268 Notes. 
puzzled, by the conditions under which I found mangroves growing 
in these regions. 
The coasts of the whole of British and German East Africa are 
composed of a hard coral limestone of peculiar properties. (For 
a full account of this see my papers in the Proc. Phil. Soc. of 
Cambridge, vol. ix, pt. vi, Part i, ‘ On the Coral Reefs of Zanzibar/) 
The erosion of the waves has cut down this rock so that at low-tide 
there is an almost perfectly plane surface of rock, sloping gradually 
from the base of the cliffs to low-water level. In creeks and sheltered 
places generally, near high-water mark, this rock plane is full of 
irregular small holes and crannies, but no loose stones or deposits, 
other than a very thin coating of fine mud, interrupt its uniformity. 
On this hard surface, sending their roots into the crannies, the 
greater number of the mangroves of Zanzibar flourish so well that 
a considerable trade is carried on from Chuaka Bay 1 in their stems. 
(These are used in the building of all the Arab and native houses of 
Zanzibar, being too hard for the jaws of the termites.) Only occa- 
sionally do we find mangroves growing in mud and see the demon- 
stration of the well-known method of planting, viz. by the impact of 
its fall forcing the root of the embryo into the mud. In the majority 
of cases one finds the embryo placed in one of the holes of the rock, 
which is usually of but slightly larger diameter than itself. Obviously 
it did not fall by chance into this position ; suitable holes are not so 
numerous, and the insertion of the radicle into them not so easy as 
this would imply. Moreover, I have often observed embryos neatly 
planted in these holes at a distance of more than a hundred yards 
from the shade of the nearest possible parent tree, and in a few cases 
at a distance of miles. 
How this planting could be done, except by human hands, remained 
for a long time a mystery to me. The solution came when I noticed 
the frequency with which I met embryos floating in the sea, being 
carried out of the bay by the strong tidal currents. Often I passed 
through fleets of them, as it were, all floating in the same peculiar 
way, viz. vertically, with the leaf-bud just projecting from the water. 
(See the figure.) A consideration of the shape of the radicle shows 
that not only is there a perfect adjustment of the specific gravity of 
the whole to that of the sea water, but a peculiar distribution of it in 
1 A large and very shallow bay on the east coast of Zanzibar island, where most 
of my time was spent. 
