310 FRUIT AND FOREST TREES. 



constitution, as will be shown in the succeeding analyses, from the wood. In this part, 

 the phosphates are deficient : altliough they are always present, still there is a great di- 

 minution in their amount, and to this statement an exception is not known. The bark, in 

 its chemical constitution, is composed mainly of lime, probably an organic salt of lime, 

 which by ignition is converted into a carbonate. There is also a great deficiency of potash 

 and soda : they are equally small in amount with the phosphates. One or two curious 

 facts, however, are observed in regard to the alkalies : it was found, for instance, that the 

 outside and apparently lifeless part of the bark (the corky part of elm bark, for example) 

 was richer in potash than (he inside bark. This may be, and probably is, an isolated 

 fact. It is well known to common observers that the elm is particularly rich in potash, 

 which pervades all its parts and organs more generally than in other trees. 



The final cause of the accumulation of lime in the bark, is to serve as an outward 

 defence of the internal or young wood : it secures a firmer covering than can be provided 

 by the other elements. In this respect there is a very close analogy with the formation of 

 the outer covering of the lower animals, which, as is well known, consists mainly of car- 

 bonate of lime, as in the Crustacea and annulose animals. As in the animal tissues, out- 

 ward defences are set up, so in the vegetable they are not left wanting. Here too a provision 

 is furnished, by which this great amount of lime is returned to the soil. The bark is 

 annually detached, and its store of soluble or partially soluble salts of lime is rapidly 

 returned to the soil. An aged oak may have regenerated its bark repeatedly from its own 

 debris : its outward covering, which is partially renewed every year, may derive a portion 

 of its supply from what previously formed part of its own organism. 



Upon this general fact is founded the law of distribution, the inorganic matters respec- 

 tively being determined to the periphery of the head and trunk. The few exceptions 

 which have been found, and which militate against this law, can hardly be regarded as 

 sufficiently numerous and important to overthrow it. 



Of the substances which appear to be more steadily and uniformly determined to the 

 outside wood, I may state the following, leaving out of the account water, which acts as 

 the medium through which every thing is conveyed into and throughout the tree. 



The phosphates abound more in the periphery than in the centre : they are found also 

 in larger quantities in the young wood of the twigs ; therefore both the outside of the tree, 

 and its growing branches, store up for the time being a larger amount of these essential 

 elements than the interior. It is by this mode of distribution that the seeds and fruit re- 

 ceive their needful supply of phosphates : at least the supply is derived more immediately 

 and directly, than jf the contrary mode of distribution prevailed. 



The final cause of this distribution of the phosphates, is to restore again to the soil those 

 important elements. They pass along the outside wood of the trunk, obtaining probably 

 an accession from the interior by the outward or lateral movement of the sap : they then 

 pass to the seed and fruit, through the small branches, twigs and leaves. All the surplus 

 amount of the phosphates in the leaves is restored to the earth by their fall and decay ; 

 and, besides, in a multitude of instances, the seed and fruit also return by their maceration 



