ARID PORTIONS OF SOUTH AUSTRALIA. 135 



indirectly by a possible reversion of the phyllodia of Acacia pycnantha, 

 for example, which are relatively large. This species apparently re- 

 quires relatively good water-relations. 



The vertical position assumed by the foliar organs of shrubs and 

 trees in the dry regions is perhaps mainly due to a reaction to condi- 

 tions accompanying a small water-supply, although possibly there is 

 here, as mentioned above, a light-relation of moment. Among the 

 species having this characteristic especially well developed are Hakea 

 multilineata, Gravillea stenobotrya, Acacia linophylla, A. tarculiensis, 

 Pittosporum phillyrceoides, and Eucalyptus spp. 



A frequent accompaniment of a condition of dryness in the environ- 

 ment, and possibly often directly related to it, is the presence of a 

 covering of trichomes which serves the end of protecting the foliar 

 organs against excessive evaporation, as well as against intense illum- 

 ination. The trichomes are of two general kinds, those which serve 

 merely as a covering and those which are glandular. A heavy covering 

 of the former usually produces a gray-green effect. Among species 

 having such a trichomal covering well developed are Acacia linophylla, 

 Eremophila longifolia, E. oppositifolia, and E. rotundifolia. In certain 

 species, as in Gravillea stenobotrya, the (mature) leaves have a covering 

 of hairs on the ventral surface only, and in other plants with furrows in 

 the leaves or phyllodia such trichomes may be confined to the furrows, 

 as in Casuarina sp. and Triodia irritans, at least in the mature organs. 



Glandular hairs are frequently observed. They are numerous, for 

 example, in Acacia linophylla, Casuarina stricta, and Dodoncea sp., but 

 may possibly be found in most perennials of the dry regions, at least 

 in the young stems and leaves, or their equivalent. In Acacia tarcu- 

 liensis may be seen what appears to be glandular trichomes, but they 

 are restricted to the margins of the phyllodia. 



The formation of resinous secretions and secretions of oil, etc., by 

 glandular trichomes as well as by internal glands, is of frequent oc- 

 currence in plants of the drier regions. It is possibly in some way 

 directly related to the dryness of the environment, although in exactly 

 what way seems doubtful. Internal glands occur in the Myrtacese, as 

 is well known, but may be found in other families, as for example, 

 in the Myoporineae. In several species of the genus Eremophila of 

 the family last named, internal secretion-glands were seen in the leaves 

 and in the cortex of the young stems. 



Other morphological features also are apparently mainly associated 

 with the occurrence of the species under dry climatic conditions. 

 In all or nearly all species of perennials, for example, the epidermis 

 is very heavy. This is usually brought about by the great thickness 

 of the outer wall, which is probably always cutinized, but in Pittos- 

 porum phillyrcBoides and Fusanus acuminatus the epidermis is in part 

 or altogether of two or more layers of cells. On the dorsal side of the 

 leaves of Gravillea stenobotrya the epidermal cells have unusual length. 



