162 THE VEGETABLE KINGDOM. 



which has thus risen is the juice of the earth in which the 

 plant grows, containing several earthy salts and vegetable 

 extract drawn from the manure or decaying vegetation 

 contained in the mould, together with carbonic acid 

 dissolved in the fluid ; this carbonic acid is changed by the 

 sun's rays as well as that which was contained in the air, 

 and the carbon uniting with the watery part of the sap, 

 forms the green substance before alluded to, called chloro- 

 phyll, which is the green colouring matter of all plants, 

 and is the basis of the wood. The altered sap descends 

 between the wood and the bark, and forms a deposit gradu- 

 ally, which at the end of the year is a complete ring of 

 wood surrounding the wood of former years. This circula- 

 tion of juices continues through the summer, until, the cold 

 weather coming on and the light being diminished, the sap 

 neither rises nor is the leaf nourished by it, when it decays 

 and falls off. 



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 pro- 

 bability, 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 Tew 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 : 



