1887.] Acacias of Southern AJrica. 273 



under the scorching sun of the desert, where all the other bushes 

 bear thick and leathery leaves." 



There can be no doubt that the acacia-leaf does possess some 

 means of protection against the detrimental effects of heat and sun, 

 and we find on examining the different species with regard to this 

 point, that the leaves are the very organs where adaptation to 

 circumstances has reached a high degree of perfection. We have 

 already seen how economical for instance the common hookthorn 

 is in the production of leaves as compared with other species of 

 moister situations, but it is not so much the quantity of the foliage 

 as the structure of the leaves, which enable them to exist under 

 such trying conditions. 



Almost all the acacias, and especially those which inhabit the 

 desert-regions, contain in their sap a certain amount of gunj and 

 the juice of their leases is more viscid and less watery than that 

 of other plants. It is obvious that this peculiarity of the leaf tends 

 to lessen the loss of moisture, for a slimy, gummy, fluid evaporates 

 more slowly than an aqueous one. 



Another very effective arrangement for the protection of the leaf 

 is its periodical motion. In the morning the leaflets open and stand 

 out almost horizontally, but towards 10 or 11 o'clock, when the 

 insolating heat of the sun becomes more intense, the leaflets rise 

 towards their rachis until the two upper sides of each pair touch 

 each other. The twofold advantage gained by this change of position 

 is apparent. The leaflets, turning their edges towards the sun 

 instead of the broad surfaces, are thus enabled to avoid the direct 

 effect of the sun's rays and as each pair then exposes only two sides 

 to the hot air instead of four, their transpiration must be less than 

 in the former position. In the afternoon, when the greatest heat 

 has passed, the leaves open a second time, closing again about sunset 

 on account of the cooler night. In A. destineus this arrangement is 

 so perfect, that even large bushes throw almost no shade in the 

 middle of the day and there is nearly as much sunshine under such 

 a bush as at the side of it. 



The most striking difference, however, between the species which 

 grow in dry localities and those which are only found where subter- 

 ranean w^ater is present, is to be observed in the anatomical structure 

 of their leaves. Taking again for comparison the karroo-thorn and 

 the camelthorn and examining transverse sections of their leaflets 

 under the microscope we find that the tissue of the former is formed 

 ^of large cells with considerable intercellular spaces, and that the 



