BY MAR.TORIK ISABEI. COLLINS. 257 



the type uitli thickened walls bearing transversely elongated 

 pits (Text-fig. 6,6'. 



At the midrib, the tissues of the leaf merge into a compact 

 chlorenchymaof rounded cells. Tlie vascular strands are strengtli- 

 ened on both the upper and lower surface Ijy a bundle of fibres. 



Crystals of calcium oxalate are found in the cells of the leaf in 

 very small quantity. 



Conclusion. 



The secietion of resin by glandular hairs in the bud and young 

 leaves, and the lacquering of older leaves by the ultimate drying- 

 up of this secretion, have been shown by Volkens(6) to be peculiar 

 to some xerophytic plants. Sccevola crdsslfoiin, being a sand- 

 dune [)lant, may be regarded as a xerophyte. 



The chief ecological factors with which a dune-plant has to 

 contend, have been summarised byCowles(2, pp. 107-111). They 

 are, (1), intense illumination, both direct and leHected; (2), the 

 great divergence in the temperature-extremes, which is still 

 further increased by the low specific heat of sand; (3), the drying 

 action of the wind, and the injurious effects of wind-blown sand; 

 (4), the porous nature of the sand, and its consequent low water- 

 capacity. The third and fourth of these factors are, perhaps, 

 the most potent. Tn South Australia, however, the first factor 

 may have some considerable importance, since there are often 

 periods of several days with cloudless sky, while the glare from 

 the sun is still further intensified by the whiteness of the coastal 

 sands. 



As we have seen, S. crassij'olia is able to survive burial in sand 

 by an upward elongation of the shoot, and by the development 

 of adventitious roots. By virtue of tlieir thick, sticky secretion, 

 the buds and young leaves are protected from desiccation, as are 

 also the older leaves, owing to their succulence. A further pro- 

 tection from intense illumination is suggested by the yellow 

 colouration of the secretion. The polished surface of older leaves, 

 caused by the partially dried secretion, also serves to refiect 

 certain rays of light. 



The low water-capacity of the sand is compensated, to some 

 extent, by the water-storage tissues of the older leaves. 



