73 
innumerable short stems and longer roots arise. The " true Mangrove " may easily be distinguished from 
its neighbours by the long aerial roots which raise the main trunk above the level of its origin and give 
the tree the appearance of being supported on stilts. These arise from the usually short stem on all sides, 
growing first for a short distance in a horizontal direction and arching down afterwards into the water. 
Soon the base of the stem, with its original roots, dies and now the only support to the upper stem and its 
branches are those stilt-roots which reach to a height of 2 or 3 yards, and which, on account of their great 
elasticity, are the best possible protective system against the continuous dashing of the waves. The tree 
may be moved forwards and backwards by the force of wind and water, but, ultimately, it will always 
assume its former position. In this way the aerial roots are like as many strong anchors which would not 
allow the tree to be carried away even by the wildest play of the waters. We may very often observe 
that the growing point of such a root loses its vitality, whereas behind the apex a forked root makes its 
appearance. It is evident that such a change of growth can have a beneficent influence only under the 
conditions of existence in a soft and muddy substratum. Another means of furnishing the tree with 
considerable resisting power is the circumstance that not seldom a row of secondary roots breaks through 
the under surface of the primary aerial root, descends immediately in a vertical direction into the mud, 
and, by a luxurious branching into roots and rootlets, helps to strengthen the primary root. (pp. 645-6). 
Father Blatter (p. 651) goes on to say that 
An interesting feature of Avicennia qfficinalis, Sonneratia acida and Ceriops Candolleana are the 
pncumatophores, which exhibit an aspect widely different from those of the Bruguieras. As soon as the 
shrub reaches a certain height, in Avicennia officinalis, e.g., 1 foot or \\ feet, there appear in great number 
around the stem within a 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 H feet above the mud or the shallow water 
and do not exceed in thickness \ or J of an inch, whereas in Sonneratia acida they reach 18 to 24 inches 
in length, by 3 inches in diameter. As they do not develop into a shrub, it is evident that they serve some 
other purpose. A transverse section of such a root-branch gives us the lookcd-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 
Bmguiera. 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 quality 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 oxygen 
from the atmosphere. That there are really graduations as to the percentage of oxygen available to the 
plant in different media, may be shown to evidence by the examination of a pneumatophore of Avicennia 
officinalis. The respiratory root is very thin at the base, where it is covered by mud ; it grows thicker, 
where it is submerged in water, and it reaches its maximum, where it is surrounded by the atmosphere. 
And if we examine the anatomical structure, we find that the various degrees of thickness are due to the 
respective development of the parenchymatous tissue, which contains the lenticels, i.e., the respiratory 
organs. The same may be observed in the species of Rhizophora. They are not possessed of spec:al 
pneumatophores, but the modified tissue of their " stilt-roots " takes upon itself the function of respiration 
and here again it is not the portion buried in the mud, but the one emerging from the mud and still more 
the upper part which is accessible to the atmosphere. 
Then follow biological notes (p. 652) on Carapa obovata Bl. (Meliacese), Lumnitzem 
racemosa Willd. (Combretaceso) and figiceras majus Gaertn. (Myrsinacefe), a well- 
known Mangrove shrub of the Sydney district and New South Wales. 
