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B O T A N Y. 



Vegetable although it is nearly universal. Some of the most 

 ysiolojjy. cur i ous f ac t s relative to it will come under the Sec- 

 * ~ * tion on Leaves. 



Bark. The bark or Cortex lies immediately below the 



cellular integument, and varies in the number of its 

 layers according to the age of the plant. Thus, it 

 consists of only one layer in a plant of one year old, 

 of two in one of two years old, and so on. The inner- 

 most layer performs some of the most important vi- 



t.ibr. tal functions, and is called liber, from some fancied 

 resemblance to the leaves of a book. Whenever a new 

 layer is formed, the liber of the preceding year be- 

 comes a lifeless envelope, and is pushed outwards with 

 the cellular integument. For a certain time, it ap- 

 pears that the older layers carry on some of the ope- 

 rations of the vegetable economy. The bark contains 

 innumerable fine woody fibres, which distinguish it 

 very essentially from the parts already described. In 

 some plants, the intersection by means of the woody 

 fibres gives it a singular and beautiful appearance.: 

 thus, in a plant of the Mezereon family, which grows 

 in Jamaica, the bark may be separated into the form 

 of very delicate lace. In some other trees, again, 

 this structure is not discernible j while in others it 

 may be seen, if the bark be exposed for some time to 

 the action of the weather. Of this last we have a 

 very good example in oak bark, which, after consi- 

 derable exposure to the action of the weather, sepa- 

 rates into thin layers, somewhat resembling the lace- 

 bark of Jamaica. The bark of many roots is re- 

 markably thick ; as in the carrot, the whole of the 

 red part of which is bark : in the parsnep and turnip 

 it is also very distinct, particularly so in the former. 



Wood. The part next to the bark is the wood ; and 



although there are many roots which have no 

 wood, yet as there are others that are chiefly com- 

 posed of it, we shall here describe its characters, 

 and afterwards mark the exceptions. The wood con- 

 sists of concentric layers, which in most trees is re- 

 markably distinct. These layers are hardest near to 

 the pith, and constitute the true wood. When 

 not quite hard, and of a different colour from the in- 



Alburmim. ternal part, it is called thealburuum by philosophers, 

 and the sap, or sap wood by workmen. This albvr- 

 nuin, according to the observations of Mr Knight, 

 appears to be generated by the action of the cortical 

 vessels, and performs important functions in the vege- 

 table economy, as will be more fully detailed here- 

 after. The -experiments of Duhamel generally con- 

 firm this opinion. Different woods exhibit consider- 

 able variety as to hardness, as well as to thickness : in 

 many trees, indeed, one side of the layers is so much 

 broader than the other, that the medulla or pith is not 

 . placed in tke common centre of the root. The tena- 

 city of the wood is owing to innumerable vessels, 

 such as will be hereafter described, that pass from 

 one part to another ; in general, they have a longi- 

 tudinal direction. These vessels perform several 

 functions, which also shall be discussed hereafter. 

 The cellular substance binds the whole firmly to- 

 gether. The colour of the wood differs very consi- 

 derably in different plants : this we have strikingly 

 illustrated in two familiar examples, the oak and the 

 ebony ; in the former of these it is brown, in the 

 latter black. 



It is the general opinion, that each of these con- Vegetal* 

 centric layers is the production of one year ; and this l'' 1 )^ ' !! 

 opinion derived much weight from the authority of ~~"V~' 

 Linnxus. It is supposed that the hard exterior 

 layer is formed by the cold of winter j so far, indeed, 

 has this opinion been carried, that it has been as- 

 serted, that the date of particularly severe winters 

 may be ascertained by the particular hardness of the 

 layers formed during them : some have gone still far- 

 ther, and add, that the northern side of a tree may be 

 known by the same means. Duhamel, Mirbel, and 

 Gerardin contend against this doctrine. The former 

 states, that a tree sometimes will not form a single 

 layer for a whole year, while at other times it will 

 form a very considerable number. Dr Smith does 

 not consider the facta adduced by the French physi- 

 ologists sufficiently strong to subvert the whole of 

 the ancient opinion ; and he adduces the uniform ap- 

 pearance of the wood of tropical trees, and of ever- 

 greens, in support of it. At the same time, he rea- 

 dily admits, that Duhamel has most completely ex- 

 ploded the belief of the influence of a northern aspect 

 already mentioned. The occasional deviations from 

 a uniform thickness may be accounted for, by sup- 

 posing the organs of the thickest side to be more 

 perfect than the others. This is probably the cause; 

 but it must be admitted, that it is a mere hypothesis. 



The manner in which this substance is formed has Fornutio 

 been long a fertile source of contention to physiolo- 

 gists. Grew and Malpighi supposed it to be form- 

 ed from the bark, and Dr Smith adopts this opinion. 

 Hales supposed that a new external layer was an- 

 nually formed by the wood itself. Linnaeus taught 

 that the pith secreted annually a new internal .layer. 

 Mirbel has given some amusing observations on it. 

 Duhamel ascertained, by making an incision into the 

 bark of a tree, and by introducing pieces of tinfoil 

 beneath it, that, after some years, the new wood is 

 exterior to the tinfoil, Dr Smith has seen the ori- 

 ginal specimens in the Museum at Paris. 



Dr Smith records another experiment made by Dr 

 John Hope, the late Professor of Botany in the Uni- 

 versity of Edinburgh, which decidedly supports those 

 of Duhamel. We shall give the account in Dr 

 Smith's own words. " The bark of a willow tree, 

 three or four years old, was carefully cut through 

 longitudinally on one side for the length of several 

 inches, so that it might be slipped aside from the 

 wood in the form of a hollow cylinder, the two ends 

 being undisturbed. The edges of the bark were then 

 united as carefully as possible, the wood covered up 

 from the air, and the whole bound up to secure it 

 from external injury. After a few years the branch 

 was cut through transversely. The cylinder of bark 

 was found lined with layers of new wood, whose num- 

 ber added to those in the wood from which it had 

 been stripped, made up the number of rings in the 

 branch above and below the experiment." 



The result of all the experiments made by Duha- 

 mel confirm the facts stated. The exception which he 

 gives of the bark being produced by the wood is so- 

 litary, and in some respects objectionable. The follow- 

 ing is the experiment alluded to. On taking off the 

 bark of a cherry stock, he observed a number of 

 gelatinous points on the surface of the woodj 



