THIRD enquiry, is into the Generation and Structure of Parts. The manner whereof I have already endeavoured to explicate[61] from the Anatomy of the Root, throughout all particulars. Some whereof I shall yet further clear.
1. §. As First, the Union of the Barque to the Body of the Tree, Contrary to the common Opinion, That they are not continuous; but that the Barque only surrounds the Body, as a Scabbard does a Sword, or a Glove the Hand. As also seemeth to be proved, by the easy Slipping of the Barque of Willow, and most other Trees, when full of Sap, from the Wood.
2. §. But, notwithstanding this, they are as truly continuous, as the Skin of the Body is with the Flesh: sc. by means of the Parenchyma; which is one entire Body, running from the Barque into the Wood, and so uniting both together; Tab. 19. as in a Branch of Vine or Corin-Tree, when the Barque is stripped off, is apparent; the Spaces between the several Parts of the Wood, being filled up with the Parenchymous, inserted from the Barque.
3. §. Now the reason why the Barque nevertheless slips so easily from the Wood, is plain, viz. Because most of the young Vessels and Parenchymous Parts, are there every year successively formed; that is, betwixt the Wood and Barque: where the said Parts newly formed, are as tender, as the tenderest Vessels in Animals. And we may imagine, how easie it were at once to tear or break a thousand Vessels or Fibres of an Embrio, of a Womb or Egg.
4. §. THE same Vessels of the Barque being always braced, and gradually falling off, together with the Parenchyma, into the utmost Rind: Hence it is, that the Barques of many Trees, are as it were, Tab. 19. latticed with several Cracks of divers Sizes, and sometimes in the Figure of Rombs: the said Fissures representing the Position and Tract of the Vessels in their Braces. Hence also it is, that the Barque of some Trees, as of Corin, Cherry, &c. falleth off in Rings, sc. because the Sap-Vessels are posited in the same manner in the Barque.
5. §. The Sap-Vessels, as they are generated at the inner Verge of the Barque: so likewise, in a small quantity, at the utmost Verge of the Pith. These being not only fed with a more vigorous Sap, but with great caution, secured within the Wood, for the propagation of the succeeding Buds.
6. §. Hence also it is, that is, by the annual accretion of these Vessels, that the Pith is sometimes less in the Trunk, than in the Branches; Tab. 18. and less in the elder Branches, than in the younger; and sometimes ’tis almost wholly filled up. By which means, as the Branches carry every year a greater burthen; so they become still more sturdy the better to support it.
7. §. SOMETIMES also the Pith breaks and shrinks up, thus making the Trunk a Pipe. The cause whereof, is either the Largeness of its Pores, or the Thinness of the Sides of the said Pores; upon both which accounts, the Pith doth more easily tear, and upon tearing shrink up, and so become hollow: as in Cichory, Lampsana, Sonchus, Teasel, Brownwort, and others; wherein the Pores of the Pith are Large, and the Sides of the Pores, Thin. Whereas, upon contrary accounts, the Piths of most Trees, remain perpetually entire.
8. §. THE Reason why Plants are made thus to become hollow, is partly, for the ripening of the Fruit or Seed; which is the better effected by a more plentiful supply of Aer continually received into their hollow Trunks. For by means of that Aer, part of the Sap, is dryed up, and the remaining part of it made warmer, and so sooner matured.
9. §. Partly, for the better determining the due Age of the Plant. Hence it is, that the greater part of Annual Trunks, are hollow: the Aer contained in that hollow, drying up the Sap, and shrinking up the Sap-Vessels so far, as to hinder the free motion of the Sap therein; from whence the Plant must needs perish. So that as the Content of the Aer-Vessels, is a kind of Vegetable Aer, whose Office is to Attenuate, and Ferment the Juyces of Plants: so the Content of these Cavities, cometh nearer to a more common Aer, designed chiefly, so soon as it is convenient, to dry them up.
10. §. AGAIN, as to the Aer-Vessels, divers questions may be asked. As how it comes to pass, that they are generally less in the Trunk of the same Plant, than in the Root? The Cause whereof is, that here in the Trunk they are more under the power of the Aer; both that which entreth in at the Trunk, and that which of its own Nature ascendeth up into it from the Root. For the Aer, as we have elsewhere said, is the Mould of the Aer-Vessels; to whose crooked or at least, Acid Parts, the Saline, and other Principles concurring to their generation, do conform. To which they do best, the smaller they are: the Fibres of the larger Aer-Vessels making greater Circles, and so coming nearer to a right Line, answerable to the Figure of the Particles, not of the Aerial, but of the Saline Principle.
11. §. Wherefore as the Aer-Vessels may be observed still to be dilated or widened towards the lower parts of the Root, the Aerial Principle being there less predominant, and the Saline more: So towards the upper part of the Trunk, to be contracted or grow smaller; the Aerial Principle being here more predominant, and the Saline less.
12. §. FOR the same cause it may be observed, That the Aer-Vessels of the Second years Growth, and the several years succeeding, are usually nearer of one Size, than those of the Second and First; all being under a less power of the Aer, than the First. For the first year the Pith being full of Liquor, the Aer-Vessels themselves, are the only Repositories of the Aer. Whereas after the first year, the Pith becoming dry, or another great Repository for the Aer; the Aer-Vessels are henceforth filled with a moister or more Vaporous and Saline Aer, and so made to grow wider.
13. §. Hence the very Size of the Pith, hath much influence upon the Aer-Vessels, and the manner of Nutrition, and the Generation of Liquors in Plants.
14. §. BUT for the most part, the Aer-Vessels are somewhat, more or less, amplified in every new Annual Ring; or at least to a certain number of years. Probably, because in the elder Branches, the Spiral Fibres, of which the Vessels consist, are more bulky; and so make a Vessel of a wider, as a more agreeable bore. Nature obtaining hereby, that the Quantity of Aer, shall always be answerable to the Growth of the Plant, or at least, be sufficient to maintain its Vegetable Life and Vigour.
15. §. And therefore, as is above hinted, it seems likely, That after a certain number of years, the Aer-Vessels are no longer amplified, but stand at a stay, and perhaps may grow smaller, according as the Tree is less or more Longæve; and that after this period, it is some way or other in its Declining State.
16. §. LASTLY, from the Content and Governing Principle of the Aer-Vessels, the Time, when they begin every year to be formed, or to appear, is always later; at least with respect to the season of the Tree. So that whereas the Sap-Vessels begin to be formed in Spring: these, not till the latter end of Summer, or there about; at least not till about that time to appear. That is, when the Sap begins to decrease, and to grow more Aery; and so more fit matter for the Generation of the said Aer-Vessels.