Because the cylindrical form is stronger than any other; _a hollow cylinder_, with moderately thick walls, _is stronger than a solid rod_, containing the same amount of material. 1267. _Why do the stalks of plants become hollow?_ Because the parallel and perpendicular fibres of the stalk are developed _more rapidly than the horizontal_. The growth of the plant, therefore, consists of a kind of _divergence from the centre_. 1268. _Why are the stomata, or pores of leaves, generally placed on their under surface?_ Because, being placed on the under surface, they are _shaded_ from the action of the _sun's rays_, and so carry on the function of respiration more actively than if subjected to direct heat; they are also protected from the injurious _effects of dust_; and are moistened by _evaporation from the earth's surface_. [Verse: "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. 1271. _What is cork?_ 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.