1890.] of South African Plants to the Climate. 35 



Proteaceae {e.g., Leucospermum, Mimetes), Legurninosae (e.g., Poda- 

 lyria, Indigofera, Aspalathus) and representatives of other orders 

 protected by a similar coat of hairs or felt. It is interesting to notice 

 how species of the same genns differ in this respect according to their 

 locality. The genns Hydrocotyle, for instance, is a good illustration. 

 Most of its species occur in moist places and possess round green leaves 

 of delicate texture. A few, however, are found on dry ground and are 

 well adapted to these different conditions of life. H. virgata, bearing 

 cylindrical leaves, belongs to our first group ; H. Centella, having 

 mostly narrow leathery leaves, to the second ; H. Solandra, which is 

 well covered with white felt, to the third. 



Some plants secrete mineral substances (Carbonate of lime, salt), 

 which form a protecting layer either over the whole leaf \Tamarix 

 articulata\ or only over the little depressions in which the stomata 

 are situated ; e.g., Statice, Vogelia africana and other Plumbagineae. 



Another peculiarity evidently serves the same purpose. The stomata 

 are often placed in depressions or in grooves of leaves and stems, or 

 the edges of the leaves are rolled so far back, that they nearly touch 

 each other, turning the leaf into a tube, which bears the stomata on 

 its inner side. All Aloes, many Proteaceae, Crassulaceae and the 

 famous Welwitschia have such deeply depressed stomata, and many 

 species of heath e.g., Erica cerinthoides, urceolaris, caff r a exhibit 

 such rolled leaves. 



More complicated and more efficient is the arrangement of the 

 stomata on several grasses, on Bobartia spathacea and Acanthosicoys 

 horrida (Naras). Leaves and stems of this Bobartia, which, as I may 

 mention, are used for making the little strawberry baskets, are long 

 and round like rushes. They are smooth but show longitudinal lines. 

 These lines are deep grooves, the sides of which possess alternate 

 projections. The stomata are situated only in the valleys between 

 these ridges. If the heat is too strong and the loss of water too great, 

 the sides of the grooves are pressed together by an automatic 

 mechanism, the ridges fit in between each other, and the communica- 

 tion with the atmosphere is interrupted in a double way. Quite 

 similar is the arrangement on the twigs of the Naras plant,* which 

 grows on the sand-dunes near Walfish Bay. 



4. The leaves assume the most favourable position 



towards the SUIl. The best example of this group is the Austra- 

 lian bluegum (Eucalyptus globulus), so largely cultivated here. It 

 bears horizontal leaves only on its younger twigs, later on developing 

 those hanging falcate leaves which turn the edge towards the sun 



* See figure in R. Marloth? ^Acantliosicyos horrida, Engler's Botanische 

 ahrbucher, vol. IX, tab. 3. 



