HE NEXT enquiry to be made, is, into the Motion and Course of the Aer. Where this question will first of all be asked; sc. Which way the Aer first enters the Plant; whether at the Trunk, Leaves, and other Parts above ground; or at the Root? I answer, That it enters in part, at them all. For the Reception, as well as Extramission whereof, the Pores are so very large, in the Trunks of some Plants, as in the better sort of thick walking Canes, that they are visible, to a good Eye, without a Glass; Tab. 19. but with a Glass, the Cane seems, as if it were stuck top full of holes with great Pins: being so large, as very well to resemble the Pores of the Skin in the end of the Fingers and Ball of the Hand.
2. §. In the Leaves of Pine, they are likewise through a Glass, a very Elegant Show; standing all most exactly, in rank and file, throughout the length of the Leaves. The Figure whereof shall be given hereafter, when we come to the Anatomy of the Leaf.
3. §. But although the Aer enters, in part, at the Trunk and other Parts, especially in some Plants; yet its chief entrance, is at the Root. Even as some Parts of Aer, may continually pass into the Body and Blood, by the Habit, or Pores of the Skin; but the chief entrance hereof, is at the Mouth. And what the Mouth is, to an Animal; that the Root is to a Plant.
4. §. Again, if the chief entrance of the Aer, were at the Trunk; then, before it could be mixed with the Sap in the Root, it must descend; and so move not only contrary to its own Nature, but likewise in a contrary Course to the Sap, throughout the Plant. Whereas, by its Reception at the Root, and so its Transition from thence; it hath a more natural and easie motion of Ascent. For while the Sap ascends, that the Aer, in the same Plant, should continually descend, cannot reasonably be supposed.
5. §. The same is further argued, From the fewness and smallness of the Diametral Portions in the Trunk in comparison with those in the Root. In which Nature hath plainly designed the same, for the Separation of the Aer from the Sap, after they are both together received thereinto. So that the Reception and Course of the Aer, is made on this manner following.
6. §. THE Aer being a Springy Body, it insinuates into all the Holes and Cranies of the Earth; and so is plentifully mixed therewith. Whereupon, as the Sap enters the Root, more or less Aer still intrudes it self together with it. The Liquid Portion of the Sap, swells and fills up the Succulent Parts of the Barque. The Aery Part, is, as was said, separated from the Liquid, into the Diametral Portions. Which running from the Barque towards the Centre of the Root, and so passing along betwixt the Aer-Vessels; do hereby convey the Aery Part of the Sap from the Barque, into the same.
7. §. Being thus received into the Aer-Vessels, and the Reception thereof, by the same means continued; it is by them advanced into the Trunk. In which advance, it is again, more or less, disbursed into all the Parts of the Trunk, as it goes. Partly, inwards to the Pith. From whence, the Pith is always, at length, filled with Aer. Partly, into the Insertions; by which it is conveyed outward into the Barque. Wherein, it is in some part, transfused through the Sap: and so the rest, with part of the Sap, remitted, in perspirations, back again into the Aer.
8. §. So that, whereas the Diametral Portions in the Root, do serve to convey the Aer from the Sap in the Barque, into the Aer-Vessels, in the Wood: on the contrary, the Insertions here in the Trunk, serve to convey the Aer from the Aer-Vessels in the Wood, into the Sap, in the Barque. Wherefore, as the Aer-Vessels advance the Aer, or the Aery Part of the Sap, and so convey it by the length of the Trunk; so the Insertions filter it, and convey it by the breadth.
9. §. AND that the Insertions have this Office or Subservience unto both Kinds of Vessels; doth yet further appear, if we consider, That the Aer-Vessels are always so postured, as to touch upon the said Insertions, or at least to stand very near them. For either they are large, and so do frequently touch upon them on both sides; as in Elm, Ash, Wallnut, &c. Tab. 28. 29. 22. Or if they are small; then they either run along in even lines collateral and oftentimes contiguous with the said Insertions, as in Holly: or at least, are reciprocally, some on one side, and some on another, inclined to them; as in Apple. 25. By all which means, the Aer is more readily conveyed from the Vessels into the Insertions.
10. §. A further evidence hereof is this, That generally, the bigger and the more numerous the Aer-Vessels be; the bigger, or at least, the more numerous also are the Insertions: Especially, if the comparison be made (as in all other cases it ought to be, as well as here) betwixt the several Species of the same Kind. So Corin, which hath small Aer-Vessels, Tab. 17. hath also very small Insertions. But the Vine, hath both very large: and so for others.
11. §. Wherefore, the Insertions minister betwixt the Aer-Vessels, and the Succiferous; in the same manner, as the Vesiculæ of the Lungs, do betwixt the Bronchiæ and the Arteries. That is to say, as in an Animal, the Bronchiæ deposite the Aer into the Vesiculæ of the Lungs; which administer it to the Arteries: so in a Plant, the Aer-Vessels deposit the Aer into the Insertions, that is into the Vesiculæ of the Insertions; by which it is gradually filtred off into the Barque and the Sap-Vessels therein.