ILLUSTRATIONS (31). MYRTLES. 345 



tnd shade wholly unknown in our deciduous-leaved forest. 

 We find that the earliest botanical travellers who visited 

 New Holland were astonished at the singular effect thus 

 produced. Robert Brown was the first to show that this 

 phenomenon depends on the vertical direction of the expanded 

 petioles (the phyllodia of Acacia longifolia and Acacia sua- 

 veolens), and on the circumstance, that the light, instead 

 of falling on horizontal surfaces, passes between vertical 

 ones.* Morphological laws in the development of the leaves 

 determine the peculiar character of the varying light and 

 shade. " Phyllodia," says Kunth, " can in my opinion 

 merely occur in families which have compound pinnate 

 leaves ; and in fact they have as yet only been met with in 

 Leguminosae (in the Acacias). In Eucalyptus, Metrosideros, 

 and Melaleuca, the leaves are simple (simplicia), and their 

 edgewise position depends on a half-turn of the leaf-stalk 

 (petiolus) ; moreover, it must be remarked, that both surfaces 

 of the leaves are of a similar character." In the scantily 

 shaded forests of New Holland the optical effects here alluded 

 to are the more frequent, since two groups of Myrtaceae and 

 Leguminosse, species of Eucalyptus and Acacia, there consti- 

 tute nearly one-half of all the greyish-green tree vegetation. 

 Moreover, between the bast-layers of Melaleuca, there are 

 formed easily soluble membranes, which force their way out- 

 wards, and by their whiteness reminds us of our birch bark. 



The sphere of distribution of the Myrtaceae is very dif- 

 ferent in the two continents. In the New Continent, and 

 especially in its western parts, this family, according to 

 Joseph Hooker,f scarcely extends beyond the parallel of 26 

 north lat., while in the Southern Hemisphere, there are in 

 Chili, according to Claude Gay, ten species of Myrtle and 

 twenty-two of Eugenia, which mixed with Proteacese (Embo- 

 thrium and Lomatia) and with Fagus obliqua, there constitute 

 forests. The Myrtaceae become more frequent from the 38th 

 degree of south lat. ; in the island of Chiloe, where a metro- 

 sideros-like species (Myrtus stipularis) forms almost impene- 

 trable underwood, which is there named Tepuales ; and in 

 Patagonia to the extremity of Tierra del Fuego in 56 lat. 



* Adrien de Jussieu, Cours de Botanique, pp. 106, 120, and 700 3 ' 

 Darwin, Journal of Researches, 1845, p. 433. 

 f Flora antarticsi, p. 12 



