136 PLANT HABITS AND HABITATS IN THE 



Usually the lateral walls as well as the internal wall of the epidermal 

 cells are considerably thickened. 



A structural characteristic of note in marked xerophytes is the 

 prominent development of ceU-walls or of tissue with heavy walls. 

 Sclerenchyma of whatever kind is, in fact, usually to be found in the 

 foliar organs and occasionally in abundance. In Triodia irritans, for 

 example, it constitutes a very large proportion of the tissue of the leaf, 

 but it is a marked feature in the leaves of Hakea muUilineata and 

 H. leucoptera, as well as in the phyllodia of Acacia linophylla and other 

 species of the genus. The pronounced development of cell-walls is a 

 direct effect of drying conditions, as shown elsewhere. 



In connection with the subject of the formation of heavy cell-walls 

 and of mechanical tissue, it is pertinent to remark here that the writer 

 was struck with the scarcity of the quality of spininess in such peren- 

 nials as he saw in the drier portions of South Australia. Aside from the 

 "needle bush," Hakea leiicoptera, the "myall," Acacia rigens, and the 

 "dead finish" A. tetragonophylla, whose leaves or phyllodia are spinose, 

 and A. continua with spinescent branches, the writer does not recall 

 having seen any perennials with very prominent spines. Other 

 acacias with spinose phyllodia or branches, or with small spines, 

 morphologically stipules, may be met with, but nowhere in the drier 

 portions of South Australia is there to be found anything like the large 

 development of spines as a characteristic of the perennial plants that 

 one encounters, for example, in southern Arizona. 



The chlorenchyma of the foliar organs is usually composed of pali- 

 sade cells. This however, is not without its exceptions; for example, 

 in Eremophila roiundifolia the chlorenchyma is of short cells. In this 

 species the leaves are heavily covered with trichomes. There thus 

 appears to be some relation between the presence of the one and the 

 development of the other. In other species, however, as in Eremophila 

 longifolia, there are both palisades and trichomes, and a similar condi- 

 tion is to be found in Acacia linophylla, not to mention others. 



Reactions to the Subterranean Environment. 



Relatively little is known concerning the direct reaction of the plants 

 of the drier portions of South Australia to the soil and soil conditions, 

 yet the subject is recognized as of much importance. Thus the presence 

 of halophytes, which constitute an important part of the flora of the 

 far north, is directly related to an excess of salts in the soil solution. 

 This feature has been brought about in some way by the development 

 on the part of the plants of the quality of tolerance to a highly con- 

 centrated soil solution. In this development an important element 

 is the formation of plant juices of high concentration and therefore 

 of high osmotic values. 



The reaction of plants to the physical nature of the soil per se is 

 probably also of importance. In coarse soils (as in sand) quick and 

 fairly deep penetration of the rains and the formation of a dust 



