Because, probably, the excessive heat of those climates would interfere with the formation of wood from the sap upon the outer surface.
The vascular structure of endogenous stems lying more abundantly towards their centre, tends to conserve the juices which in hot climates are so highly valued. Palm-wine is a delicious and cooling beverage, and is procured from various kinds of palms, but especially from the cocoa-nut palm. Even the fresh sap is very refreshing. The juice is procured by cutting the tree in the upper part, and attaching a vessel to the opening, to receive the sap. Its flow is increased by cutting off a slice of the wood daily.
"I have caused thee to multiply as the bud of the field, and thou hast increased and waxen great, and thou art come to excellent ornaments."—Ezekiel xvi.
1282. Why have endogenous stems no bark?
Because, one of the chief functions of the bark in exogenous trees, is to protect the sap from which the wood is formed on the outward surface; and as there is no such external flow of sap in endogenous trees, the bark is unnecessary to them, and is therefore withheld. They are furnished instead with a thin cuticle.
1283. Why do endogenous stems grow to a great height?
Because, as the stem grows from the centre, it soon reaches that limit of diameter which its vascular structure is calculated to support; and, therefore, the wood-forming sap is deposited chiefly at the top of the stem, causing it to grow to a considerable height.
1284. Why do the various vegetable fruits ripen in succession?