FIRST, as to the Course of the Sap, there are Three Parts in which it moveth; sc. the Pith, the Wood and the Barque. First the Pith; in which the Sap moveth the First year, and only the First year. Or, it is Proprium quarto modo, to the Pith of every Annual Growth, and to the Pith of such a Growth only, To be succulent. That is, whether of a Sprout from a Seed, or of a Sucker from a Root, or of a Cyon from a Branch; The Pith is always found the First year full of Sap. But the Second year, the same individual Pith, always becomes dry, and so it continues ever after.
2. §. One cause whereof is, that the Lymphæducts in the Barque, being the first year adjacent to the Pith; they do all that time, transfuse part of their Sap into it, and so keep it always Succulent. But the same Lymphæducts, the year following, are turned into Wood; and the Vessels which are then generated, and carry the Sap, stand beyond them, in the Barque. So that the Sap being now more remote from the Pith, and intercepted by the new Wood, it cannot be transfused, with that sufficient force and plenty as before, into the Pith; which therefore, from the first year, always continues dry.
3. §. THE SECOND Part in which the Sap moves, sub forma liquoris, is the Wood. Which yet, it doth not in all Plants, but only in some; and visibly, in very few; as in the Vine: In a Vine, I say, the Sap doth visibly ascend by the Wood. And this it doth, not only the first year, but every year, so long as the Vine continues to grow. But although this ascent, in or through the Wood, be every year; yet it is only in the Spring, for about the space of a Month; sc. in March and April.
4. §. There are many other Trees, besides the Vine, wherein, about the same time of the year, the Sap ascendeth, though not so copiously, yet chiefly, in the Wood. For if we take a Branch of two or three years growth, suppose of Sallow, and having first cut the same transversely; if the Barque be then also transversely, and with some force, pressed with the back of the knife, near the newly cut end; the Sap will very plainly rise up out of the utmost Ring of Wood. And if it be pressed in the same manner, or a little more strongly, about an Inch lower, the Sap will ascend out of every Ring of Wood to the Center. Yet at the same time, which is to be noted, there ariseth no Sap at all out of the Barque.
5. §. Whence appears the Error of that so Common Opinion, That the Sap always riseth betwixt the Wood and the Barque. The contrary whereunto is most true, That it never doth. For the greater part of the year, it riseth in the Barque, sc. in the inner Margin adjacent to the Wood, and in Spring, in or through the Wood it self, and there only.
6. §. THE THIRD Part in which the Sap ascends, is the Barque, as was above hinted, and may be observ’d in almost any Branch, if cut cross, in the late Spring and in Summer; either as the Sap issueth spontaneously, or upon pressing, as aforesaid. So that when the Sap ceaseth to ascend, sub forma liquoris, by the Wood, then it begins to ascend by the Barque.
7. §. Besides the difference of Time, the Organical Parts likewise, in which these two Saps ascend, are divers. For in the Barque, it ascendeth visibly, only in the Succiferous, whereas in the Wood, it ascendeth only by the Aer-Vessels.
8. §. FROM what hath been said, we may understand, what is meant by the Bleeding of Plants. If we take it generally, it properly enough expresses, The eruption of the Sap out of any Vessels. And so, almost all Plants, in Summer time, do Bleed, that is, from Sap-Vessels, either in the Barque, or in the Margin of the Pith: the Saps they Bleed, having either a Sower, Sweet, Hot, Bitter, or other Tast. At which time, the Vessels also, in the Barque of a Vine-Branch, do Bleed a Sower Sap.
9. §. But that which is vulgarly called Bleeding, as in a Vine, is quite another thing; both as to the Liquor which issueth, and the Place where it issues: that is to say, it is neither a Sweet, nor Sower, but Tasteless Sap; issuing, not from any Vessels in the Barque, but from the Aer-Vessels in the Wood. So that there is as much difference betwixt Bleeding in a Vine, or the Rising of the Sap in any other Tree, in March, and in July; as there is betwixt Salivation and an Hæmorrhage; or betwixt the Course of the Chyle in the Lactiferous Vessels, and the Circulation of the Blood in the Arteries and Veins.
10. §. NOW the Cause from whence it comes to pass, that the early Spring-Sap of a Vine, and other Trees, ascendeth by the Wood, is, In that the Generation of the young Sap-Vessels in the Barque, by which the Sap ascendeth all the Summer; is, in the beginning of Spring, but newly attempted. So that the Sap having not yet these Vessels to receive it, it therefore (pro hac vice) runs up the Aer-Vessels in the Wood. But so soon as the said Vessels in the Barque begin to be considerably encreased, the Sap, declining the Aer-Vessels, betakes it self to These, as its most proper Receptacles.
11. §. THE CAUSE also, why the Vessels of almost all Plants, upon cutting, do yield Sap, or Bleed; is the Pressure which the Parenchyma makes upon them. For the Pith and other Parenchymous Parts of a Plant, upon the reception of Liquor, have always a Conatus to dilate themselves. As is manifest from Sponges, which are a Substance of the same Nature, and have a somewhat like structure. As also from Cork, which is but the Parenchyma or Barque of a Tree. I say therefore, that the Parenchyma being fill’d and swell’d with Sap, hath thereby a continual Conatus to dilate it self; and in the same degree, to press together or contract the Vessels which it surroundeth. And the said Vessels being cut, their actual Contraction and the Eruption of the Sap, do both immediately follow.
12. §. IT may be also noted, That the Trunk or Branch of any Plant being cut, it always bleeds at both ends, or upwards and downwards, alike freely. Which, as well as divers other Experiments plainly shews, That in the Sap-Vessels of a Plant, there are no Valves.
13. §. FROM what we have now above, and elsewhere formerly said, we may also understand the manner of the Ascent of the Sap. As to which, I say, First, That considering to what height and plenty, the Sap sometimes ascends; it is not intelligible, how it should thus ascend, by virtue of any one Part of a Plant, alone; that is neither by virtue of the Parenchyma, nor by virtue of the Vessels, alone. Not by the Parenchyma alone. For this, as it hath the Nature of a Sponge or Filtre, to suck up the Sap; so likewise, to suck it up but to a certain height, as perhaps, about an Inch, or two, and no more.
14. §. Nor by the Vessels alone, for the same reason. For although we see, that small Glass-Pipes immersed in Water, will give it an ascent for some Inches; yet there is a certain period, according to the bore of the Pipe, beyond which it will not rise. We must therefore joyn the Vessels and the Parenchyma both together in this Service; which we may conceive performed by them in the manner following.
15. §. Let A B be the Vessel of a Plant. Let C E D F be the Bladders of the Parenchyma, wherewith, as with so many little Cisterns, it is surrounded. Tab. 39. I say then, that the Sap, in the Pipe B A, would, of it self, rise but a few Inches; as suppose, from D to L. But the Bladders D P, which surround it, being swelled up and turgid with Sap, do hereby press upon it; and so not only a little contract its bore, but also transfuse or strain some Portion of their Sap thereinto: by both which means, the Sap will be forced to rise higher therein. And the said Pipe or Vessel being all along surrounded by the like Bladders; the Sap therein, is still forced higher and higher: the Bladders of the Parenchyma being, as is said, so many Cisterns of Liquor, which transfuse their repeated Supplies throughout the length of the Pipe. So that by the supply and pressure of the Cisterns or Bladders F D, the Sap riseth to L; by the Bladders Q L, it rises to M; by the Bladders N M, it rises to I; by the Bladders O I, it rises to K; by the Bladders P K, it rises to E; and so to the top of the Tree. And thus far of the Motion of the Sap.