Hooke describes the needle structure on a nettle leaf and discusses its similarity to a syringe
A Nettle is a Plant so well known to every one, as to what the appearance of it is to the naked eye, that it needs no description; and there are very few that have not felt as well as seen it; and therefore it will be no news to tell that a gentle and slight touch of the skin by a Nettle, does oftentime, not onely create very sensible and acute pain […] These observations, I say, are common enough; but how the pain is so suddenly created, and by what means continued, augmented for a time, and afterwards diminish’d, and at length quite exstinguish’d, has not, that I know, been explain’d by any.
The needle structure
And here we must have recourse to our Microscope, and that will, if almost any part of the Plant be looked on, shew us the whole surface of it very thick set with turn-Pikes, or sharp Needles […] which are visible also to the naked eye; […] it is of substance very hard and stiff, exceedingly transparent and cleer, and, as I by many trials certainly found, is hollow from top to bottom.
This I found by this Experiment, I had a very convenient Microscope with a single Glass which drew about half an Inch, this I had fastned into a little frame, almost like a pair of Spectacles, which I placed before mine eyes, and so holding the leaf of a Nettle at a convenient distance from my eye, I did first, with the thrusting of several of these bristles into my skin, perceive that presently after I had thrust them in I felt the burning pain begin; next I observ’d in divers of them, that upon thrusting my finger against their tops, the Bodkin (if I may so call it) did not in the least bend, but I could perceive moving up and down within it a certain liquor, which upon thrusting the Bodkin against its basis, or bagg, I could perceive to rise towards the top […]; this I did very often[…]
I could plainly perceive […] the liquor of the Plant, which was poisonous, and those small Bodkins were but the Syringe-pipes, or Glyster-pipes, which first made way into the skin, and then served to convey that poisonous juice, upon the pressing […], into the interior and sensible parts of the skin, which being so discharg’d, does corrode, or, as it were, burn that part of the skin it touches; and this pain will sometimes last very long, according as the impression is made deeper or stronger…
The chief thing […] is, how this Plant comes, by so slight a touch, to create so great a pain; and the reason of this seems to be nothing else, but the corrosive penetrant liquor contain’d in the small baggs or bladders, upon which grow out those sharp Syringe-pipes […]; and very consonant to this, is the reason of the pain created by the sting of a Bee, Wasp, &c. as I elsewhere shew: For by the Dart, which is likewise a pipe, is made a deep passage into the skin, and then by the anger of the Fly, is his gally poisonous liquor injected; which being admitted among the sensible parts, and so mix’d with the humours or stagnating juices of that part, does create an Ebullition perhaps, or effervescens, as is usually observ’d in the mingling of two differing Chymical saline liquors, by which means the parts become swell’d, hard, and very painfull; for thereby the nervous and sensible parts are not onely stretch’d and strain’d beyond their natural tone, but are also prick’d, perhaps, or corroded by the pungent and incongruous parts of the intruded liquor…
Comparison with syringes and wider observations
And the ingenious Invention of that Excellent person, Doctor Wren of injecting liquors into the veins of an Animal, seems to be reducible to this head: I cannot stay, nor is this a fit place, to mention the several Experiments made of this kind by the most incomparable Mr. Boyle […]: But I shall rather hint, that certainly, if this Principle were well consider’d, there might, besides the further improving of Bathing and Syringing into the veins, be thought on several ways, whereby several obstinate distempers of a humane body, such as the Gout, Dropsie, Stone, &c. might be master’d, and expell’d […]