FLORA OF ADEN. 
57 
3. Adaptation. 
In order to give a more complete aspect of the flora of Aden, a few 
notes may be added which are apt to give a better insight into the life- 
functions of a vegetation that has to thrive under such unfavourable 
conditions as are those of Aden. The account will, naturally, be 
fragmentary. On many interesting points of ecology no observations are 
available, and we do not wish to repeat in this place a number of 
morphological details which will be found below in the descriptive part 
of the book. 
We need scarcely mention that the flora of Aden is of a distinctly 
xerophytic character and that nearly all of its members show some 
effect or other or adaptation to the abnormal conditions of soil and 
climate. 
In most xerophytes we observe a deep-seated root system, which enables 
them to draw water from the lower moist soil. We cannot say to what 
extent modifications of roots have been developed in Aden. Things 
like these can only be observed on the spot, and it is quite useless to 
speculate upon the function of long and short, thick and thin, vertical 
a. d horizontal roots that you may find in a herbarium, if you are not 
told at the same time, where they were growing, at what depth water 
was to be found at a certain period of growth, whether dew might have 
influenced the development of roots at a certain time of the year, etc. 
The same holds good if we want to know some details regarding the 
absorption of moisture and dew by subaerial organs, whether the process 
consists in the condensation of moisture by secretion of hygroscopic 
salts or in the absorption of rain and dew by trie homes. In both cases 
minute hygrometric observations and careful experiments on the living 
plant are required. 
With regard to transpiration we may safely state at the outset 
that such abnormal thermometric and hygrometric conditions as prevail 
at Aden must necessarily lead to excessive evaporation from the plants 
and consequently to their destruction, if there are not special protective 
modifications in the organs of the plant-body counteracting the adverse 
influences. Another factor, in addition, should not be overlooked, viz. 
insolation. The vegetation is exposed to its influence almost through- 
out the whole year, and the edaphic formation of Aden can only increase 
its intensity. Statistical accounts as to the values of insolation in Aden 
are entirely wanting, as far as we are able to ascertain, and much less 
are we allowed to expect observations regarding the degree to which 
the heat of the soil raises the temperature within the plant. 
