MODIFICATIONS OF FORM 



177 



Additional protection is given in some cases by woolly coverings of hair, as 

 in the Swiss " Edelweiss " ; while certain xerophytes bear glandular hairs 

 secreting essential oils, as in Rosemary, Lavender, and many other Labiatae 

 of Southern Europe. Such secretions are believed to be effective as a heat- 

 screen. 



A very peculiar specialisation occurs in the Eucalyptus-trees of Australia- 

 Instead of exposing their leaf-surfaces upwards in the usual di-hehotropic way, 

 they hang hi a vertical plane, exposing their edges to the zenith. This is 

 brought about by a half-turn of the petiole ; but it only appears in Blue-Gum 

 Trees after growth is considerably advanced. In the first leaves the surfaces 



Fig. 133. 

 Part of a transverse section of the xerophytic leaf of Hahea, showing a stoma 

 greatly depressed below the well-developed epidermis, which has greatly thickened 

 outer walls, covered by a thick, continuous cuticle. ( x 150.) F. O. B. 



face upwards, thus shomng the character of the ancestrj^ By quite different 

 means the same advantage, that of not exposing the leaf-surface to the full 

 blaze of mid-day, is attained by certain Acacias, also largely represented in 

 Australia. In them the petiole itself becomes flattened in a vertical plane, 

 without torsion, while the lamina is abortive. Such leaves are termed 

 phyllodes. In the Acacias also the first plumular leaves are often atavistic, 

 developing like ordinary Acacia leaves, while the phyUodes usually appear 

 later. Since Eucalyptus belongs to the Myrtaceae, and Acacia to the 

 Leguminosae — two quite distinct famihes — these cases give a good example 

 of homoplasy, or parallel development, in distinct families of plants under like 

 conditions. Another example of homoplasy has already been seen in the 

 succulent Euphorbias and Cacti. Further instances may easily be multipUed 

 among plants which show special adaptations. 



