119 



whicli is5 always cflecled by a luiioii ol" tlic libcr ol' the gial'i, 

 witli the stock.* 



" There is no fixed or definite period (as Keitli observes,) 

 that can positively be assigned, as necessary to tlie complete 

 induration of the wood or bark, although it seems to require 

 a good many years, before any particular layer is converted, 

 from the state of alburnum to that of perfect wood."t In 

 respect to baric, there is not any circumstance, which hastens 

 this period so much, as the exposure of trees to the elements, 

 even at an early period of their age. In the same way, as 

 the action of the air multiplies or thickens the branches and 

 spray, the cellular tissue and cortical layers are thickened, 

 for the protection of the proper vessels. The inner layers 

 being always the softest, the outer by this means gradually 

 increase in solidity and thickness, and become visibly indu- 

 rated ; so that with some trees it even sloughs, and splits 

 into chinks and fissures, as in the case of the sycamore, the 

 elm, and the fir. In this condition, we may conceive, how 

 well adapted such a mass of non-conducting matter is, to 

 protect from cold the ascending, and still more, the descend- 

 ing or proper vessels, as already mentioned. 



In order to assist the reader, in forming a clear conception 

 of the great value of a proper thickness of bark to trees in- 

 tended for removal, it will be necessary to inquire a little 

 into the means, by which the sap-vessels minister to the 

 sustenance of plants. In the warmer latitudes, the sap flows 

 in certain plants, during the whole yeai- ; but in those that 

 are more temperate, the functions of vegetables are suspended, 

 or nearly so, during the winter season. Early in the spring, 

 however, it begins to rise in woody plants, and continues to 



* Senebier,Phys. Veget. T. I. pp. 177, 178 — Keith, Vol. II. p. 299. 

 — Knight, Philosoph. Trans. 1803. — Elhs, Aniit. Veget. in Suppl. 

 Encyclop. Britan. 



t Physiolog. Bol. Vol. II. p. 231. — Sec also Kiei;er, Organis. dca 

 Piantes, ch. II. pp. 95, 96, et seqq. Also p. 153, «S:c. 



