116 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



plants of quite different orders, as in Mundtia spinosa (Polygalece) 

 and Aspalathus (Lcquminosa), both being common South African 

 plants. 



"\Ye reach a further stage in the loss of the spiny tip. The leaf is 

 then reduced to a minute size and is closely adpressed to the shoot. 

 This type may be called cupressoidal, as it resembles the form seen in 

 Cuprcssus, Thuya, and Juniperus of the northern hemisphere. 



It is, however, imitated in widely different plants ; but to judge by 

 coincidences it is brought about either by a cold or hot atmosphere, 

 provided it be dry ; for it is found in arctic and antarctic regions, as well 

 as on Alpine and in hot deserts. It is even seen in our own Ling 

 (Calluna vulgaris). 



As illustrations from South Africa the following may be mentioned : 

 Selago, Elytropappus, Crassula lycopodioides, and some species of 

 Heaths See. 



Lastly, the leaf may be totally arrested. In this case the stem under- 

 takes all the function of leaves, as in the Euphorbias and Stapelias, 

 already alluded to. 



We have seen how fleshiness or succulency occurs in many stems, but 

 it as frequently, if not more so, is found in leaves. 



The genus Mesembryanthcmum, which has some 300 species in 

 South Africa, is the most familiar example there, the leaves assuming 

 various forms, as cylindrical, deltoid, hatchet-shaped, &c. ; but in all cases 

 they store up water. 



In England that genus is not known except under cultivation ; but 

 our Sedums, Cotyledon (a large South African genus), and the House Leek 

 are familiar to us as having thick succulent leaves, in correlation to their 

 habit of life, growing on walls, rocks, and roofs of houses &c. 



Another type of plant characteristic of Africa is the Aloe (species of 

 Aloe, Liliaccce). In Mexico is the so-called American Aloe, familiar to the 

 residents of the Riviera, where it often flowers, bearing huge candelabra- 

 like flower-stems, some twenty or more feet in height. This genus, how- 

 ever, belongs to the order Amaryllidacece. The two plants very closely 

 resemble each other, having similar long massive leaves terminating in 

 a hard point. 



The leaf acts as a storehouse of water ; as by far the greater part of 

 the interior consists of large cells filled with water, but charged with a 

 resinous matter which supplies the " bitter Aloes " of druggists. 



Spincsvrurc is eminently characteristic of dry places and hot deserts. 

 In the fleshy Euphorbias the leaves are rarely present, being represented 

 by tufts of spines. It is the same with the Mexican Cactacea?. 



We have seen how the ericoidal type easily becomes spinescent ; thence 

 it passes into the cupressoidal ; but it is of common occurrence to find 

 both forms of leaf on the same bush of Juniper &c, the spiny leaf 

 representing tlx younger, and the non-spinescent the older type. These 

 thus correspond with their evolutionary history. 



In some pla its the stipules only are spinescent. This is character- 

 istic of the genu Acacia, of which A, horrida is a well-known shrub in 

 South Africa. It supplies one of the several plants known as the 

 * Wait-a-bit' thorns, being useful for hedges. 



