FLORA OF ADEN. 
59 
Reduction of surface is also brought about by the division of the 
leaf blade into narrow linear or thread-like segments. The decrease in 
exposed surface is considerable in this case. 
In a great number of plants the amount of leaf surface exposed to 
dry air is reduced by rolling or folding of the leaf. The edges are 
usually rolled up with the lower side inward, owing chiefly to the 
greater turgidity of the upper. In many plants the leaves are permanent- 
ly rolled or folded. In these cases the stomata lie in a chamber that 
is permanently and more or less completely closed. In consequence of 
this the protection against drought is very effective. Examples: 
Eragrostis mucronata, E. cynosuroides , Sporobolus spicatus, Aeluropus 
villosus , Aristida Adscensionis, A. plumosa , A . hirtigluma , Poly car pcea 
fragilis, Zygophyllwn simplex. 
In many species the leaves either fall off early or are reduced to 
functionless scales. The stems are usually thin, erect, and rod-like. 
A characteristic example is Ephedra. Virgate stems may be seen in 
Maerua Tkomsoni , Hibiscus micranthus , 1L Welsh ii, Commiphora abyssi - 
nica, C . opobalsamum. 
In Cocculus cebetha the bigger leaves fall off very soon whilst a few 
of the smaller ones remain. The former are characterized 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. 
In too bright and intense illumination the leaflets of Cassia oboe at a 
change their transverse position into a line more or less parallel with 
the direction of the rays of light. By assuming this perpendicular 
position the upper sides of each pair of leaflets cover each other and thus 
reduce the transpiring surface considerably ; in this case, besides, only 
the margins of the leaflets are exposed to the rays of the sun. 
Many Aden plants have greyish-white stems and leaves > This 
colour is often due to a very thin layer of wax which covers all the green 
parts of the plant, thus protecting the tissues against too profuse 
transpiration. We mention only Capparis spinosa, in which the 
chlorophyll of the leaves shines faintly through the coat of wax. 
In a few plants transpiration is diminished by the deposition of a 
mucilaginous substance in the epidermal cells, v. g. in. Zizyphus spina 
Christi , Moringa aptera , and Cassia obovata. 
We must not forget to mention another characteristic feature of 
desert plants, the production of spines. [ ( Though in many cases/ - * 
