London Horticultural Society and Garden, 509 



centre, is so loose, that it may be pulled out in strings ; but which, near 

 the outer bark, becomes by compression a dense substance, nearly as hard 

 as ebony. As the centre fills up, this dense substance gradually increases ; 

 till, at last, it will admit of no farther addition, and the whole trunk is 

 changed into an almost impenetrable mass. After this period the tree 

 begins to decay ; and, consequently, endogenous trees rarely attain any 

 great age. Nearly all the timber trees of this class are natives of tropical 

 climates. 



Exogenous plants are those which increase by concentric layers, de- 

 posited between the inner bark and the alburnum, or outer surface of the 

 soft wood. The substance enclosed by the bark being thus enlarged, the 

 outer bark frequently cracks, and flies off in flakes, its place being supplied 

 by a new bark formed beneath it. The diameter of exogenous trees thus 

 increases every year ; and as, under favourable circumstances, this process 

 may continue an indefinite length of time, no certain period can be fixed 

 for their decay. When creepers twine round the trunk of an exogenous 

 tree, they are frequently found, in the course of a few years, buried in the 

 bark, and they thus often destroy the tree which supports thein, by pre- 

 venting the proper circulation of the sap. This, of course, is not the case 

 with endogenous trees, which are never injured by the tropical climbers, 

 though these frequently attain the thickness of a man's wrist, and are 

 sometimes found with their folds growing together, from the principle of 

 adhesion, so as to form a complete shell. 



The centre of exogenous trees is called the heartwood, and is, in conse- 

 quence of the solidification of its vessels, unfitted for the circulation of 

 sap, like the other parts of the tree. It is always the first to decay; and 

 instances are often seen, where the trunk of a tree is completely hollow, 

 and yet the branches thrive, and bear leaves, flowers, and fruit. 



It is generally supposed that the age of exogenous trees may be counted 

 by the number of concentric layers which they contain ; but this rule Mr. 

 Lindley declared to have many exceptions. Sometimes two zones form 

 in one year, and sometimes, from the situation of the trees, or other acci- 

 dental circumstances, the zones are nearer together on one side than on 

 the other. Thus, calculations made by comparing the diameter of the tree 

 with the distance between the two outer zones, and thus endeavouring to 

 estimate the number' of zones in the interior, have been often found erro- 

 neous : and there is little doubt but that the enormous age attributed to 

 some trees, especially the baobab and the deciduous cypress, may be ex- 

 plained in this manner. 



Mr. Lindley concluded by saying that want of time obliged him to 

 postpone the consideration of roots till his next lecture. 



Lecture III. On the Roots of Plants, arid on the Formation of Timber. — 

 In the present lecture, the professor observed, he should commence with 

 -that portion of his former lecture which he had been obliged to postpone 

 for want of time, viz., some description of the structure and functions of 

 the root. The distinction between roots and stems appeared at first sight 

 so obvious as scarcely to need definition ; and yet it was difficult to dis- 

 tinguish, by mere external appearance, between detached portions of the 

 two. A piece of old root, sawed in two, and polished, strongly resembled 

 a piece of stem timber ; and a detached root of the screw pine might easily 

 be taken for the trunk of some kind of cane. Potatoes and other tubers 

 were also often called roots, when they were, in fact, as he should explain 

 hereafter, only portions of buried sterns. It was necessary, therefore, for 

 the botanist, or rather the physiological horticulturist, to look beyond mere 

 external appearances for the true difference between roots and sterns.^ 

 These might be found in the position, structure, growth, colour, mode of 

 extension, power of propagation, and functions of the root, all of which 

 Mere essentially different from those of the stem, 



