THE AUSTRALIAN GREY MANGROVE. 



265 



and these can be easily traced on the plates of these roots 

 showing transverse and longitudinal structure* From the 

 nature of this arrangement it is evidently in this portion 

 of the pneumatophore much photosynthesis of the tree is 

 carried out. 



The epidermis or outer ring is composed of irregularly 

 shaped thin-walled cells with a tomentum identical in 

 structure with that on the underside of the leaf, but much 

 smaller. In this cuticle, formed by a bulging out of the 

 epidermis and hypodermal layers, are found fairly numerous 

 papillose projections, or special pneumathodes, a section 

 showing these layers of cells to be raised over what is a 

 vacant cavity or air space in direct communication with 

 the ventilating system. 



Externally, these appear like so many raised black spots 

 scattered over the surface, with a circular depression on 

 the summit, and supporting what looks to be a circular 

 valve or disc, which from above is apparently made up of 

 three or four concentric growths or rings, but these are 

 lost in a cross section, which shows continuous cells with 

 but yet differentiated from, the contiguous epidermis and 

 hypodermal cells. A section through one of these, when 

 this cap is removed, is not unlike the air pore found in 

 Marchantia. It may be that these discs act like a clack 

 valve of a steam engine, and close the orifice when the tide 

 rises and submerges the root, and receding when the air 

 plays round it. But this requires further investigation, as 

 similar markings occur on the serial roots found on the 

 stem six or ten feet above high water mark. Apparently 

 then, these are pneumathodes or they may be secondary 

 organs of ventilation, of the same nature or function as 

 lenticels. 



The tip (Plate XXXIV, figure 8) of this portion of the root 

 is composed of thin walled nucleated parenchymatous cells 



