AGE OF EXOGENOUS TREES. 101 



remains in the newest layer of bark, it will be gradually 

 brought to the surface and will fall out. Such experiments 

 have been tried, for the purpose of showing the mode, in which 

 these two parts of the stem respectively grow. In the same 

 manner, it is not unfrequently found, when a tree is cut 

 through, that the ends of the lateral branches, which had 

 either died, or had been cut off, at an early period of its life, 

 are covered by a number of layers of wood, corresponding to the 

 number of years that have subsequently elapsed. 



139. As the bark is sufficiently distensible, to admit of the 

 increase of diameter of the interior of the stem, there is no 

 necessary limit to the age of Exogens ; and there are many 

 unquestionable examples of such trees having attained an 

 enormous longevity. In this country, the Oak and the Yew 

 appear to be the longest-lived. At Ellerslie, the birthplace of 

 "Wallace, exists an Oak, which is celebrated as having been a 

 remarkable object in his time; and which can scarcely, there- 

 fore, be less than 700 years old. Near Staines, there is a Yew 

 tree older than Magna Charta ; and the Yews at Fountains 

 Abbey, in Yorkshire, are probably more than 1200 years old. 

 Eight Olive trees still exist in the Garden of Olives at Jerusalem, 

 which are known to be at least 800 years old. But the rate of 

 increase in old trees is by no means the same as in young ; so 

 that, when they are grown for the profit to be derived from their 

 timber, it is not advantageous to let them pass a certain age. 

 Thus the rate of growth in the Oak diminishes greatly, after 

 about seventy years ; that of the Larch, after sixty ; and that of 

 the Elm, after about sixty-five. 



140. It is in the vessels and woody tubes of the alburnum, 

 that the fluid absorbed by the roots, is transmitted to the 

 opposite extremity of the stem ; and these vessels communicate 

 with those of the leaves, which receive it from them. In the 

 liber, on the other hand, the fluid which has been converted in 

 the leaves into nutritious sap, descends again through the trunk, 

 for the purpose of nourishing its different parts. Of this de- 

 scending sap, a part is carried inwards by the medullary rays; 

 which thus diffuse it through the whole stem, as also through 



