THE FLORA OF ADEN. 67 



thomsoni, Cadaba glandulosa, Polygala erioptera, Folycarpcea corym- 

 bosa, Sphcerocoma hookeri, Corchorus antichorus, Greivia populi folia, 

 Zyqophyllum simplex, Balsamodendron opobalsamum, Moringa aptera, 

 Indigofera semitrijuga, T. arabica, I. paucifolia, I. parvula, Tavemiera 

 glauca, Rhynchosia minima, var. memnonia, Oldenlandia schimperi, 

 Help>tropium strigosum, Convolvidus ylomeratus, Linaria macilenta, 

 Schiceinfurthia pterosperma, Campylanthus junceus, Lavandula setifera, 

 Saltia papposa, Suwda fruticosa, S. vermiculata, Euphorbia arabica, 

 E. cuneata, E. systyla, Ephedra foliata, var. ciliata. The aridity of the 

 climate has also favoured the production of spines and spinous struc- 

 tures. Of the many species which show these modifications I need only 

 mention : Capparis galeata, Fagonia cretica, Z>zyphus lotus, Zizyphus 

 spina Christ i, Sphcerocoma hookeri, Acacia edgeworthii, A. ebumea, A. 

 hamulosa, Acacia spirocarpa, A. nubica, Lycium europamm^ Blepharis 

 edulis, Jatropha spinosa^ Alhagi maurorum. 



In Cocculus leceba we observe that the bigger leaves fall off very 

 soon, whilst a few of the smaller ones remain. The former are charac- 

 terized not only by their greater size, but also by their anatomical 

 structure. They are usually thinner, more pointed, and entirely devoid 

 of hairs. The stomata are almost superficial and equally distributed 

 over both surfaces. The remaining leaves on the other hand are 

 covered with thin-walled hairs which, even when the leaf is dry, absorb 

 water very easily. 



Another kind of reduction of the evaporating surface is shown in 

 leaves which are rolled up lengthwise or have become cylindrical, as 

 e. g. in Zygophyllum simplex, Eragrostis mucronata, E. cynosuroides, 

 Sporobolns spicatus, sEluropus villosus, Aristida adscensionis, A. 

 plumosa, A. hirtigluma. In too bright and intense illumination the 

 leaflets of Cassia obovata change their transverse position more or less 

 in a line with the direction of the rays of light. By assuming this 

 perpendicular position the upper sides of each pair of pinnae cover each 

 other and thus reduce the transpiring surface by one-half; besides, in 

 this case, only the margins of the leaflets are exposed to the rays of 

 the sun. 



Many of the Aden plants have greyish-white stems and leaves. This 

 colour is due to a very thin layer of wax which covers all the green parts 

 of the plants, thus protecting the tissues against too profuse trans- 

 piration ; I mention here only Capparis spinosa, in which the choloro- 



