THE MANGROVE OF THE BOMBAY PRESIDENCY. 651 



pedicel. The cushion-shaped fruit is 1 to 2 in. in diameter. The cup- 

 shaped, persistent calyx bears the persistent base of the style, which is 

 about 1 in. long. 



An interesting feature of Avicennia officinalis, Sonneratta acida and 

 Ceriops candolleana are the pneumatophores, which exhibit an aspect 

 widely different from those of the Bruguieras. As soon as the shrub 

 reaches a certain height, in Avicennia officinalis, v.g., 1 ft. or 1^, there 

 appear in great number around the stem within a rather large circle 

 erect shoots with a soft, elastic texture like cork. They resemble very 

 much the young shoots of Asparagus, except in colour, which, in our 

 case, is a brownish black. They are very seldom observed developing 

 leaves and growing up into bushes. If we follow them downwards we find 

 the point of origin to be the subterranean roots of Avicennia officinalis, 

 of which they are the negative-geotropic branches. In this plant they 

 reach 1 to 1\ ft. above the mud or the shallow water and do not exceed in 

 thickness \ or f of an inch, whereas in Sonneratia acida they reach 18 to 

 24 in. in length, by 3 in. in diameter. As they do not develop into a 

 shrub it is evident that they serve some othsr purpose. A transverse 

 section of such a root-branch gives us the looked-for explanation. In 

 Avicennia officinalis our attention is drawn to a large, white ring which 

 occupies nearly the whole plane of the section, leaving room only for a 

 small, darker ring in the centre and a comparatively disappearing, 

 protective skin. The white, loose portion is easily recognised as the 

 parenchymatous tissue of the primary cortex and in it the naked eye is 

 able to distinguish little holes which, by microscopic examination, 

 prove to be lenticels. Those roots, therefore, represent respiratory 

 organs like the over-ground roots of Bruguiera. But why do the 

 pneumatophores reach beyond the water-level, as there is oxygen in the 

 water ? We must admit that the air dissolved in water shows, on the 

 one hand, a higher percentage of oxygen than the atmosphere, but, on 

 the other also a higher percentage of carbonic acid. In consequence of 

 it the quantity of oxygen available to the plant is much smaller in the 

 water than in the air. Besides, the air diffuses very slowly in water 

 and thus it may easily happen that the slow movement of the water 

 causes a want of oxygen. It is for this reason that woody plants, the 

 stem-bases and roots of which are submerged in mud and stagnant 

 water, are furnished with special adaptations for the absorption of oxy- 

 gen from the atmosphere. That there are really graduations as to the 



