60 JOURNAL, BOMB A Y NATURAL HISTORY SOCIETY, Vol. XVIII. 



These tables show that there exists a great preponderance of natural 

 orders and genera compared with the number of species. The pro- 

 portion of natural orders, genera and species is 1 : 2'42 : 4'17, whilst 

 on the neighbouring Socotra, an island in the Indian Ocean, the 

 vegetation shows the following proportion according to Krause : 1 : 4 : 7. 

 This seems to indicate that the paucity of species in the flora of Aden 

 is not the result of situation or isolation, but the necessary consequence 

 of the meteorological factors combined with unfavourable edaphic con- 

 ditions. Of the 196 species, 35 are Monocotyledons, chiefly represented 

 by the Gramiuese, and 160 Dicotyledons which owe the high number 

 especially to the Leguminosse with 28 species and to the Capparidacese 

 with 17 species. 



With regard to the geographical distribution of the Aden plants 

 we may distinguish five groups. 1 To the first belong the species 

 which are endemic in Aden. Anderson considered 14 out of his 94 

 plants to be endemic, viz., Cleome paradoxa, R. Br. ; Cleome pruinosa, 

 T. Anders. ; Mcerua thomsoni, T. Anders. ; Sphcerocoma hookeri, T. 

 Anders. ; Hibiscus welshii T. Anders. ; Sterculia arabica, T. Anders ; 

 Tavernieria glauca, Edgew. ; Acacia edgeworthii, T. Anders.; Ptychotis 

 arabica, T. Anders. ; Convolvulus sericophyllus , T. Anders. ; Anar- 

 rhinum pedicellatum, T. Anders. ; Campylanthus junceus, Edgew. ; 

 Lavandula setifera, T. Anders. ; Euphorbia systyla, Edgew. Later 

 botanical explorations, especially in South-Arabia, Eritrea, Somaliland 

 and Socotra, have shown that only three of the 14 are endemical in 

 Aden, viz., Hibiscus welshii, T. Anders. ; Mcerua thomsoni ; T. Anders. ; 

 Cleome pruinosa, T. Anders. Besides these the following species seem 

 to be confined to Aden : — Albuca yerburyi, Ridl. ; Crotalaria schicein- 

 furthii, Defl. ; Fagonia glabra, Krause; Polygala thurmanniana, 

 Chod. ; Heliotropium adenense, Guerke. It is more than probable that 

 further explorations of the neighbouring countries will reduce even this 

 limited number of endemic plants, for it is very little we know, v. g., of 

 the interior of Arabia and its vegetation. 



The plants belonging to the second group belong geographically to 

 two narrow strips of land along the Red Sea, beginning at about 23° N. 

 Lat., the one on the African side passing over into the coast region of 

 Somaliland, whilst the other one comprises Tehama and goes as far as 

 the boundaries of Yemen and Hadramaut. Aden has the following 



1 Cf . Krause, 1. c. 



