The other class is called endogenous,
and increases by layers applied to the inside; and when the hollow there
is full, the growth is stopped - the tree must die. Any injury
is felt most severely by the first class on the bark; by the second
on the inside; while the inside of the exogenous may be removed,
and the outside of the endogenous may be cut, without stopping the growth
in the least. The mowana possesses the powers of both. The reason is that
each of the laminae possesses its own independent vitality;
in fact, the baobab is rather a gigantic bulb run up to seed than a tree.
Each of eighty-four concentric rings had, in the case mentioned,
grown an inch after the tree had been blown over. The roots,
which may often be observed extending along the surface of the ground
forty or fifty yards from the trunk, also retain their vitality
after the tree is laid low; and the Portuguese now know that the best way
to treat them is to let them alone, for they occupy much more room
when cut down than when growing.
The wood is so spongy and soft that an axe can be struck in so far
with a good blow that there is great difficulty in pulling it out again.
In the dead mowana mentioned the concentric rings were well seen. The average
for a foot at three different places was eighty-one and a half of these rings.
Each of the laminae can be seen to be composed of two, three, or four
layers of ligneous tubes; but supposing each ring the growth of one year,
and the semidiameter of a mowana of one hundred feet in circumference
about seventeen feet, if the central point were in the centre of the tree,
then its age would lack some centuries of being as old
as the Christian era (1400). Though it possesses amazing vitality,
it is difficult to believe that this great baby-looking bulb or tree
is as old as the Pyramids.
The mopane-tree (`bauhinia') is remarkable for the little shade
its leaves afford. They fold together and stand nearly perpendicular
during the heat of the day, so that only the shadow of their edges
comes to the ground. On these leaves the small larvae of a winged insect
appear covered over with a sweet, gummy substance. The people collect this
in great quantities, and use it as food;* and the lopane -
large caterpillars three inches long, which feed on the leaves,
and are seen strung together - share the same fate.
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* I am favored with Mr. Westwood's remarks on this insect as follows:
"Taylor Institution, Oxford, July 9, 1857.
"The insect (and its secretion) on the leaves of the bauhinia,
and which is eaten by the Africans, proves to be a species of Psylla,
a genus of small, very active Homoptera, of which we have
one very common species in the box; but our species, Psylla buxi,
emits its secretion in the shape of very long, white, cotton-like filaments.
But there is a species in New Holland, found on the leaves
of the Eucalyptus, which emits a secretion very similar
to that of Dr. Livingstone's species.
