PECULIARITIES OF THE CAFE FLORA. 



115 



hair. Such is often assisted by secretions of substances (such as tannin, 

 gum, mucilage, resinous matters, salts, &c.) which thicken the water, and 

 so render it less capable of being lost. 



Lastly, the thick cuticle and wax are themselves induced to be formed 

 by the great heat and light. 



Hence all these features, which thus bring about the very best 

 structures to enable the plants to survive the injurious effects of the 

 climate, are simply the direct outcome of the excessive heat and light, 

 coupled with the deficiency of water. 



They illustrate the general responsive power possessed by protoplasm 

 to build up such structures as are required by the plant under whatever 

 circumstances it may find itself. They illustrate what Darwin called 

 " definite results produced by the direct action of the conditions of life." 



Leaves. — It is in the leaves where the greatest diversity resides, but 

 in all cases the forms and structures are the result of the external con- 

 ditions of life. 



Commencing with large and broad leaves as occur in members of the 

 order Proteacece, they are of a leathery or "coriaceous " texture, with a 

 thick cuticle, often waxy, and with the stomata or breathing pores sunk 

 at the bottom of pits or grooves, which are more or less closed by hairs. 

 At least these features are common in many plants with quite differently 

 shaped leaves. 



The arrest of cellular tissue between the ribs and veins of the frame- 

 work forming the so-called skeleton of a leaf causes the leaf to be often 

 deeply lobed or dissected. Thus in the genus Hakea (Proteacece) a 

 commonly cultivated species has a thick, lobed leaf, looking like a double 

 or enlarged tooth-comb. Several members of the Umbelliferce have 

 dissected leaves, apparently from the same cause. 



It must be observed, however, that any other cause which arrests 

 growth may produce similar forms. Hence submerged dicotyledonous 

 plants have almost always dissected leaves, as our English Water Crow- 

 toot (Banuncuhis aquatilis). In this case degeneracy affects the fibro- 

 vascular bundles as well ; whereas under drought the tendency is for 

 them to become rigid, and even spinescent. Such, for example, is seen 

 in the "ericoidal " or Heath-like type of foliage. 



Besides the true Heaths, of which there are some 500 species at the 

 Cape, many other plants of very different orders assume a similar form of 

 leaf, such as the members of the family Brmiiacece, peculiar to South 

 Africa, Phylica (Rhamnacece), Gnidia, &c. (Thymelacece), and many 

 others. 



The advantage accruing is the great reduction of surface to be exposed 

 to the sun. A not uncommon feature is the rolling of the margins back- 

 wards, so that the stomata are concealed within the tube. In Grasses the 

 two halves of the blade sometimes remain conduplicate when the stomata 

 are on the upper side. 



Our English Empetrum nigrum has leaves of which Sir J. D. Hooker 

 observes : — " The recurved portion conceals the pubescent under-surface 

 and forming a tube closed at both ends." A still further reduction of the 

 size of the leaf from that of the Heath is obtained by shortening it, when 

 the apex becomes hardened and spinescent. This is well seen in several 



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