NATURAL HISTORY 



82f 



l^ral branches to be produced, or 

 *i!l pass into those already existing. 

 The forms of such branches will be 

 similar to that of the trunk ; and 

 the growth of the insulated tree on 

 the mountain, will be, as we always 

 find it, low and sturdy, and well 

 cakulated to resist the heavy gales 

 to which its situation constantly ex- 

 poses it. 



Let another tree of the same kind 

 be surrounded, whilst young, by 

 others, and it will assume a very 

 different form. It will now be de- 

 prived of a part of its motion, and 

 another cause will operate : the 

 leaves on the lateral branches will 

 be partly deprived of light, and, as 

 I have remarked in the last paper 

 I had the honour to address to you, 

 little alburnum will then be gene- 

 rated in those branches. Their vi- 

 gour of course becomes impaired, 

 and less sap is required to support 

 their diminished growth; more, 

 in consequence, remains for the 

 leading shoots ; these, therefore, ex- 

 ert themselves with increased energy, 

 and the trees seem to vie with each 

 other for superiority, as if endued 

 with all the passions and propensi- 

 ties of animal life. 



An insulated tree, in a sheltered 

 valley, will assume, from the fore- 

 going causes, a form distinct from 

 either of the preceding*; and its 

 growth will be more or less aspiring, 

 m proportion to the degree of pro- 



tection it receives from winds, and 

 its contiguity to elevated objects, 

 by which its lower branches during 

 any part of the day are shaded. 



When a tree is wholly deprived 

 of motion, by being trained to a 

 wall, or, when a large tree has been 

 deprived of its branches to be re- 

 grafted, it often becomes unhealthy, 

 and not unfrequently perishes, ap- 

 parently owing to the stagnation of 

 the descending sap, under the rigid 

 cincture of the lifeless external 

 bark. 1 have, in the last two years, 

 pared off this bark from some very- 

 old pear and apple trees, which had 

 been re-grafted with cuttings from 

 young seedling trees ; and the effect 

 produced has been very extraordi- 

 nary. JNIore new wood has been 

 generated in the old trunks, within 

 the last two years, than in the pre- 

 ceding twenty years ; and I attri- 

 bute this to the facility of commu- 

 nication which has been restored 

 between the leaves and the roots, 

 through the inner bark. I have had 

 frequent occasion to observe, that 

 wherever tiie bark has been most 

 reduced, the greatest quantity of 

 wood has been deposited. 



Other causes of the descent of 

 the sap towards the root, I have 

 supposed to be capillary attraction, 

 and something in the conformation 

 of the vessehs of the bark. The al- 

 burnum also appears, in my former 

 experiment, to expand and contract 



very 



* Not only ihe external form nf the troe, but the internal character of the wood, 

 will be affected by the situation in which the tree throws; and hence, oak timber 

 which grew ii\ crowded forests, apj)ear.s to have Ijc.eii mistaken, iu old buildings,, 

 for Spanish cliesnut. But I have found the internal organization of the oak and 

 Spanish chcsiiut to be very essentially different. The silver giain, and general 

 character of the oak and Spanish chcsiiut, are also so extremely dissiniihir, that 

 the two kinds of wood can only be tiiistaken for each other, by very careless ob- 

 Bcrvers. Alany piece* of wood found in the old buildings of London, and suppo&ed 

 to be Spanish chcsnut, have been put into niy hands, bat ihcy were aiiuioht eer- 

 ulidy foreiit oak. 



