66 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, Vol XV J II. 



the plants are able to draw the necessary supply of water even from the 

 upper layers of the ground. After a very short time the rain water 

 partly evaporates, partly sinks deeper, and either flows off on the hard 

 inclined rock, or is collected in subterranean reservoirs. It would be 

 interesting to examine and compare the various modifications of roots 

 of the Aden-plants, which must be considered as so many adaptations 

 to the respective conditions of water-supply. But such investigations 

 can be made on the spot only, and it is quite superfluous to speculate 

 upon the use of long and short, thick and thin, vertical and horizontal 

 roots that you may find in a herbarium, if you are not told at the same 

 time where they are growing, at what depth water is to be found at a 

 certain period of growth, whether dew might influence the development 

 of roots at a certain time of the year, etc. The same holds good if we 

 want to know some details about the absorption of moisture and dew by 

 subaerial organs, whether the process consists in the condensation of 

 moisture by the secretion of hygroscopic salts or in the absorption of rain 

 and dew by trichomes. In any case, minute hygrometric observations 

 and careful experiments on the living plant are wanted. 



With regard to transpiration we may say beforehand 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 do not arise special 

 protective modifications in the organs of the plant-body counteracting 

 the adverse influences. There is, besides, another factor, which must 

 not be neglected, viz., the insolation. The vegetation is exposed to its 

 influence almost throughout the whole year, and the edaphic forma- 

 tion of Aden can only increase its intensity. Statistical accounts as 

 to the values of insolation at Aden are, as far as I am able to ascertain, 

 entirely wanting, and still less can we be expected to know to what 

 degree the heat of the soil raises the temperature within the plant. In 

 spite of the deficiencies in our knowledge, an examination of the flora 

 of Aden as to the special contrivances for regulating the water-supply 

 will reveal some interesting facts. 



The most efficient protection from too great a loss of water by 

 transpiration is obtained by the reduction of the evaporating surfaces. 

 The following plants have their leaves reduced in size, and sometimes 

 also in number : — Fartetia longisiliqua, Cocculus leceba, Dipterygium 

 glaucum, Cleome papillosa, CI. brachycarpa, CI. paradoxa, Mcerua 



