188 MEDULLARY SHEATH. [BOOK n. 



p. 44. &c.) ; and Braschet imagines it and its processes to 

 constitute the nervous system of plants ! 



The MEDULLARY SHEATH seems to perform an important 

 part in the economy of plants; it diverges from the pith 

 whenever a leaf is produced ; and, passing through the petiole, 

 ramifies among the cellular tissue of the blade, where it 

 appears as veins : hence veins are always composed of bundles 

 of woody tissue and spiral vessels. Thus situated, the veins 

 are in the most favourable position that can be imagined for 

 absorbing the fluid which, in the first instance, is conducted 

 to the young pith, and which is subsequently impelled up- 

 wards through the woody tissue. So essential is the medul- 

 lary sheath to vegetation in the early age of a branch, that, 

 although the pith and the bark, and even the young wood, 

 may be destroyed, without the life of a young shoot being 

 much affected ; yet, if the medullary sheath be cut through, 

 the pith, bark, or wood, being left, the part above the wound 

 will perish. It may be supposed, considering the large pro- 

 portion of oxygen it contains, that its office is in part to 

 convey that gas to places inaccessible to the external air, 

 where it may combine with the carbon of such parts, and 

 cause the production of carbonic acid ; without a power of 

 composing and decomposing which, no part exposed to light 

 can long exist ; or it may be the natural means of conveying 

 into the atmosphere the oxygen produced by the decompo- 

 sition of carbonic acid or of water in the innermost recesses 

 of the stem. 



The BARK acts as a protection to the young and tender 

 wood, guarding it from cold and external accidents. It is 

 also the medium in which the proper juices of the plant, in 

 their descent from the leaves, are finally elaborated, and 

 brought to the state which is peculiar to the species. It is 

 from the bark that they are horizontally communicated to 

 the medullary rays, which deposit them in the tissue of the 

 wood. Hence, the character of timber is almost wholly 

 dependent upon the influence of the bark, as is apparent 

 from a vertical section of a grafted tree, through the line of 



