120 PLANT HABITS AND HABITATS IN THE 



selves, but their position, as well as the presence of trichomes, ap- 

 parently suffices in this regard. The outer surface of the epidermal 

 cells is sharply arched and trichomes of characteristic sorts, both 

 glandular and protective, spring between these cells; the former have 

 relatively widely expanded heads which collapse in age, and the latter 

 are apparently only short, two-armed, giving an appearance much like a 

 section of shield hairs. The glandular hairs are especially abundant. 



Acacia contintja. 



According to Tate, Acacia continua is one of three species of the 

 genus in South Australia which has neither leaves nor phyllodia. It is 

 a shrub with short branches, 2.5 to 5 cm. in length, which resemble 

 spines. The species was observed at Quorn, where it occurs under 

 fairly good conditions, so far as moisture is concerned. 



A cross-section of one of the short branches is somewhat angular 

 in outline (fig. 15). The general arrangement of the tissues is ap- 

 parently about as in cylindrical phyllodia of other species. That is, 

 there is a large central portion composed of chlorophyll-free paren- 

 chyma, a peripheral band of chlorenchyma, masses of sclerenchyma 

 extending in from the angles, and an epidermis with a well-developed 

 cuticle. Trichomes appear to be wholly wanting, and no secretion of 

 any kind was seen to cover the epidermis. A leading departure from 

 the usual condition to be found in phyllodia (which appears to be a 

 quantitative variation, however) is in the development of tissue with 

 especially heavy walls, either fibrous or otherwise. The hypoderm of 

 the material studied was Uttle developed, although it was regularly 

 present; also, there is relatively little of the fibrous tissue between it 

 and the more deeply placed fibrovascular systems. Finally, the cell- 



EXFLANATIONB OF FIGURES 22 TO 31. 



Fio. 22. Ftuanua ticuminalus, fragment of leaf showing chlorenchyma and a etoup of tracheids, 

 X350. 



Fia. 23. Same. Cross-section of leaf to show the heavy epidermis consisting ot two layers of 

 cells, X350. 



Fio. 24. GravilUa stenobotrya, semi-diagrammatic transverse section of leaf. The various tissues 

 are as indicated. Trichomes and stomata are confined to the ventral side, X52.5. 



Fio. 25. Same. Detail of leaf, dorsal side, in cross-sectioU to show the greatly elongated epi- 

 dermal cells and well-marked palisades, X350. 



Fio. 26. Hakea leucopiera, leaf fragment, in transverse section, with very heavy epidermis and 

 deeply sunken stoma and papillate processes in stomatal canal. The presence of 

 sclerenchymatous fibers in the palisade chlorenchyma is shown. X350. 



Fio. 27. Hakea muUUineata, semi-diagrammatic cross-section of leaf. The prominent devel- 

 opment of mechanical tissue and dorsiventral nature of the leaf structure are 

 indicated. XS2.5. 



Fio. 2S. Same. Fragment of leaf, cross-section, to show heavy epidermis, deeply sunken stoma, 

 and pronounced palisade character of the chlorenchyma, X350. 



Fio. 29. Pitlosporum phiUyrceoidea, fragment of dorsal side of leaf, transverse section, to show 

 the 2- or 3-layered epidermis, X350. 



Fio. 30. Same, ventral side of leaf. The heavy outer epidermal wall, the single cell layer of the 

 epidermis, and the superficially placed stoma are indicated. X350. 



Fio. 31. Triodia irritans, transverse section of leaf, semi-diagrammatic, showing its infolded 

 condition and the position and relative abundance of the main tissues, X85. 

 The tissues are designated as follows: ch, chlorenchyma: ep, epidermis;/!, fibro-vascular tissue 



M, sclerenchyma. 



