112 PLANT HABITS AND HABITATS IN THE 



in a great variety of ways, such as the cuticularization of exposed 

 surfaces, the formation of trichomes, the marked formation of scleren- 

 chyma, the development of resinous or other secretions which cover 

 the shoots at least in part, the formation of water-storage organs or 

 cells, a modification in the direction of growth in some manner related 

 to light and, not to extend the list, the size and the form of the chloro- 

 phyllous organs themselves. The precise steps, however, by which 

 such modifications may have taken place, even if they are apparently 

 related directly to differences in the water relations, for example, are 

 necessarily obscure and are subjects for physiological research. 



A survey of the leaf characters of representative perennials of South 

 Australia shows a strong tendency to develop leaves, or phyllodia, 

 which are relatively long. This is most marked in the very dry regions. 

 In this type of leaf modification there is a reduction in the amount of 

 chlorenchyma of whatever kind, and the leaves may be reduced to 

 little more than midribs with narrow wings. In the case of phyllodia, 

 however, the leaf-stalks on which they are based may become very 

 long, which apparently is also a secondary development. Under 

 more moist conditions of growth, however, the latter undergo a ter- 

 tiary modification by becoming relatively wide, thus increasing the 

 surface to a marked extent. Such reversion, in effect if not in fact, 

 occurs only under relatively good water relations. The phyllodia of 

 Acacia pycnantha, the ''golden wattle," illustrate the feature last 

 referred to. The transpiring surface is also increased by the increase 

 in length alone, so that there is in this particular an apparent conflict 

 in the direction of development, as well as a real exception to the 

 general tendency of species to undergo a reduction of the leaf-surface 

 with a decrease in the amount of available water. 



These features can be illustrated by a few examples of leaf or phyllodia 

 sizes. Heterodendrum oleasfoUum is a small tree and occurs in arid- 

 semiarid regions. The leaves are about 110 mm. in length by about 

 14 mm. in width, and have an area, one side, of approximately 1,080 

 sq. mm. The ratio of length to width, therefore, is nearly 8 to 1, 

 while the ratio of area to length is approximately 19 to 1. Acacia 

 stenophylla was found in the desert-arid regions. The phyllodia of the 

 species may become of great length, some 375 mm. long; they measured 

 5 mm. in width and had a surface, one side, containing about 1,700 

 sq. mm. The ratio of length to width is in the latter instance about 

 75 to 1, and that of area to length is about 4.5 to 1. In this connection 

 it can be remarked that in simple leaves, circular in shape and thus 

 with the greatest possible area, the ratio of length to breadth is unity 

 and that of area to length varies with the diameter and increases in 

 geometrical ratio directly with the increase in diameter. In the 

 case of leaf sizes about equal to the "microphylls" of Raunkiaer 

 (Fuller, 1918), 2,025 sq. mm., the ratio of area to length (diameter), 

 in circular leaves is about 40 to 1. 



