492 ECOLOGICAL STUDY OF SALTMARSH VEGETATION. 



on the mud but would be awash at high tide. The Azolla is 

 usually found in fresh water pools and was not noted elsewhere 

 in the Port Jackson district in a saline habitat. Though the 

 plants were vigorous, they had not collected into the close 

 layer which they form in a pool, the thinness of the sheet re- 

 flecting the disturbing tidal influence. The diminutive fern-like 

 plants float on the surface and are dispersed by currents. 



3. The Fluvial Zone. 



The line of demarcation between the dry salt plain and the 

 fluvial zone is frequently indicated in the local marshes by the 

 Swamp-Oak, Casuarina glauca Sieb., which follows the boundary 

 of the plain in belted formation or spreads into an irregular 

 forest in the fluvial swamp frequently formed at the head of 

 the bay, or on the swampy river flats. Its movements are re- 

 stricted to the vicinity of the tidal waters owing to its partiality 

 for a saline environment and its objection to a dry station, in 

 exceptional cases a grove of these trees has ascended the hill- 

 side to a distance of 1000 yards from the waters of the estuary, 

 e.g., the clump near the Mitchell Library, and a straggling 

 colony which has reached the Pittwater road at Deewhv. A. 

 forest formation of Casuarina glauca on the Parramatta Rivet- 

 is depicted in a photo by R. H. Cambage.* Belts of these 

 pine-like trees line the banks of the Curl Curl lagoon on the 

 Manly-Narrabeen Road and ornament long stretches of the banks 

 of Cook's River at Undereliffe and Duck River at Clyde with 

 shady avenues. The Casuarina is exceptionally well equipped 

 to withstand the xerophytic conditions obtaining in the estuary, 

 its vertically held cylindrical branchlets, arranged in switch-like 

 tufts and practically aphyllous, offering a minimum surface for 

 subjection to direct insolation. 



The equisetoid branchlets are laxly jointed, and consequent 

 upon the advent of a sudden dessicating factor such as the 

 passage of a hot wind over the forest, they are easily dismem- 

 bered and fall (simulating the leaf- fall of a mesophytic tree 

 under a like provocation ) , affording the root system a speedy 

 measure of relief from the strain of the accelerated evaporation. 



The bedding of fallen branchlets under the trees has been 

 compared by several writers to the carpet of Pine-needles in a 



*in Maiden, J. H., The Forest Flora of N.S.W., ii.. 

 1907, p. 96. 



