BY A. A. HAMILTON. 155 



Halophily. — Succulence is the predominating character noted 

 in species growing in saline situations, which affects their leaf- 

 morphology; and it has been demonstrated that the leaves of 

 certain species are increased in size and thickness by their prox- 

 imity to tidal waters. Cliaracters evolved by the necessity for 

 provision against the absorption of an undue quantity of sodium 

 chloride have, in the case of many beach and estuarine plants, 

 induced a degree of similarity in the leaves of plant-associations 

 adopting a common device, and variation in groups dependent 

 upon differing contrivances to obviate this danger. 



Humous acidity. — The swamp-dwelling fraternity of plants 

 supplies instances of similarity of foliage arising from the com- 

 munal use of xerophytic devices tending to decrease the absorp- 

 tion of the swamp-water, which contains the elusive, deleterious 

 compounds, known as humous acids. 



Insolation v. Shade. — Among plant-associations growing in 

 open sandstone-country incapable of carrying an arboreal vegeta- 

 tion, a similarity in foliage is displayed by species belonging to 

 differing families, due to the common necessity for a reduced 

 leaf-surface, with accompanying characters, calculated to mini- 

 mise the injurious action of unbroken sunlight. The shade, and 

 luxuriant food-supply afforded by the conditions obtaining in 

 the " Brush" forest, have permitted an increase in the size and 

 flacciditv of the leaves of its inhabitants, which has resulted in 

 many resemblances in leaf-characters, in plants widely separated 

 in relationship. 



Juvenility v. Adolescence. — The variation attributable to the 

 transition from juvenile to adult growth affects pilosity, viscidity, 

 armature, marginal division, and axial altenation; the division 

 of the young leaves (pinnate) in the genus Acacia, as opposed 

 to the phyllodic character of the adult foliage, and the opposite 

 or alternate arrangement of the juvenile or mature leaves of the 

 genus Eucalyptus, is common knowledge. Characters, arising 

 from protective devices adopted by the young leaves, are dis- 

 carded when no longer necessary. Scabridities, and asperities 

 become more conspicuous as the leaves age, owing to shrinkage 

 in their texture. The foliage of young plants is frequently 

 larger than that of older growths. 



