Mediterranflora. Engl. bot. Jahrb. Bd. 38, 
1907, p. 410. 
