210 MR. В. $. ADAMSON ON THE COMPARATIVE ANATOMY 
The most peculiar case is that of. V. monticola, where the layers of large cells 
would seem to have this function. These cells contain very little chlorophyll, 
and it appears possible that they may also serve as a screen against the 
damage of intense insolation. 
The hypodermal layer below the upper surface of the leaf in those species 
with reduced leaves probably also acts as a water-storage tissue. V. Hulkeana 
alone of the species examined shows a water-storage tissue on both surfaces 
of the leaf. 
This tissue seoms one which is affected in its development by environmental 
conditions. In comparing cultivated plants with those grown in their own 
country, where water-tissue is present, it 15 almost always better developed in 
the latter. This is noticeably the case with that on the lower surface in 
V. Hulkeana. Also native plants of V. subalpina showed some indications 
of this tissue, which do not appear in the leaves of cultivated plants. 
Relations of the Groups. 
Structurally the species examined fall into two series of increasing 
xerophily, leaving Group F out of consideration for the present. The long- 
leaved forms of Groups A and B form the first series. Those of Group A are 
only slightly xerophilous, but still have protected stomata and a reduced 
lacunar system, Water-tissue is developed in V. Lewisii, and V. elliptica 
has specially protected hydathodes. 
The species of Group B agreeing in external features with those of Group А 
have thick cuticle and homogeneous mesophyll structure. 
The short-leaved forms show a similar series of stages. In Group C such 
forms as V. vernicosa and V. anomala show little special adaptation ; they 
possess hydathodes and normal mesophyll A further stage is reached in 
V. Traversii, with its protected stomata, somewhat homogeneous mesophyll, 
and reduced hydathodes. In V. pimeleoides and the species of Group D 
further adaptations have taken place ; the mesophyll is quite homogeneous 
and palisade-like, with a very small lacunar system, and hydathodes have 
heen suppressed. From a study of the reverted forms of V. cupressoides etc., 
it would seem that these forms, Group E, are modified from those of Group D; 
so that Group E would form the final stage in the series of increasing 
xerophily. 
As was mentioned above, the conditions of environment of the groups 
show the same stages of increasing xerophily. It is interesting to note how 
similar the structure reached in V. rakaiensis (Group B)—the culmination 
of the xerophilous evolution of the larger-leafed forms—is to that occurring 
in some forms of Group D, such as V. buzifolia. 
