Besides which use, the nitro-aerious particles may there enter, to keep up the vital ferment or flame.
Mr. Papin shows, that air will pass in at the leaves, and out through the plant at the roots, but water will not pass in at the leaves ; and that if the leaves have no air, a plant will die, but if the leaves (being left on the outside of the receiver, parted by a hole, cemented with wax) have air, though the root remain in water in vacuo, the plant will live and grow.
Dr. Grew, in his Anatomy of Plants, mentions vessels which he calls network, cobweb, skeins of silk, &c. But above all, the multitude of air-bladders in them, which I take to be of the same use in leaves as the vesiculę are in lungs. Leaves being as lungs inverted, and of a broad and thin form, their vesiculę are in contact with the free open air, and therefore have no need of trachea, or bronchia, nor of respiration.
Lungs being situate within the animal's body, their vesiculę could have no communication with the atmosphere, without the trachea and bronchia ; and even there, the aerial influence would be intercepted by the fuliginous and recrementitious steams, were they not thence expelled by the systole in expiration ; the want of which is the cause of an asthma, a disease leaves are not affected with, because their vesiculę are always contiguous to the nitrous air, which continually presses against them.
Sanctorious, who, by his static chair, found five-eighths of the nourishment, or a weight equal to it, taken by a man, passes off by insensible perspiration ; could he have invented any method to calculate the quantity of that part of those perspirations which pass off through the trachea from the lungs, I believe he would have found the most of it to pass that way*.
When the blood enters the lungs from the right ventricle of the heart, it is so full of this fuliginous matter, that its colour is blackened with it. This is all discharged in passing through the lungs ; for when the blood arrives at the heart's left ventricle, being purified of its recrements, it is become of a pure, florid, red colour ; and in cold weather these steams may be seem to issue out from the trachea in great quantities, which are constantly supplied by the nourishment taken in at the lacteals of the guts. Sanctorius's other three parts were but as the soil from whence the five parts are extracted.
Since leaves do so much resemble lungs in the anatomy of their organs, it is very reasonable to believe they imitate them in their office ; though the fineness of the vegetable vessels, and slow motion of the sap, will not admit a demonstration of the sap's circulation by ligatures, but we have other reasons which do sufficiently prove it.
Sap is a word, which custom has made proper to signify the blood of plants.
Roots is the word used for the guts of a plant.
Leaves the words used for the lungs of a plant.
* See Mr. Papin's Experiments of the Pueumatic Engine ; it appears that water will pass out at the leaves, but not from without into them ; and that nothing can be found to pass in or out by the bark, unless the bark be cut.
Pabulum the word used for the food of plants, before taken in by the roots ; and also for the chyle, which is the most proper word for it, after taken in, before it is mixed with the sap in the leaves.
Pabulum is sometimes the word used for the nourishment that the chyle gives to the sap, and sometimes for the nourishment and increase that the sap gives to a whole plant.
I believe that the whole stress of the arguments against the circulation of sap, consists in the mistake of making no distinction between chyle and sap which is no less than between the chyle and blood of an animal : and this distinction, I think, may be fully demonstrated by repeating the experiment of my mint G, p. 15, wherein the liquor which entered the roots at the bottom of the glass, and issued out at the other roots in the box from near the middle of the stalk (which being lopped was not above nine inches), appeared, both in taste and colour, to be no way altered from the water at the bottom of the glass, from whence the lower roots imbibed it. The colour of the juice of the bruised mint, when pounded and strained, was greenish, and would have been greener, no doubt, if it could have been separated from all mixture of that other liquor which I call chyle.
The juice or sap of mint, though mixed with the chyle, has a strong aromatic flavour ; but the chyle, which is in the root, and all that issues out of a root is insipid, which you may prove by sucking a mint-root, when taken out of water, except you chew or bruise it, for then it is possible there may be some very small quantity of sap expressed from the capillary vessels, that run in the coats of the root to nourish it. Yet I could never find any taste in mint-roots that way neither, the proportion of sap therein being too minute to affect my palate.
But if sap can be tasted in chewing the fibrous roots, as doubtless it may in some strong-scented plants, then let us consider what will follow upon what Equivocus affirms, viz. " That sap is not made in the root, and that the root contains a liquor different from the sap or liquor of the stem." From hence he argues against the circulation of the sap : for, says he, " if the sap should descend from the stem into the root, what a jarring would there be between these two different liquors or juices?" To which I answer, that the chyle and sap being contained in different vessels, the chyle in the larger cavities of the root, the sap in the fine capillaries, which supply nourishment to the vessels of the root, they never mix there, and therefore there can be no jarring betwixt them : and if, on chewing the fibrous roots (which I suppose is best done when they are dry), sap is tasted in them, it will be a proof, that sap not being made in the root, descends thither from that part of the plants where it is made : and that part must be it that has the most secretory ducts, which send off what is dissentaneous to sap ; and I think the leaf is allowed to be this part ; and it is proved to be so, when the root being in water without, and the rest of the plant being within the exhausted receiver, nothing is found to issue out from any other part, but from the leaves. There may be yet another proof, if the like experiment to mine of mint G be made with some plant that has a red sap, as the red beet hath.
For further proof, I would propose to the curious, that a good quantity of this liquor, which may easily be obtained from an upper root, after it has passed through a great part of the stalk, be distilled, and an equal quantity of sap expressed from the pounded stalks of mints. I am confident the different quantities of spirit, so drawn from these distillations, will convince them of the difference there is between chyle and sap.
There may be yet another proof, if the like experiment to mine of mint G be made with some plant that has a red sap, as the red beet thath.
When the chyle has thus passed through the body of such a red plant, and yet retains its white colour, there will be no more room for arguments against the circulation of the sap, taken from the motion of this chylurs liquor.
The roots that supplied the earth in my trough with moisture sufficient to maintain my mints plants (marked HH in p. 16) therein a whole summer, give me great reason to believe, that such a quantity of liquor, imbibed by these roots, was not converted to sap in them. And, indeed, if sap could be so soon made by the roots, great profit might be made of some plants, by obtaining more sap from such upper roots in a few hours, than the whole sap of the same plant amounts to.
When the roots of a plant are set in water without, and its leafy part included in the exhausted receiver, water will distill from the leaves, and forming drops thereon, fall down into the receiver ; but when the leafy part is in water without, and the roots included within the exhausted receiver ; no liquor will come out from the roots, as in the experiment in " Philosophical Transactions." Here, if water enter the leaves, it is a proof, that when the chyle vessels have delivered their liquor to the sap of the leaves, there are a sort of valves which prevent both sap and water from entering the chyle vessels from the leaves, for if either of them did, it must have issued out at the roots in the empty receiver, where there was no resistance to balance the external pressure ; therefore, what water entered the leaves must have remained only in the sap ; and in all probability circulated with it, as doth the water which enters the capillary veins by the pores of the skin of men that swim or bathe, and also of washerwomen.
There can be no other valves in the chyle-vessels, except the above-mentioned, because the chyle can move both ways, as it is proved by a willow growing in an inverted posture.
Since no sap is ever found in the cavity of a root, or of any of its appendages (as may be proved by my said mint G, &c. and such appendages will come out all over a plant, as at the end of a brier) we may conclude, that sap is not made in the root, nor in passing out of the root into the stem, as Equivocus asserts, because the chyle continues to be chyle all over the body of the plant in its proper vessels, until it reaches the leaves, and in them it is certainly mixed with the sap, because both the mint-sap and garlic-juice were tasted in the leaves of my mint E, in p. 15.
If the garlic-juice that was taken in by mint E, had been made mint-sap, by being purified by the root, stem, or bark of the mint, it could not have remained garlic-juice in the mint-leaves, as by the taste it plainly did.
It is very probable, that the chyle mixes with the sap at the entrance into the leaf, and that the taste of mint and garlic in the same leaf, is from a mixture of them there, in the sap-vessels only, as the chyle and blood are mixed in the blood-vessels of animal lungs ; for no chyle-vessels can ever be found in a leaf, as they may be in every other external parts of a tender-rind plant, and pure chyle drawn from them.
It is certain the chyle must enter the sap-vessels somewhere, else how could the sap be diluted or nourished ? And since the leaf is the only part free from chyle-vessels, though chyle is there tasted, I see no reason to doubt of the chyle's being in the sap-vessels of the leaf, and not there in their own proper vessels.
The reason why the chyle (or water) passed out at the roots that were in earth in the trough, and in sand in the box, was because those roots had not so equal a pressure to their whole superficies from the earth and sand as the other roots had from their water ; for earth and sand have larger pores than water, some of those pores were filled only with air, whose weight was no counterpoise against the water, which, having lesser pores, pressed against more of the
superficies of the roots contained in it, and thereby caused the chyle to issue out at the superficies into those pores of the earth and sand where was les resistance.
And the same reason may be given, why some of the chyle, taken in by roots which are in water and earth may pass out at other roots of the same plant that have less water in the earth wherein they are included.
Now this chyle (by some mistaken for sap) entering at the roots, has, doubtless, a progressive motion only, and doth all, except in case of unequal pressure just mentioned, march to the leaves, thence never to return, save such parts of it as are proper and sufficient to dilute and nourish the sap ; all the rest, I think, is universally agreed to perspire off from the leaves.
And this motion of the vegetable chyle agrees with that of the animal chyle, which, likewise, is only progressive, it ascending to the subclavian vein, not by pulsion, andy more than the vegetable chyle.
Roots, indeed, differ from guts in this, and the cavities of roots continued quite through a plant, serve as chyle-vessels, which office the cavities of guts cannot supply in an animal, because these carry the mass from whence the chyle is imbibed by the lacteals, and which is carried from their insides or cavities
outwards ; and therefore it was necessary for guts to have other vessels to carry the chyle to the blood.
But roots taking in their chyle from without, needing no other vessels, serve by themselves for both uses, viz. to separate the vegetable chyle from the mass of earth wherein they are included, and to carry it in their own cavities up to the leaves, where it is mixed with the sap.
True sap never passing out nor in at the leaf, nor at any other part of the plant, unless wounded, must be made (of chyle) within the plant.
And must either circulate or stagnate ; and stagnation of sap is as sure death to a plant, as stagnation of blood is to an animal, for without motion it would corrupt and putrify ; and this motion must be circular, because, it being proved that the chyle is joined with the sap in the leaves, and allowed that sap is made of, or rather nourished by, chyle (which, I believe, nobody who considers will deny), it follows, that the sap passes from the leaves to all parts of the plants, as blood doth from the lungs of an animal for the nourishment of the whole body ; and if such part of it as is not spend in nourishing the plant were not returned back to the leaves, there could be no sap in them to mix with the chyle. This motion from the leaves, and returning to them, is what I call circulation, by what means soever it is performed.
There may be other ways by which this circulation of sap is performed, besides pulsion : I am inclined to think trusion the most likely ; and, as I remember, Mr. Bradley has accounted for it this way, viz., as heat rarefies the sap in one part of a plant more than in another, it must require more room, and consequently expand itself, and move further, thrusting or pushing on that which is next it ; but far be it from me to attempt explaining the manner of it.
If the analogy there is in other respects between a plant and an animal, holds between sap and blood, there must be in a plant vessels analogous to arteries and veins, and even to capillaries ; for it is said by the learned, that blood does not nourish the vessels by passing through their cavities, but by that which is sent out of the capillary arteries into the parenchyma. How very unlikely is it then, that the plant should be immediately nourished by the crude chyle passing once from the root through the cavities of the chyle-vessels up to the leaves, and thence all into the atmosphere ?
The argument brought against this circulation, from the great quantity of water imbibed and perspired in a short time by the; sunflower, will be answered by the very short and direct passage
which that liquor has from the root to the leaves, which perform the office of kidneys to the redundant aqueous part of this chyle.
The chyle in an animal has but a short passage from the lacteals to the blood ; but yet much longer than the passage of the vegetable chyle.
The animal chyle, though not moved by pulsion, arrives soon at the subclavian vein ; and there joining with the blood, goes with it immediately to the heart ; whence it is by pulsion driven through the lungs, being therein more intimately mixed, and also purified ; and that which is not thence thrown off by expiration, hath not a long journey, by the emulgents to the kidneys, which separate and send down a greater or lesser quantity of urine, and quicker or slower, in proportion to the quantity of liquor drunk ; and this is sometimes much more than is necessary. As I remember two Swiss soldiers at Montpellier were carried before the governor by their landlady, for refusing to pay for fifty-six pots of very strong wine, which they drank at one sitting : the dispute was about the odd pots, for they said, they never used to drink more than fifty in that time ; but the woman insisting on her proof, the governor paid for the odd. A Montpellier pot contains three English pints.
Now I suppose this quantify is vastly greater than is necessary for a man to drink in that time ; yet not so much unnecessary as the quantity of water was to Mr. Hale's sunflower ; for I am in no doubt, but that it would have thrived well the fiftieth part it imbibed ; because I have seen a sunflower grow very well in dry rich ground in a dry summer ; and then it might drink no greater quantity, than a man in his regular way of living, bulk for bulk.
The lungs cannot do the office of kidneys in an animal, because being at such a distance from the open air, so great a quantity of liquor necessary to be sent off, though rarefied to vapour, would cause suffocation in the bronchia and trachea ; but leaves being in contact with the air, can execute the office of kidneys without that danger.
Nature has other ways of discharging the aqueous part of the blood, besides kidneys, even in some animals, as in fowl ; for to them their feathers serve as kidneys, having no other, and yet they drink plentifully.
Fishes also have their lungs almost without their bodies, like plants, and seem to have no other passage for discharging their urine but their lungs, though fishes are accounted great drinkers.
No body doubts of the circulation of blood in all animals, though in many very small ones it cannot be proved by demonstration ; and there is no more reason to doubt of it in plants than in oysters, mites, and in many species of insects too minute to be seen by the naked eye.
The argument taken from the liquor issuing plentifully out of the lower part of a notch, or a disbarked gap of a tree or branch set in water, and not from the upper part of it, is answered by showing, that the greatest part of that liquor passes out of the leaves without descending ; and so cannot issue out at the upper part of the gap ; and the sap being thicker, and in less quantity, has probably a much slower motion, and is not so apt to pass out at a cut, as the aqueous chyle is ; for a plant never bleeds to death, but when the sap is very much diluted by a great mixture of chyle.
As to what is offered by Equivocus against the circulation, from the same stocks producing different sorts of pears, it may be answered, that the ovaria of plants are a part of their very substance, and do no fluctuate or circulate in their juices ; so that each scion or bud, contains, actually adhering to itself, all the fruit and plants that ever will proceed from it ; and though the same juices may so agree with the stock and the scions, as to nourish them all, the scions being different from the stock and from on another, yet the juices cannot change the sort of fruit, that being an organical part, only nourished, extended, and increased by the juices.
Yet we see that when the nature of a stock is very different from the scion, the juices made by their different vessels are so disagreeable to each other, that one or both, but always the scion, will die.
It is true, that the juice of a stock, mixing with that of the scion, may a little alter the flavour of its fruit ; as a pear grafted upon a quince may be mended, but if grafted upon a white thorn will be worsted : but this may very well be from that little alteration the sap receives in circulating through the vessels of the stock.
We find by inoculation, that a bud is an entire little tree, containing within itself its proper seed, and all the trees that ever can proceed from it ; for to suspect that all the individuals of plants and animals did not actually exist within the first of each of their respective species, would be to suspect that there is an equivocal generation of them.
The last objection I shall speak to is this :
It is asked by Equivocus, How goes on the circulation, when a part is cut off from a plant ? Why, I say, it goes on as the circulation of blood does in a man upon the amputation of a leg or an arm.
As for the part cut off from the plant, provided it be at a proper season stuck into the ground, if it have a spongy rind it will grow, the roots, being the chyle-vessels, passing all over the plant, are sent out from that part of the bough which is in the ground, and doing that for it which all fibrous roots do in the earth ; the bough sends out leaves also, which are contained all over it, which are explained in the air, and then the bough becomes a tree.
Why the roots should choose to strike out in the earth rather than in the air, and the leaves in the air rather than in the earth, I cannot tell : it is by an unknown sort of mechanism, or rather instinct, which I can no more pretend to explain, than I can the cause of gravitation.
But I can see no reason to believe, that a plant is a mere Thermometer, nor that the vegetable life can be carried on any more than the animal life, without a circulation of that juice, which is necessary to nourish and maintain it.
I might urge another argument, against those who assert that the sap is made by the bark in its ascent only ; which argument is, that if it were so, the sap must be more pure the higher it ascended, and pass off into the atmosphere in its greatest perfection ; which would intimate, that sap was not designed by nature for nourishment of plants, but to be thrown away as useless, when it was made the most useful for that purpose.
The young potato is nourished from the plant*, at the end of a white string, by vessels passing from the bottom of the plant ; at the same time, when salt being bound to this string, passes by other vessels of the same string, contrary to the other, into the body of the plant, and may be tasted in the leaves.
It must be the chyle-vessels that imbibe the salt, as they would have imbibed the chyle, had the string been in contact with earth, as it was with the salt, all fibrous roots being parts of the system of chyle-vessels, as leaves seem to be of the sap-vessels ; the former carrying their contents to the leaves, that were not able to separate or discharge the salt from the agreeable part of the chyle, nor to carry the salt back to the stem, in their sap-vessels ; which, it is probable, were soon corroded by the saline acrimony. The salt appeared to remain in the leaves, by their tasting almost as strong as pure crude salt.
* It must have its nourishment from the mother plant, because, the young potato and string being laid on tiles, could have no nourishment from the earth, and yet it will grow large, and have no taste of salt in it, that being stopped in the leaves, and kills the mother potato. If the salt did enter the inmost vessels, and carry the nourishment to the young one, that would taste of the salt, which is applied nearer to it then to the mother.
Thus, it is no wonder that there should not be salt enough carried from the leaves to the young potato to be tasted in it ; and it could not be carried to it, immediately from the string, without first passing the leaves : because the sap-vessels never sent out roots, and therefore could not imbibe the salt at the string : the chyle-vessels only sending out roots, as the sap-vessels only send out leaves. Neither could any salt pass to the young potato in the chyle-vessels, they always carrying their liquor towards the leaves, but never from them ; except when they supply roots that happen to be empty, as in the case of mint HH, mentioned in page 16.
A quantity of matter, nearly equal to that received by the roots, is constantly carried off, as appears by Dr. Woodward's experiments ; and I believe nobody every doubted but that it had its chief exit from the leaves.
It is not likely that all those curious vessels which appear in the texture of the leaf, should be designed for the recrements and sap to pass once through them, and thence to fly away together : they might as well pass off without the use of the leaves, at the place where they are inserted into the plant, if the leaves were off*.
* Mr. Hales in his Vegetable statics, found that a plant in summer imbibed and perspired less water when its leaves were pulled off, that when they were on ; but this might be partly from the contraction of the vessels by the air, at the wound where the leaves were broken off. He also proves that this quantity of liquor, that passes through a leafless plant in summer (though it be less than what passes through a plant that has its leaves on) is vastly greater than what passes through the same plant in winter ; and yet the plant with the great quantity of liquor (or nourishment) in summer will die, and yet will live with the least in winter. Hence it appears, that the taking in and passing of never so great a quantity of pabulum, with its vehicle, through a plant, will not keep it alive unless it have leaves in proportion to that quantity, as all plants have that live in winter. It cannot then be denied, that leaves are absolutely necessary to the life of a plant, and if they are, it must be either on account of their conveying something from it, or sending something to it : it is plain it cannot be only by the former, because that can be done without leaves ; they must therefore be necessary by the latter.
And as the lungs would be of no benefit to an animal, if the blood, after it was secerned and purified in them, were not returned to the body, as well as received from it ; so the secretion made by leaves would be of no benefit to the plant, if none of the sap there secerned were returned back to it; neither could the air taken in by the leaves be of any use to the rest of the plant, unless it did pass from the leaves along with the purified sap into the plant, by some vessel like the aorta. And I think that whoever proves that the air passes from the leaves into the plant, sufficiently proves the circulation of the sap, because if the sap did move always from the plant to the leaves, the air could not pass against the stream of it.
It seems that the chief arguments that give Mr. Hales a suspicion against the circulation, are taken from the quick passage of liquor from the root through the plant, and his supposing that liquor to be sap ; which I think almost as unreasonable as to suppose the wine we drink and which passes out, to be blood. The more we drink, the quicker the liquor will generally pass ; and the man who drank out a large vessel at one draught, and discharged it without taking his mouth from the tap until it was finished, had as quick a passage for liquor as any plant in all Mr. Hales's experiments, and yet is no proof against the circulation of the blood.
Vide Mr. Hales of Vegetation, page 324, 325 ; where he says, that it is probable dew, rain, &c. are imbibed by the leaves, and are the materials of which the more subtle and refined principles are formed. And also, 'That leaves do, in some measure, the same office for the support of vegetable life, that lungs do for the support of the animal life ; plants drawing through their leaves some part of their nourishment from the air.'
If this nutritive matter did enter at the excretory ducts of leaves, while the plant was in an imbibing state (as he seems to think), then they must be expelled again at the same ducts, by the force of the perspiring stream, as soon as the perspiring state returns ; and thus could be of little or no use to the sap, not going far in.
And is it not more probable, that the sulphureo-aerial particles which, he proves, are so plentifully in leaves, should invigorate the pure sap returning into the plant, than to invigorate only that recrementitious sap, that is just making its exit at the excretory ducts ? how could this invigorate the plant, or help to nourish it.
And to think that the nourishment and sap of a plant pass off together, is no less absurd, than to think that the blood and chyle pass off together in perspiration.
Or to what purpose should the sap be depurated in the leaves, if it were not to be returned back by other vessels, like arterię venosę, into the stem or stalk of a plant ?
If no circulation, the sap and all other juices must pass off together, and then there would be no manner of use of the vessels of the leaves as strainers.
It would be very strange, if what is pure earth and water when it enters the roots, should be transformed to such different juices, by passing once through a plant, as from an alembic, and in so short a time.
In such case, either it must become perfect sap in the root, or else, when it first passed thence, it would not be much different from earth and water near the root, and the higher it went the more different, and the more altered it would be ; but we find the sap at the bottom and top of a plant to be the same, and as full of spirit at the bottom, which could not be supposed to be made in so short a percolation, if by percolation at all.
If leaves did not perform this necessary work of succification, the lives of plants would not, in all probability, so entirely depend on the use of leaves, as they appear to do. And this is always found tree (though too late) by those who kill their St.-Foin, by suffering it to be indiscreetly fed by sheep : and to caution them against that injury, is the reason of my writing this chapter.
Leaves being so necessary, nature has, in all perennial plants, provided a reversionary stock of them ; wherefore leaves are always formed, as Dr. Grew observes, in autumn, though they are not usually explained till the following spring, which then open and increase gradually, in proportion to the motion of the sap, and quantity of pabulum it then receives to be circulated.
These may also, though not wholly appearing out of the bud, be sufficient for the extreme small motion of life the sap of perennial plants, which drop their leaves, has in winter.
Besides these autumnal leaves of Dr. Grew's, there is another set of them formed in the spring, which appear and are explained about midsummer ; these save the lives of the mulberry-trees, when the first leaves are taken off for the food of silkworms ; but these second leaves alone would not suffice to purify the sap, or save the trees, if the first leaves were stripped off downwards ; but as those who gather them, pull and strip them upwards, there always remain some of the tails or foot-stalks, with a little part of the leaves behind unpulled ; by help of which remaining parts, the trees make a shift to live for some time, till the new leaves grew large enough : as men have been found to live (but not long I suppose) by a small part of their lungs, the rest having been wasted and dried away in consumptive or asthmatical cases.
This is certain from all experience, that no vegetable whatever can live long without leaves, but will very soon lie, if the leaves are pulled off as fast as they appear.
The reason why natural grass may seem an exception to this is, that when it is fed by cattle, there is never any great quantity of it (especially of stalks) growing at once, and so less sap to be
purified ; and has not only greater proportion of leaves, but also many successions of them, still ready to supply the loss of those that are eaten ; and many of these leaves are so small, short, and low, that the cattle cannot come at them to bite them off close ; many more also come out of the very roots of natural grass.