416 



Margin thickened or strongly marked — continued- 



E. Foelscheana. In the young leaves some of the margins are distinctly thickened 

 and rounded as if corded; it is a device to secure the stability of so large a 

 leaf. 



E. haematoxylon. E. patens. 



E. Kybeanensis. E. Preissiana (We have a rich 



red marginal line in this 

 species also). 



E. marginata. E. Le Souefii. 



E. nitens (S.) 



For a note on glandular leaf-margins, or those the work of insects, see under 

 E. nitens, Part XIX, p. 272. 



Twisting of the Petiole. 



" Grisebach, in his account of the ' Vegetation of Australia,' dwells on the close relation of inter- 

 dependence which exists between the tree vegetation and the coating of grass which covers the ground 

 beneath it ; and remarks that the amount of light allowed by the trees to reach the ground beneath them is 

 rendered more than usually great by the vertical position in which their leaves grow. Hence the growth 

 of the grass beneath is aided. 



It may be that this permitting of the growth of other plants beneath them, and consequent 

 protection of the soil from losing its moisture, besides other advantages to be derived, is the principal 

 ireason why, as is familiarly known, two widely different groups of Australian trees, the Eucalypti and 

 Acacias, have arrived at a vertical instead of a horizontal disposition of their leaves by two different 

 methods. 



The Acacias have accomplished this by suppressing the true horizontal leaves and flattening the 

 leaf stalks into vertical pseudo-leaves or ' phyllodes.' The gum-trees, on the other hand, have simply 

 twisted their leaf-stalks, and have thus rendered their true leaves vertical in position. There must exist 

 some material advantage, which these different trees derive in common, from this peculiar arrangement, 

 and the benefit derived from relation to other plants by this means may be greater and more important 

 than that arising from the fact that the vertical leaves have a like relation to the light on both sides, and 

 are provided with stomata on both faces." (Moseley's " Challenger," p. 229). 



Schimper (" Plant Geography," p. 9) remarks : — 



" Other leaf -bearing xeroplrytes have their leaves or leaf-like cladodes arranged parallel to the 

 incident rays of sunlight, and are consequently less intensely heated and illuminated. In some plants, 

 such as Eucalyptus, the position has become hereditary." 



A condition of equilibrium between the Eucalypts and the grass in an open 

 forest has been arrived at, and the twist of the leaf-stalk aids the provision of light. 

 Eingbarking the trees promotes the growth of grass still further; in the first place, the 

 leaves fall, and the gaunt limbs of the tree but little obstruct the light ; and secondly, 

 the dead tree no longer absorbs moisture, portion at least of which goes to improve the 

 condition of the grass. See also Eingbarking, Part LIT, p. 92. 



Where the Eucalyptus petiole is markedly twisted, it is usually notably flattened. 

 The twisting can be seen in a number of figures, e.g., figure 3a, Plate 127, E. pellita. 



There is a reference to the use of the transverse sections of petioles as aids in the 

 determination of species at Part I, p. 6, of the present work. 



