X FLOBTJLA. ADENEtfSSI. 



Natural Orders. Genera. Species. 



Aden 41 79 94 



Hong-Kong 122 560 965 



Ischia 86 372 794 



Gibraltar 68 243 456 



These tables show that the great preponderance of natural 

 orders and genera, relatively to the number of species, is not ne- 

 cessarily a distinguishing mark of the vegetation of similar local- 

 ities ; since it is not the result of situation or isolation, but is 

 entirely due to climatic causes. In regions favourable to vegeta- 

 tion, as in Hong-Kong, Ischia, and Gibraltar, the balance is pre- 

 served by a constant struggle for existence going on between the 

 species. At Aden, however, no such struggle takes place ; but 

 all the species have to strive against conditions tending to the 

 entire extinction of vegetable life. Viewed in this light, the Flora 

 appears a collection of desert species, selected from widely dif- 

 ferent natural orders and genera, and all alike contending with the 

 excessive heat and drought. 



Before noticing some facts in the geographical distribution of 

 the species, I have only to advert to the unequal proportion borne 

 by monocotyledonous to dicotyledonous plants. But eleven out 

 of the ninety-four species are monocotyledons, of which nine are 

 grasses. Of the remaining two, one is an exceedingly rare plant 

 at Aden {Pancratium tortuosum), of which I found a solitary clump. 

 The eutire absence of Palms and Polygonacece seems also worthy of 

 record. In so dry a climate Ferns and other Cryptogamia, with the 

 exception of Lichens, are quite unknown. 



3. The geographical distribution of even so small a number of 

 species possesses several points of interest, particularly when re- 

 garded with reference to the extension of the Arabic Flora over 

 the arid regions of the earth. Of the ninety-four species com- 

 posing the Florula, fourteen, or a little less than a sixth, are en- 

 demic, and one constitutes a new genus, confined to Aden. They 

 are as follows : — 



Cleome paradoxa, R. Br. Acacia Edgeworthii, T. Anders. 



pruinosa, T. Anders. Ptychotis Arabica, T.Anders. 



Mserua Thomsoni, T. Anders. Convolvulus sericophyllus, Tenders. 



Sphaerocoma Ilookeri, T. Anders. Anarrhinumpedicellatum, T.Anders. 



Hibiscus Welshii, T. Anders. Campylanthus junceus, Edgew. 



Sterculia Arabica, T. Anders. Lavandula setifera, T. Anders. 



Tavernieria glauca, Edgew. Euphorbia systyla, Edgew. 



The remaining eighty species have an extensive geographical 



