The comparative anatomy of trunks together with an account of their vegetation grounded thereupon; in two parts: the former read before the Royal Society, Feb. 25. 1674/5; the latter, June 17. 1675. The whole explicated by several figures in nineteen copper-plates; presented to the Royal Society in the years 1673. and 1674. By Nehemiah Grew, M.D. and fellow of the Royal Society.

CHAP. II.

THE NEXT enquiry to be made, is into the motion and course of the Air. Where this question will first of all be asked; sc. which way the Air first en∣ters 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 inception, 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, with∣out a glass, 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 these pores of the skin in the ends of the fingers and ball of the hand.

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 I come to the Anatomy of the Leaf.

But although the Air 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 Air, may

continually pass into the Body and Blood, by the habit, or pores of the skin; but chiefly, at the mouth. And what the mouth is, to an Animal; that the Root is to a Plant.

Again, if the chief entrance of the Air, was at the Trunk; then, before it could be mixed with the Sap in the Root, it must de∣scend; and so move in a contrary course to the Sap throughout the Plant. As by its reception at the Root, and so its ascent from thence, it hath a more natural and easy motion of ascent: for that the Sap of the same Plant should continually ascend, and the Air descend, would be somewhat strange.

The same is further argued from the few∣ness and smallness of diametral Portions in the Trunk, in comparison with those in the Root: which nature hath plainly there designed, for the separation of the Air from the Sap, as they are both together re∣ceived into the Root. So that the reception and course of the Air, is made on this man∣ner following.

The Air being a springy body, it insinu∣ates into all the Pores and cranies of the earth; and so is perfectly mixed with the soyl. Whereupon, as the Sap enters the Root, more or less Air still intrudes it self

together with it. The liquid portion of the Sap swells and fills up the succulent parts of the Bark. The Airy part is, as was said, separated from the liquid into the Diame∣tral portions. By which it is conveyed from the Bark, and so into the Air-vessels; be∣twixt which Vessels, the said Diametral portions run from the Bark towards the center of the Root. The Air thus received into the Air-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 Air. Partly into the Insertions; by which it is conveyed outward into the Bark. Wherein, it is in part transfused through the Sap; and so the rest, with part of the Sap, remitted in perspirations back again into the Air. So that, whereas the Diame∣tral portions in the Root, do serve to con∣vey the Air from the Sap into the Air-vessels: on the contrary the Insertions here in the Trunk serve to convey the Air from the Air-vessels into the Sap. Wherefore, as the Air-vessels advance the Air; or the Airy part of the Sap, and so convey it by the length of the Trunk; so the Insertions filter

it, and convey it by the bredth.

AND that the Insertions have this office of subservience to both kinds of Vessels; doth yet further appear, if we consider, that the Air-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. Or if they are small; then they either run along in even lines colla∣teral 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. By all which means, the Air is more readily conveyed from the Vessels into the Insertions.

Again, a further evidence hereof is that generally, the bigger and the more nu∣merous the Air-vessels are; 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 Air-vessels, hath also very small In∣sertions. But the Vine hath both very large: and so for others.

Wherefore, the Insertions minister be∣twixt the Air-vessels and the succiferous; in

the same manner, as the Vesiculae of the Lungs, do betwixt the Bronchiae and the Arteries. That is to say, as in an Animal, the Bronchiae deposite the Air into the Ve∣siculae of the lungs; which administer it to the Arteries: so in a Plant, the Air-vessels deposit the Air into the Insertions, that is into the Vesiculae of the Insertions; by which it is gradually filtred off into the Bark and Sap-vessels.