W. Watson. 
(7). Arrangement of the Leaves on the Stem. 
i 68 
This is one of the most important methods of protection 
against dryness. During moist conditions the leaves quickly absorb 
water and open out; with a drier environment, they place them¬ 
selves close to the stem, or inroll, or curl, or display some other 
movement which makes the surface of evaporation less. The 
leaves which perform these movements have larger and less 
chlorophyllous cells at the base; these are often provided with 
small pores, are more active in absorbing and losing moisture than 
the upper leaf-cells, so that their expansions and contractions may 
be regarded as the causes of leaf-movement. 
The following are the main changes in form and position of the 
leaves during the changes in humidity. 
(а) Imbrication of the Leaves in the Dry State. The leaves, on 
the approach of dryness, lay themselves on the stem, so as to fit 
closely together like the tiles of a house, the chief parts exposed 
being the upper portions (which are sometimes hyaline), the thick 
nerve, or the revolute margins. This method is very common 
amongst the mosses, being well shown in many species of Bryum 
(especially B. ccespiticium v. imbricatmn, B. argenteum and B. 
provinciate), Grimmia, Rhacomitrium, Polytrichum, Andrecea, Pseudo- 
leskea, Myurella, in Ulota Hutchinsice, Hedwigia imberbis, Barbula 
fallax, Leucodon sciuroides, Leskea nervosa, Brachythecium albicans, 
and Camptothecium sericeum. It is also met with amongst the liver¬ 
worts as in Aplozia crenulata, Chandonanthus setiformis , Lophozia 
bicrenata, Madotheca platyphylla, Plagiochila punctata v. minuta, 
P. asplenioides v. minor, Cephalozia bicuspidata v. conferta, Gymnomi - 
triuni spp., and other plants of dry ground. In Aplozia crenulata 
and Odontoschisma Sphagni the chief parts exposed are the thickened 
cells of the leaf-margin. 
(б) Spiral Twisting of Leaves in the dry State. The drying 
leaves are not only in opposition to the stem and to each other, but 
are also spirally twisted round the stem, so that their apices and 
nerves are the chief portions exposed. This is a more efficient 
method than mere imbrication, in a greater or less degree is 
common amongst the acrocarpous mosses, and is well shown in 
Tortnla muralis, T. ruralis, T. Icevipila , Barbula revoluta, B. 
Hornschuchiana, B. convoluta, B. unguiculata , Trichostomum fragile, 
Grimmia funalis, and Encalypta vulgaris. In Bryum capillare the 
leaves are usually strongly spirally twisted, but in the variety 
