But these Pores, although they are a free and open way to the ascending Sap; yet that meer Pores or Vessels should be able of themselves to advance the Sap with that speed, strength & plenty, and to that height, as is necessary, cannot probably be supposed. It follows then, that herein we must grant the Pith a joynt service. And why else in the smaller parts of the Root, where the Pith is often wanting, are the Pores there greater? Why is the Pith in all primitive growths the most Sappy part, why hath it so great a stock of Sap, if not after due maturation within it self still to be disbursed into the Fibres of the Lignous Body? Why are the annual growths of all both Plants and Trees with great Piths, the quickest and the longest? But how are the Pores of the Pith permeable? That they are so, both from their being capable of a repletion with Sap, and of being again wholly emptied of it, and again, instead thereof fill’d with Ayr, is as certain as that they are Pores. That they are permeable, by the breadth, appears from the dilatation of the Lignous Body, and from the production of Branches, as hath been, and shall hereafter be said. And how else is there a Communion betwixt this and the Cortical Body? That they are so also, by the length, is probable, because by the best Microscope we cannot yet observ, that they are visibly more open by the breadth, than by the length. And withal are ranked by the length, as those of the Insertions by the breadth of the Trunk. But if you set a piece of dry Elder-Pith in some tinged Liquor, why then doth it not penetrate the Pores, so as to ascend through the Body of the Pith? The plain reason is, because they are all fill’d with Ayr. Whereas the Pith in a Vegetating Plant, as its Parts or Pores are still generated, they are at the same time also fill’d with Sap; which, as ’tis gradually spent, is still repair’d by more succeeding, and so the Ayr still kept out; as in all primitive growths, and the Pith of Elder it self: Yet the same Pith, by reason of the following Winter, wanting a more copious and quick supply of Sap, thus once become, ever after keeps dry. And since in the aforesaid Trial the Liquor only ascends by the sides of the Pith, that is of its broken Pores, we should thence by the same reason conclude that they are not penetrable by the breadth neither, and so no way; and then it need not be ask’d what would follow. But certainly the Sap in the Pores of the Pith is discharged and repaired every moment, as by its shriv’ling up; upon cutting the Plant is evident.
We suppose then, that as the Sap ascendeth into the Trunk by the Lignous Body, so partly also by the Pith. For a piece of Cotton with one end immers’d in some tinged Liquor, and with the other erect above, though it will not imbibe the Liquor so far as to over-run at the top, yet so as to advance towards it, it will; so here, the Pith being a porous and spongy Body, and in its Vegetating state its Pores also permeable, as a curious Filtre of Natures own contrivance, it thus advanceth, or as people use to say, sucks up the Sap. Yet as it is seen of the Liquor in the Cotton; so likewise are we to suppose it of the Sap in the Pith; that though it riseth up for some way, yet is their some term, beyond which it riseth not, and towards which the motion of the ascending Sap is more and more broken, weak and slow, and so the quantity thereof less and less. But because the Sap moveth not only by the length, but breadth of the Pith; at the same time therefore as it partly ascendeth by the Pith, it is likewise in part pressed into the Lignous Body or into its Pores. And since the motion of the Sap by the breadth of the Pith not being far continued, and but collateral, is more prone and easie than the perpendicular, or by its length; it therefore follows, that the collateral motion of the Sap, at such a height or part of the Pith, will be equally strong with the perpendicular at another part, though somewhat beneath it; and that where the perpendicular is more broken and weak, the collateral will be less; and consequently where the perpendicular tendency of the Sap hath its term, the collateral tendency thereof, and so its pressure into the Pores of the Lignous Body will still continue. Through which, in that they are small, and so their sides almost contiguous, the Sap as fast as pressed into them will easily run up; as betwixt the two halves of a Stick first slit, and then tied somewhat loosely together, may also any Liquor be observed to do. And the sides of the said Pores being not smooth, but by the intrusion of the smallest insertions made somewhat rough; by that means the higher and more facile ascent of the Sap therein will farther be promoted. By all which Advantages the facility and strength of that ascent will be continued higher in the said Pores than in the Pith. Yet since this also, as well as that in the Pith will have its term; the Sap, although got thus far, would yet at last be stagnant, or at least its ascent be very sparing, slow and feeble, if not some way or other re-inforced. Wherefore, as the Sap moving by the breadth of the Pith, presseth thence into the Pores of the Lignous Body; so having well fill’d these, is in part by the same Collateral motion disbursed back into a yet higher Region of the Pith. By which partly, and partly by that portion of the Sap, which in its perpendicular ascent was before lodged therein; ’tis thus here, as in any inferiour place equally repleat. Whereupon the force and vigour of the perpendicular motion of the Sap herein will likewise be renew’d; and so its Collateral motion also, and so its pressure into the Pores of the Lignous Body, and consequently its ascent therein; and so by a pressure from these into the Pith, and from the Pith into these reciprocally carried on, a most ready and copious ascent of the Sap will be continued from the bottom to the top, though of the highest Trunk.
An Appendix.
Of Trunk-Roots and Claspers.
The distinct Parts whereof these are constituted, are the same with those of the Trunk, and but the continuation of them.
Trunk-Roots are of two kinds: Of the one, are those that vegetate by a direct descent: The place of their Eruption is sometimes all along the Trunk; as in Mint, &c. Sometimes only at its utmost point, as in the Bramble.
The other sort are such as neither ascend nor descend, but shoot forth at right Angles with the Trunk; which therefore, though as to their Office, they are true Roots, yet as to their Nature, they area Middle thing betwixt a Root and a Trunk.
Claspers, though they are but of one kind, yet their nature is double; not a mean betwixt that of the Root and that of the Trunk, but a compound of both; as in their Circumvolutions, wherein they often mutually ascend and descend, is seen.
The use of these Parts may be observed as the Trunk mounts, or as it trails. In the mounting of the Trunk, they are for support and supply: For support, we see the Claspers of Vines; the Branches whereof being very long, fragile and slender, unless by their Claspers they were mutually contain’d together, they must needs by their own weight, and that of their Fruit, undecently fall, and be also liable to frequent breaking. So that the whole care is divided betwixt the Gardener and Nature; the Gardener with his Ligaments of Leather secures the main Branches; and Nature with these of her own finding, secures the less. Their Conveniency to which end, is seen in their Circumvolutions, a motion not proper to any other Part: As also in their toughness or strength, though much more slender than the Branches whereon they are appendent.
For Supply, we see the Trunk-Roots of Ivy: For mounting very high, and being of a closer Constitution than that of a Vine, the Sap could not be sufficiently supplied to the upper Sprouts, unless these to the Mother-Root were joyntly assistant. Yet serve they for support likewise; whence they shoot out, not as in Cresses, Brook-lime, &c. reciprocally on each side, but commonly all in one; that so they may be fastened at the nearest hand.