THE Fifth Head, shall be, of the Figuration of Trunks. Which also, as well as the making of Liquors, dependeth upon the Structure of the Parts. As First, almost all Shrubs (cæteris paribus) have a greater number of Aer-Vessels; and those of a smaller Size; and consequently much spread abroad, as most easily yielding to the magnetick Power of the Aer, according as we have more fully demonstrated, in speaking of the Vegetation of Roots: as in Elder, Hazel, Fig, Sumach, and the like. By which spreading, the said Aer-Vessels do sooner, and more easily strike into the Barque, and so produce collateral Buds and Branches, and that upon the first rising of the Body from the Root: that is, the Plant becomes a Shrub.

2. §. BUT if the said Aer-Vessels are very large, they will not yield so easily to shoot out collaterally; and so the Trunk grows up taller and more entire: as in Oak, Wallnut, Elm, &c. wherein they are exceeding large, is seen. Hence also the Vine, if supported, will grow to a prodigious length. And Hops and Bryony, are some of the tallest, amongst all Annual Growths: the Aer-Vessels of all which, are very large. Whereas Borage, and many other like Plants, although the Pores of their Parenchyma, are vastly wide, and filled with Sap; yet because their Aer-Vessels are small, they are therefore but Dwarf-Plants. Wherefore the tallness or advancement of a Plant or Tree, dependeth not upon the Plenty of Sap, how great soever, but on the Largeness of the Aer-Vessels.

3. §. AGAIN, as a Plant or Tree grows either Shrubby, or Tall and Entire, according to the Size of the said Vessels: so from their Position, doth it grow Slender or Thick. So, where they keep more within the compass of a Ring, as in Elm, and Ash, the Tree, in proportion, usually grows taller, and less thick. But where the said Vessels are spread more abroad, and especially are postured in Rays, as they are in Oak, the Tree grows very thick. Because the said Vessels thus standing all along nearer to the Insertions, there is a more ready and copious passage of the Aer out of the one into the other; and so the Diametral growth of the Wood is more promoted.

4. §. LASTLY, from the same general cause it is, That the Trunks of Vegetables are either Round or Angular. Those of all Trees are Round. Because the Barque, being here thicker, and the Aer-Vessels bound up with a greater quantity of Wood; the Aer hath not sufficient power to move them, and the Barque with them, into those various Positions or Figurations, as the Trunks of Herbs do yield to.