CHAP. III. STEM. 257 



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 that, in the first instance, is conducted to 

 the young pith, and that is subsequently impelled upwards 

 through the woody fibre. So essential is the medullary 

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

 well known, 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, bai'k, or wood being left, the part above 

 the wound will pei'ish. It may be supposed, considering the 

 large proportion of oxj^gen it contains, that its office is to 

 convey that gas to parts inaccessible to the external air, and, 

 parting with it to the carbon of such parts, to cause the pi'o- 

 duction of carbonic acid, without a power of composing and 

 decomposing which no part exposed to light can long exist. 



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 tlie 

 bark that they are horizontally communicated to the medullary 

 rays, by them to be deposited 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 tlie line of union of the 

 stock and scion. This line will be found so exactly drawn 

 that the limits of the two are determined in the oldest spe- 

 cimens as accurately as if they were fixed by rule and line : 

 the woody tissue will be found uninterruptedly continuous 

 through the one into the other, and the bark of the two 

 indissolubly united ; but the medullary rays emanating from 

 the bark of each will be seen to I'emain as different as they 

 were at the time when the stock and scion were distinct indi- 

 viduals. It is remarkable that the bark has only a limited 

 power of impelling secreted matter into the medullary rays ; 

 and that there are certain substances which, although abundant 

 in bark, are never found elsewhere; as, for instance, gum 



s 



