BY SELF-ADAPTATION TO THE ENVIRONMENT. 249 
the above peculiarities of stems the great thickness of the 
cuticular surface of the epidermis, which is not infrequently 
clothed with hair like that of the foliage; the great depth of the 
palisadic layer, which often consists of three, four, or more 
zones of elongated cells (e. g. Zilla), the cortex acting as a 
storage-tissue for water, subsequently passing over into cork. 
Now, when we observe how often the different members of the 
tissue of stems of woody desert plants are thus variously disposed 
and constructed, while at the same time there are certain general 
features very commonly to be seen throughout, I think we cannot 
fail to arrive at the conclusion that these structures are simply 
the outcome of the environment in which such plants live. This 
view is corroborated by experiments, for they prove that the 
great tendency to lignification of the tissues, as already shown 
for the spinescent features, is a result of a deficiency of water, 
and they at once tend to disappear when desert plants grow in 
an ordinary prepared soil of cultivation. 
This, for example, is well seen in several species which frequent 
both the Desert and the Nile Valley, and in the plant of Zilla 
myagrotdes already alluded to, which was raised from seed in 
Cairo. The spines were quite flexible, the pericycular scleren- 
chyma, which is very dense and thick-walled in the desert plants, 
being very greatly reduced under cultivation. 
We have also seen, from Ducbartre’s experiment with Batatas, 
that the absence of sufficient water is a direct cause—in con- 
junction with the responsiveness of protoplasm—of a comparative 
increase of lignification. To this we may add the great defection 
of foliage in the hot summer months, when the formation of 
tissues is proportionally arrested. We can thus realize how 
anomalies in the structure of the stems may well be expected, 
though we may not be able to explain in the case of every 
individual stem the direct connection between cause and effect. 
XI. Water-storage Tissues. 
One of the most characteristic features of desert plants is 
their system of storing water. Commencing with the epidermis, 
certain of the cells form rounded or oval elongated bladders 
bulging on both sides; or they may elongate into hairs, the lower 
part acting as a storehouse of water, or they may assume the 
form of bladders supported on short pedicels. As an example is 
the familiar ice-plant which occurs about Alexandria, being so 
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