ARTICLE II.

On the Hypothesis of Mr. Knight, accounting for the Direction of the Radicle and Germen. By the Rev. Patrick Keith.

SIR,

(To Dr. Thomson.)

Bethersden Vicarage, Kent, Jan. 29, 1819.

IN In your number for April, 1817, you have given an account of the proceedings of the Linnæan Society of London, by which it appears that, on Tuesday, March 4, preceding, a paper, by T. A. Knight, Esq. was read to the Society, containing a defence of what is called my attack upon his hypothesis, of which I unfortunately exhibited an inaccurate representation. This inaccuracy I have since admitted and apologized for in the proper quarter, but not to the prejudice of any future inquiry. If Mr. Knight's hypothesis is founded in truth, it will suffer nothing from my investigation; and if it is founded in error, the sooner the error is detected the better. On this account, Í have given it a second perusal, and I am desirous of communicating the result of it to the public through the medium of your Annals. But that I may not again exhibit an incorrect view of Mr. Knight's hypothesis, I will give it in his own words, as it occurs in an inference drawn from the two following experiments.

Exper. I.-On the circumference of a vertical wheel performing 150 revolutions in a minute, by which the influence of gravitation was conceived to be wholly suspended, beans were placed in all directions. The result was, that the radicles uniformly turned their points outwards from the circumference of the wheel; and in their subsequent growth receded nearly at right angles from its axis. The germens, on the contrary, took the opposite direction; and in a few days their points all met in the centre of the wheel. They even extended beyond it; but the same cause which first occasioned them to approach its axis still operating, their points returned, and met again at the

centre.*

Exper. II.-In consequence of some slight objections which Mr. Knight anticipated as likely to be alleged against the conclusion he was inclined to draw from the foregoing experiment, a second experiment was instituted, by adding to the former machinery a horizontal wheel, which was made to perform 250 revolutions in a minute, and to the circumference of which, beans were fastened as before. The issue was, that the radicles were protruded outwards and downwards, about 10° below, and the germens inwards and upwards, about 10° above the plane of the wheel. But when the rapidity of the wheel's motion was diminished, the radicles were more perpendicular, and the ger

Phil. Trans. 1806. Part I. p. 100, 101.

mens more upright; 80 revolutions in the minute giving an elevation and depression to the stem and root respectively of 45°.*

From the foregoing experiments, Mr. Knight infers, "that the. radicles of germinating seeds are made to descend, and their germens to ascend, by some external cause, and not by any power inherent in vegetable life; and that there is but little reason to doubt that gravitation is the principal, if not the only agent employed in this case by nature."+

With regard to the first experiment, it may be remarked, that the anticipated objection is not quite so slight as Mr. Knight seems to have imagined; for as the radicles were, at least during the one-half of their circumvolution, in their natural position, or nearly so, while the artificial centrifugal force was operating rather in conjunction with gravitation, or in the direction in which radicles naturally grow, so as to do more than counterbalance its effect in the other half of the circumvolution, in which the force of gravitation was opposed to it, it may be said, that there is no new case put from which any inference can be drawn; and the moment the stems passed the centre, it was to them the same thing as growing downwards, which it is known that they cannot do. But the experiment seems to me to be liable to a much more serious objection than that which Mr. Knight had anticipated; for, as in this case the influence of gravitation was conceived to be wholly suspended, and the radicles subjected to the agency of the centrifugal force alone, they ought surely to have been protruded in the direction of that force. Now the direction of the centrifugal force in question" must of necessity have been oblique, as being the simple effect of circular motion; and not the reverse of that of gravitation, like an arrow shot from a bow perpendicularly upwards. Why then were the radicles protruded at right angles to the axis of the wheel? If one of the beans had by any accident lost its hold, would it have been thrown off from the circumference of the wheel in that direction? Unquestionably not. It would have been thrown off in the direction of a tangent to the orbit which it was describing; and in this direction also the radicle ought to have been elongated, the direction of the plumelet being the

reverse.

The second experiment is thought to be the most decisive, and we may fairly allow it to be the most plausible of the two; though the account that is given of it by Mr. Knight leaves a desideratum that greatly diminishes its importance. We are told that the radicles were protruded outwards and downwards at about 10° below, and the germens inwards and upwards at about 10° above the plane of the wheel's orbit; but we are not told whether this approach or recession was in the plane of the

Phil. Trans. 1806. Part I. p. 102, 103.

+ Ibid. 103,

wheel's axis, or otherwise; and if otherwise, then we are not told any thing with respect to the degree of its deviation, or how it was affected by the increased or diminished velocity of the wheel; all which seems to be absolutely indispensable to Mr. Knight's conclusion; for if the velocity had been such as to counteract the force of gravitation completely, then, upon Mr. Knight's principles, it is evident, that the radicle ought to have been protruded, not merely outwards and downwards, but horizontally; and not yet merely horizontally, but in the direction of a tangent to the orbit of the bean, like the drops of water that flew off from the rim of Mr. Knight's main wheel; or (to take a more familiar example), like the drops of water that fly off from the tags of a twirled mop. It ought, therefore, to have been making approaches to this direction according to the degree of velocity with which the wheel's motion was accelerated. Will it be said that the resistance of the air prevented it from approaching, or from assuming that direction? Then the resistance of the air ought, for the very same reason, to have aeted upon the radicles of the beans that were fixed to the circumference of the vertical wheel, and to have affected their direction also. But of this we find not the slightest intimation; and if it had even done so, there is no reason to believe that its action would have stopped just at right angles to the axis of the wheel. Hence it is evident that Mr. Knight's conclusion does not legitimately follow from the premises which his experiments present.

We do not, however, deny that gravitation, or a power counteracting gravitation, may affect the growth of plants, and influence the direction of the root or stem; or that the effect produced by a foreign force will be in proportion to the amount of the force impressed; but we contend that the vegetating plant possesses energies capable of counteracting the influence of gravitation when necessary; and that gravitation is not the sole, nor even the principal agent employed by nature to give direction to vegetables. It is indeed a grand trial of our faith to have to believe that the roots of plants grow downwards and the stems upwards merely by the agency of gravitation. But if it were even granted, still the phenomenon would remain an incomprehensible paradox till duly explained, notwithstanding the result of the two experiments; and accordingly Mr. Knight endeavours to point out the means by which gravitation may produce the diametrically opposite effects which his hypothesis ascribes to it.

He begins by saying, that "the radicle of a germinating seed (as many naturalists have observed) is increased in length only by new parts successively added to the apex, or point, and not at all by any general extension of parts already formed; so that the matter added being fluid, or changing from a fluid to a solid state, may be supposed to be sufficiently susceptible to the

influence of gravitation to give an inclination downward to the point of the radicle."

Whether Mr. Knight takes this supposed fact entirely upon the credit of others, or whether he confirmed it by his own observation, I cannot positively decide; though the parenthesis contained in the above quotation renders the former part of the alternative the most probable. There is no doubt that many naturalists have been of this opinion, particularly Du Hamel, who gives a minute account of the experiment by which he seemed to have ascertained the fact. Having passed several threads through the root of a plant, and noted the distances, he then immersed the root in water. The upper threads retained always their relative and original situation, and the lowest thread, which was placed within a few lines of the end, was the only one that was carried down. Hence he concluded that the root is elongated merely by the extremity.*

Resting upon this high authority, I confess that I did till lately assume the fact without examination. But the result of the following experiment will show that the opinion is still incorrect, in spite of all the authorities by which it has been backed.

On Oct. 1, 1818, I sowed some tick beans in a small earthen pan filled with garden mould.

On the 4th, the radicle of the most forward had protruded about 4th of an inch beyond the integuments, when I marked it with ink at the point, in the middle, and at the base, as clearing the integuments; so that the marks were about th of an inch from each other.

On the 5th, the radicle was 4th of an inch in length, and the marks nearly as before with regard to their relative distances, but removed evidently from the integuments, so as to admit of a fourth or additional mark again adjoining the integuments. The radicle, which was originally upright, was now bending down.

On the 6th, the radicle was an inch in length, the first mark being within two or three lines of the point; the second at about 4th of an inch from the first; the third at about 4th of an inch from the second; and the fourth at about 4th of an inch from the third; as well as perceptibly removed from the integuments. On the 7th, the radicle was 4th of an inch in length. The first mark was still within two or three lines of the point; the second was at the distance of of an inch from the first; the third was at the distance of of an inch from the second; and the fourth was at about the distance of 4th of an inch from the third, being but little more than its original distance, but removed to the distance of 4th of an inch from the integuments.

On the 8th, the radicle was one inch in length, the first mark being still near the apex; the second at the distance of about

Phys, des Arb, lib, i. chap. v.

d of an inch from the first; the third at the distance of about id of an inch from the second; and the fourth nearly as before.

On the 9th, the radicle was 14 inch in length, the three marks next the base being nearly as before, and the mark next the apex being the only one that was carried down.

On the 10th, and as long as any further observations were made, it was still the lower extremity of the radicle, and that only, which was carried down. But enough had been previously observed to show that the assumed peculiarity of the elongation of the radicle is founded in a mistake; and that the root in its incipient state, like the stem in its incipient state, is augmented by the introsusception and deposition of additional particles throughout its whole mass; or "by a general extension of parts already formed;" though it may afterwards, like the stem, become so firm and compact as no longer to admit of augmentation in that way. I suspect, therefore, that Du Hamel's experiment was not instituted at a sufficiently early period of the radicle's or root's growth; or that it was somehow or other unnaturally affected by being placed in water; or that there are exceptions to the rule, which my experiment establishes.

The bean, which was the subject of the above experiment, grew, as has already been stated, in garden mould, and was taken up and planted again at every observation. My observations were not, however, confined to that single bean; they were extended to many others, as well as to the radicles of mustard, cress, and radish seed, all which gave similar results; so that if there are any exceptions to the rule which my experiment establishes, the radicle of the bean, on which Mr. Knight's two experiments were made, is not one of them.

Thus it is proved that the facility with which the germinating radicle might be influenced by the agency of gravitation from the supposed peculiarity of its mode of growth is wholly imaginary; and if it were even the fact, still the particles by which it is augmented, though originally fluid, or changing from the fluid to the solid state, are contained within an epidermis which bounds and confines them, and are not committed to the influence of gravitation merely, like the trickling drops of water that are added to the point or surface of an icicle.

If any other evidence were wanted to prove the fact that the root is augmented by the introsusception and deposition of additional particles throughout its whole extent, I would adduce the case of the garden radish when past the stage of germination. In taking up young radishes that are just fit for the table, it is no uncommon thing to meet with an individual that is elevated at the collar by at least an inch above the surface of the soil. But how is this elevation to be accounted for except upon the princíple now assumed? It must not be said that the base of the root has been pushed upwards, because the apex could not get downwards; for the apex has been descending all the while, and

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