328 PROCEEDINGS OF SECTION D. 



were in process o£ extension to an unoccupied mudbank. Having 

 secured a number of specimens I carried them home for further 

 examination. This powder by the aid of a microscope was found 

 to consist of cells, iodine tinting them brown. This substance 

 could have no relation to the reproductive system of the Aviceniiia 

 tree as the flowers are high up on the branches, followed by fruits 

 like garden-beans. 



The aerial roots of Avicennia are from a foot to eighteen inches 

 long, covered with green epidennis, on which the tides deposit 

 mud and confervas. They never throw out leaves, but occasionally 

 become forked. The muddy bank around the Avicennia stem is 

 covered by a brush of these roots to a distance of from four to six 

 yards from the bole of the tree. 



This brush, by entangling debris, protects the bank from de- 

 struction by stream or tide. The roots are as thick as a penholder, 

 and are covered with pores, five hundred or more to be counted on 

 a single specimen. The pores just opened are surrounded by broken 

 epiderm, looking like the sepals of a flower, but having no regu- 

 larity. The horizontal portions of the root system to which the 

 aerial upright parts just described are attached, are white, pithy, 

 and full of air, and though living in undrained mud are quite free 

 from any waterlogged condition. As the upright roots appear to 

 rise out of the mud to obtain air, could tlie powder-discharging 

 pores contribute anything towards aeration ? Might they be 

 mouths to admit air ? After considering how this could be 

 determined, I attached the indiarubber head of a pipette used for 

 eye-drops to the cut part of a root, tied it, and immersed the aerial 

 portion in water, On compressing the rubber cap, air was found 

 to issue freely from the pores, and at no other parts. 



This, then, seems to me to be the function of the pores, to 

 supply air to the root system of the mud-inhabiting Avicennia 

 tree ; the office of the discharged powder being to establish a 

 communication between the air vessels of the plant and the outer 

 atmosphei^e, by bursting open the cuticle of the root. 



Some time later I made an excursion to the estuary of the 

 Brisbane River, and examined other shore plants. 



Rhizophora ■mucronata, a true mangrove, throws up no aerial 

 roots, but on those sent downwards tripod like apertures are seen 

 with elevated edges, circular, one-twentieth of an inch in diameter, 

 and tilled with reddish-brown powder. 



jEgiceras niajus, a small shrub, has white spots on the roots and 

 stems extending up among the leaves. 



ExccHcaria agallocha, has a large well-formed aperture, in which 

 a brown powder is to be seen. I can blow air by the mouth 

 applied to the cut stem, through all these apertures, but tind the 

 bellows of Paqnelin's thermo-cautery a very convenient instrument 

 for such experiments. 



