"The trees of the Lord are full of sap: and the cedars of Lebanon which he hath planted."—Psalm civ.
1269. Why have plants a formation of pith in their centre?
The pith is the chief organ of nutriment, especially in the young plant. It is the structure which first conveys fluids to, and receives them from, the newly-formed leaf. It communicates with every branch, leaf, bud, and flower; and also with the bark, through the medullary rays, which radiate horizontally from the centre of the plant. It is the centre of the movements of the sap which occur in the horizontal vessels; and it holds an important influence over the life of the plant.
1270. Why are trees covered with bark?
Because the bark serves to protect the woody structure, and also to give a passage to the descending sap which flows abundantly in the spring, and out of which the woody fibre is formed. It is also, from its peculiar nature, well fitted to endure the changes of the seasons for many years; and from its non-conducting properties it serves to maintain the equal temperature of the vital parts of the tree.
Cork is the bark of a description of oak-tree, which grows in great abundance in Spain, Italy, and France.
1272. Why does the cork-tree release its own bark?
Because it possesses a bark which is exceedingly useful to man; and it seems, therefore, to have been the design of providence that the tree should cast it off, to be applied to the wants of the human family; for the cork-tree does not discharge its bark by the mere cracking, or exfoliation, of its substance; the tree retains the bark for a number of years, until it has attained that consistency and thickness which renders it useful, and then the tree forms within the bark a series of tabular cells, which cut off the connection of the bark with the internal structure, after which it peels off in large sheets.