THE FLORA OF ADEN. 65 



colonum, Setaria verticillata, Setaria viridis, Cynodon dactylon, Era- 

 grostis ciliaris. 



It is evident from the above tables that a great number of species is 

 oommon to Aden and the countries bordering the Red Sea on the 

 opposite side. It is just this similarity of vegetation which induced some 

 scientists to assume an old continent as the centre of origin of this flora. 

 At an early period, they say, this continent must have disappeared, 

 when and in what way we are not told. At the same time, perhaps, 

 or at a later period a great depression of land took place which resulted 

 in the formation of the Red Sea and, consequently, in the separation 

 of Arabia from Abyssinia. Along both shores of the Red Sea and of 

 the Gulf of Aden there is known to be a great development of volcanic 

 rocks. There is every probability that these, so far as they are known, 

 belong to one series; and as some active volcanoes still exist in the sea, 

 and cones quite unchanged in form and evidently of very recent date 

 abound in many places along the coast, it is clear that the series is still 

 in process of formation, and that it is, in part at least, of recent date. 

 Although there can be little doubt of the recent date of a largo portion 

 of this series, the great amount of denudation which parts of it have 

 undergone around Zulla and Massowa and also at Aden seem to prove 

 that these portions of the series are of a more ancient date, but not older 

 than the Pliocene rocks. It is, therefore, impossible that the flora of 

 Aden developed independently during a geological period of long dura- 

 tion ; we must on the contrary conclude, that it is the product of species 

 imported from the neighbouring countries. This view is strengthened 

 by the fact that the plants endemic in Aden are only very few. 



In order to give a more complete aspect of the flora of Aden, I 

 examined the morphological structure of a number of plants, which is 

 apt to give a more adequate insight into the life -functions of a vegeta- 

 tion that has to thrive under such unfavourable conditions as are those 

 of Aden. The account will, naturally, be fragmentary, and I am 

 making an attempt in the hope only that there will be found some who 

 are willing to continue and complete the work. 



As scarcity of water is the most prominent feature in the climate of 

 Aden, we shall, in the first place, see what special adaptations there 

 are to be found in the organs of absorption and transpiration. 



The rain being very scanty, it is on the whole only after a rainfall 

 that the soil of the valleys is uniformly moistened throughout, so that 



