PER
Hoot of the little Plant be turn'd downwards, and the Stem upwards, and even 'Perpendicularly upwards ; 'tis eafy to conceive that the little Plant coming 'to unfold itfelf, 'its Stalk and Root need only follow the Direction they have, to grow 'perpendicularly, But 'tis known the Seeds of Plants whe- ther ibwn of themfelves, or by the Help of Man, fall in the Ground at random ; and among an infinite Number of Situa- tions with regard to the Stalk of their Plant, the Perpendicti- lar one upwards is but-one. See Semination.
In all the reit therefore, 'tis neceflary the Stalk redrefs or rectify itfelf, in order to get out of the Ground : But what Force is it, that effects this change, which is certainly a violent Action ? Is it that the Stalkfinding a lefs Load of Earth above it, goes naturally that way where it finds the leaft Obftacle ? Were this fo, the little Root when it happens to be uppermoft, mult for the fame Reafon tollow the fame Direction, and mount on high.
M. tDo'daftj therefore, to account for two fuch different Actions ; has recourfe to another Syffem : He fuppofes that the Fibres of the Stalks are of fuch a Nature, as that they contract and fhorten by the Heat of the Sun, and lengthen out by the Moifture of the Earth ; And on the contrary, that the fibres of the Roots contract by the Moifiure of tie Earth, and lengthen by the Heat of the Sun.
When, then, the Piantule is inverted, and the Root a- Top; the Fibres which compofe one of the Branches of the Root are not equally expofed to the Moifture of the Earth 5 the lower Part is more expofed than the Upper. The Lower therefore raull contract: the mole ; which Contraction is again promoted by the lengthening of the Upper^ whereon the Sun ads with the greatefi Force. Of Confcquence, therefore, this Branch ot the Root mull recoil towards the Earth, and iniinuating thro' the Pores thereof get underneath the Bulb,@fa By inverting this reafoning, 'tis eafy to mew how the Stalk comes to get uppermoft.
In a Word, we may imagine that the Earth attracts the Root to itfelf, and that the Sun contributes to its De- scent ; and on the contrary, that the Sun attracts the Stem, and the Earth, in fome mcafure, fends it towards the fame.
As to the fecond Strcigluning, viz. that of the Stalks in the open Air; he takes it ro arifefrom the Imprcffion of ex- ternal Gaufes, particularly the Sun and Rain. For the up- per Part of a Stalk that is bent, is more expofed to the Rain, Dew, and even Sun, SfTc. than the under. Now both thefe Caufes, in a certain Structure of the Fibres, tend equally to fireighten the Part molt expofed, by the Shortening they fuc- ceflively occailon it ; for Moifture fhortens by fwclling, and Heat by Diflipating. Indeed, what that Structure is which gives the Fibres fuch different Qualities 5 or whereon it de- pends, is {till a Myfiery.
M. de let, Hire Accounts for the 'perpendicularity of the Stems or Stalks of Plants thus : He imagines that in Plants, the Root draws a coarfer and heavier Juice ; and the Stem and its Branches a finer and more volatile one. And, in ef- fect, molt Naturaliits conceive the Root as the Stomach of the Plant, where the Juices of the Earth are fubtiliz'd, fo as to become able to rife thro' the Stem to the Extremity of the Branches. This Difference of Juices fuppofes larger Pores in the Roots than the Stalk, £jye. and in a Word, a different Contexture; which Difference mufi be found even in the lit- tle inviiible Plant inclofed in the Seed : In this Piantule, therefore, we may conceive a point of Separation ; fuch, as that all on one Side JE.gr. the Root, mail be unfolded by the groffer Jukes, and all on the other Side by the more fubtile Juices.
. Suppofe, now, the Piantule when its Parts begin to unfold, to be entirely over-turn'd ; the Root a Top, and the Stalk below : the Juices which enter the Root will {till be coarfeif, and when they have open'd and enlarg'd the Pores, fo as to admit Juices of a determinate Weight, thofe Juices {till pref- ifing the Root more and more, will drive it downwards, and this the more, as the Root more extended or enlarged : For the Point of Separation being conceived as the fixed Point of a Lever, they will act by the longer Arm. At the fame Time the volatile Juices having penetrated the Stalk, will tend to give it a Direction from below upwards; and by reafon of the Lever, will give it more and more every Day. Thus is the little Plant turn'd on its fix'd Point of Separation ; till it be perfectly erect.
The Plant thus erected; the Stalk, we know, fhou'd con- tinuetorife Perpendicularly, to give it the more firm Biding and enable it to withfiand the Effort of Wind and Weather. The Manner wherein this is effected, M. Parent lays down thus: The nutritious Juice being arriv'd at the Extre- mity of a rifing Stalk; if it evaporate, the Weight of the Air which encompafles it on all Sides, will make it afcend verti- cally; and if it do not evaporate, but congeal, and remain fix'd to that Extremity whence it was ready to go off; the Weight of the Air will give it the fame Direction : fo that the Stalk will have acquir'd a very little new Part, vertically laid over it : Juft as in a Candle held any how obliquely to the Horizon, the Flame (till continues Vertical, by the Preffure of the Atmofphcre. The new Drops of Juice that fucceed, will
C 791 )
PER
follow the fame Direffion ; and as all together form the Stall*; that muft ot courfe be Vertical, unlefi^fome particular Cir- cumftance intervene. r
As to the Branches, which are at firft fuppofed to proceed Laterally out of the Stalk in the firft £mb F r P of the P p knt tho they thou d even come out in a horizontal Direction, yer, mult they raife emfelves upwards by the conftant Ditefflon ot the nutrmousjmce; which at fat fcarce meets any Re- finance ma tender, fupple Branch ; and even afterwards, tho the Branch grow more firm, yet will it act with the more Advantage; fince the Branch being become longer fteuJb.es it with a longer Arm of a Lever. The (lender AeHofI ota little Drop becomes very confiderable, by its Continuity ; and by the Affiftance of fuch favorable Circumftances Hence may be accounted for, that regular Situation and DirefluM of the Branches, which all, and always, nearly, make the Cane conftant Angle of 45 ? with the Stem and one another. See Branch.
M.Mruc, accounts for the 'Perpendicularity of the Stems, and their Redrefling themfelves ; on thefe two Principles: i°. That the nutritious Juice arifes from the Circum- ference of the Plant, and terminates in the Pith. 2 . That Fluids contained in Tubes either parallel or oblique to the Horizon, gravitate on the lower Part of the Tubes, and not at all on the Upper.
For hence it eafily follows, that in a Plant pofited either obliquely or parallel to the Horizon, the nutritious Juice will act: more on the lower Part of the Canals than the upper, and by this Means, infinuate more into the Canals communicating therewith, and be collected more copioufly therein; thus the Parts on the lower Side will receive more Accretion, and be more nourim'd than thofe on the upper; the Confequences where- of muft be, that the Extremity of the Plant will be oblig'd to bend upwards.
The fame Principle brings the Seed into its due Situa- tion at firft : In a Bean planted upfide down, the Plume and Radicle are eafily perceiv'd with the naked Eye, to (hoot, at firft, directly for about an Inch; but thence forth they begin to bend, the one downward, and the other upward. The like is feen in a heap of Barley, to be made into Malt ; in a Quantity of Acorns, laid to fprout inamoift Place, igc. each Grain of Barley in the firft Cafe, and each Acorn in the fe- cond, has a different Situation; and yet, all the Sprouts tend directly upward, and the Roots downward, and the Curvity or Bend they make is greater or lefs "as their Situation ap- proaches more or lefs to the direction wherein no Curvature at all wou'd be neceflary. Now, two fuch oppofite Motions cannot arife wirhour fuppofing fome confiderable Difference between the two Parts : The only one we know of, is, that the Plume is fed by a Juice, imported to it by Tubes parallel to its Sides, whereas the Radicle imbibes its Nourifhment at all the Pores in its Surface. As oft, therefore, as the Plume is either parallel, or inclin'd to the Horizon, the nutritious Juice feeding the lower Parts more than the upper, will determine its extremes to turn upward, for the Reafons already affign'd. On the contrary, when the Radicle is in the like Situation, the nutritious Juice penetrating more copioufly thro' the upper Part than the under ; there will be a greater Accretion of the former, than the latter ; and confequently the Radicle will be bent downwards. And this mutual Curvity of trie Plume and Radicle muft continue, till fuch Time as their Sides are nourifh'd alike, which cannor be till they are Perpendicular. Memoirs de I Mead. Roy ale des fcien. An. 1 7 08.
PERPETUAL, fomething that endures always, that lafts for ever. See Eternity.
The Term is fometimes alfo ufed for a Thing that kits, or holds, during a Perfon's Life.
Thus Offices, (jc. held durante vita, are fometimes call'd 'Perpetual Offices: In this Senfe U.Foutenelle is faid to be Perpetual Secretary of the Royal Academy of Sciences. Hence the French call him abfolutely M. le Perpetuel.
Perpetual Motion, in Mechanicks, is a Motion which is fupplied and renew'd from itfelf, without the Interven- tion ot any external Caufe ; or it is an uninterrupted Commu- nication of the fame Degree of Motion from one Part of Mat- ter to another, in a Circle, (or other Curve returning into it- felf) fo as the fame Momentum ftill returns perpetually undi- minifhed upon the firft mover. See Motion.
To find a perpetual Motion, or to conftrufl an Engine, iSc which fhall have fuch a Motion, is a famous Problem that has employ'd the Mathematicians of two thoufand Tears 3 tho' none perhaps have profecuted it with Attention and Earncftnefs equal to thofe of the prefent Age.
Infinite are the Schemes, Defigns, Plans, Engines, Wheels,- ££c. to which this Jong'd for perpetual Motion has given Birth: 'Twere as endlefs as impertinent to give a Detail of 'em all.
Nor does any of 'em deferve particular Mention, fince they have all equally prov'd Abortive. It wou'd rather be of the Nature of an Affront than a Complement, to diftingulm the Pretenders hereto ; when the very Thing they are commemo- rated for, carries with it fo difagreable an Idea.
- it
