BY SELF-ADAPTATION TO THE ENVIRONMENT. 243 
level of the outer surface of the leaf and may occur on both sides. 
The depression in which they lie is either in consequence of the 
great thickness of the outer wall of the epidermis (Allium Cra- 
meri, Pityranthus tortuosus, &c.), or because the epidermis itself 
lines cavities in the parenchyma which are either naked or 
clothed with hairs covering over the stomata, as in the grass 
Danthonia Forskali, very like the well-known case of Nerium 
Oleander, to which M. Vesque adds Capparis Breynia. The 
Oleander has narrow, rigid, more or less erect leaves which are 
well suited to live in a dry atmosphere. In a specimen growing 
at Cannes there was a thick cuticle and two layers of thick- 
walled hypodermic cells, a palisade-tissue of two layers on the 
upper side and of one layer on the lower. A lax mesophyll of 
green cells fills up the central space. The stomata are on the 
bottom of the epidermal cavities. 
In a leaf gathered from a tree in Cairo, there were slight 
differences, in that there happened to be no palisade-cells on the 
lower side at all, the lax mesophyll reaching to the hypodermic 
layer. Such differences are probably accidental ; but they show 
clearly how easily the anatomy of a leaf conforms to slight 
differences of illumination, &e. 
In desert plants the guard-cells are often so thick-walled that 
the lumen is nearly obliterated ; and, contrary to what is gene- 
rally supposed to be the case in temperate regions, Dr. Volkens 
shows that they often close during the day and are open at 
night*. Perhaps the arrested moisture due to the check to trans- 
piration may cause turgescence by day, which closes the slit, while 
the cessation at night brings about a relaxation ; or it may be the 
result of a more complicated action between the guard-cells and 
the adjacent epidermal cells. It is, however, difficult to say 
without a very close investigation into the phenomena on the 
living plants in their natural conditions. 
IX. Assimilative Tissues. 
The chlorophyll-tissue of an ordinary dorsi-ventral leaf is 
typically differentiated into a palisadic layer below the upper 
epidermis and a spongy layer above the lower epidermis. It is 
well known that in those plants in which the leaf is normally 
reversed in position, as Als¢rwmeria, the relative positions of 
* Die Flora der aegypt.-arab. Wiiste, p. 47. 
