DEPARTMENT OF BOTANICAL RESEARCH. 81 



of the phyllodia of Acacia aneura, and others of the genus, as well as 

 that of the ultimate branches of Casuarina stricta, to mention three 

 species only, is brought about very largely by a segregation of the 

 mechanical tissue as well as of the chlorenchyma. The distribution 

 of the conductive tissue also contributes to the same effect. Another 

 characteristic of such chlorophyll-bearing organs is the pronounced 

 development of the cuticle, which often is of relatively great thickness. 

 Stomata may or may not be present either in young or in old organs. 

 Stomata are usually protected by a depressed position, as in grooves 

 or furrows, by the in-rolling of leaves, or by heavy cuticle. In the 

 last-named condition the stomata lie at the bottom of pores which, in 

 Hakea leucoptera, may be interconununicating, with the effect that sto- 

 matal chambers are formed between the stomata and the external open- 

 ing of the stomatal (outer) canals. 



The fact that such organs of varied morphological relations, as above 

 referred to, exhibit a strong paralleUsm in the nature of the structure, 

 as they often do, suggests on the one hand the potent influence of the 

 impinging environment, and on the other a parallelism in capacities 

 for adjustment on the part of the reactive organs themselves. Thus 

 in the plants considered the chlorenchyma is usually composed of cells 

 whose long axis is placed at right angles to the siuface of the organ of 

 which they form a part. Where exceptions occur they are usually to 

 be associated with the presence of trichomes or other outer protective 

 structures. But the most striking characteristic of the inner morphol- 

 ogy of the chlorophyll-bearing organs has to do with the formation of 

 cell-walls of relativelj^ great thickness. This mechanical tissue may 

 in many species serve the obvious purpose which its name implies. 

 Thus, in Triodia irritans and Hakea multilineata, to mention no other 

 species, the upright habit, which may be a reaction to light, is made 

 possible through the presence of tissue of this character. But scleren- 

 chyma is often to be found in organs where mechanical support does 

 not appear to be of the first importance. Further, if not of universal 

 occurrence in the chlorophyll-bearing organs of the perennials of South 

 Australia, mechanical tissue, and other tissues with heavy cell-walls, 

 are at any rate very generally to be found. It appears, therefore, that 

 the formation of heavy cell-walls is closely associated with aridity — 

 the most striking feature of the environment. This conclusion finds 

 support in the fact that under arid conditions polysaccharids may be 

 converted into anhydrids, or ceU-waU material.^ 



1 Carnegie Inst. Wash. Year Book for 1919, pp. 99, 100. 



