light being diminished, the sap neither rises nor is the leaf nourished by it, when it decays and falls off. [Illustration: FIG. 16.--PERPENDICULAR SECTION OF EXOGENOUS WOOD (ELM), A, the pith, B B, the woody rings, C C, the bark.] [Illustration: FIG. 17.--PERPENDICULAR SECTION OF EXOGENOUS WOOD (CEDAR), MAGNIFIED.] [Illustration: FIG. 18.--TRANSVERSE SECTION OF EXOGENOUS WOOD (ELM), SHOWING THE GROWTH OF NINE YEARS.] [Illustration: FIG. 19.--TRANSVERSE SECTION OF EXOGENOUS WOOD (CEDAR) MAGNIFIED, SHOWING THE GROWTH OF FIVE YEARS.] The age of exogenous wood can be ascertained--where the centre has not decayed--by counting the rings, one only being deposited every year; and it is truly astonishing to find that some trees will continue to live and flourish for several thousand years! There does not, in truth, appear to be any limit assigned to the life of an exogenous tree if it escape accidents; for, although decay inevitably attacks the heartwood, and a cavity is the result, yet, the new wood continuing to be deposited on the outer part, the vitality of the tree is kept up, and its size continues to increase. The Baobab or Monkey Bread-fruit trees, growing at the mouths of the Senegal, have been estimated by Adinson to be upwards of six thousand years old, and are, in all probability, the oldest relics of organic life existing at the present time. The cedars of Lebanon are supposed to have existed longer than the records of history. The Yew at Braburn, in Kent, is at least three thousand years old; and that of Fortingal nearly as much. Dr. Livingstone, describing the Mowana or Baobab tree (fig. 20), thus comments upon its power of withstanding injury:-- [Illustration: FIG. 20.--THE BAOBAB TREE.]