ARID PORTIONS OF SOUTH AUSTRALIA. 133 



CERTAIN REACTIONS AND ADJUSTMENTS OF PLANTS OF 

 THE MORE ARID PORTIONS OF SOUTH .AUSTRALIA. 



Any conception of the nature of the reactions of living plants to 

 their physical environment, as, for example, to any particular feature 

 of their environment like the oxygen-supply or that of water, has to 

 deal with factors widely different from a simple or direct reaction in 

 which non-living substances only take part. Aside from and in addi- 

 tion to the chemical complexity of the living organism, the feature of 

 its heredity must be taken into account. This, to put it in simple 

 terms, merely means in the course of its phylogeny the organism has 

 for untold ages reacted to its environment, so that the living plant, 

 which to-day occupies its place in the desert, is in itself a resultant, 

 so to speak, of all of the reactions of its long past. One of the results 

 is that living plants with unlike histories, in the sense above used, 

 may react along different lines to a certain extent, according to the 

 direction of the development they may be taking. But in the present 

 study only existing plants and the present-day environment are taken 

 into consideration, and the following paragraphs give some of the most 

 striking reactions of the plants to their environment. Other reactions 

 are referred to in the course of the paper. 



REACTIONS TO LIGHT. 



Among the possible reactions to light may be mentioned the vertical 

 position assumed by the chlorophyll-bearing organs of many or most 

 perennials of the drier portions of the state. This is attained in part 

 by an upright position of the branches, but mainly by a vertical posi- 

 tion taken by the individual organs themselves and in a measure 

 independent of the position of the branches which bear them. In 

 Hakea muUilineata, Gravillea stenobotrya, and Acacia linophylla, to 

 mention no others, the vertical position is attained because of the 

 upright habit of the leaves, and in Pittosporum phillyrceoides and 

 Eucalyptus spp. it is attained by their dependence. 



The presence of a heavy epidermis, or of a covering of trichomes, 

 or of resinous substances, or the elongation of the cells containing 

 chlorophyll bodies in a diiection at right angles to the surface of the 

 leaf, may all (in part at least) be related to an adjustment of the organ 

 to the intensity of the light. The evidence for certain of these con- 

 clusions, it must be said, is merely inferential. For example, in Ere- 

 mophila rotundifolia the leaves have a heavy covering of trichomes, and 

 in them the palisade cells are poorly developed. The palisades appear 

 to be about the same in E. neglecta, in which species there is a heavy 

 epidermis with glandular hairs and with a marked resinous coating. 



Whether, however, the light-screen of whatever kind is a cause or 

 effect is another question, and the observations throw no light on its 

 solution. The interactions and interrelations are so complex that 



