] 807.J 



Proceedings of Learned Societies. 



571 



(lefccndcd tlirougli their burk. Subfe- 

 quent expeiiinciUs have confirmed clus 

 tlieory. It is genersdly admitted, that 

 the matter which enters into tlie com- 

 polition oF the radicles of the germinat- 

 ing feeds, exiltfc'd previoully in their 

 cotyledons ; it follows, that the firft mo- 

 tion of tlie tiue f<ip at this period is 

 dowimaids. Aud as no al burnous tubes 

 exill in the radicles of {germinating feeds 

 during the ewlier perioils of their t^rowth, 

 the fap in its deicent mull eitlier pafs 

 through the hark or the medulla. But 

 the rucduUa does not apparently contaia 

 any veflTels calculated to carry the dc- 

 •tceijding fap ; whilft the cortical veiVels 

 are, during this period, much dillendcd 

 and full of iiioilUue : and as the medulla 

 certainly does not cany any fluid in 

 Uems or branches of more than one year 

 old, it can fcarcely be fulpected that it 

 at any period conveys the whole current 

 of the (ielcending fap. As the leaves 

 grow and enter on their otfice, cortical 

 velfels, in every refpett apparently fnni- 

 lar to tho'c which defcended from the 

 cotyledons, are found to defceiid from 

 the bales of the leaves; and there ap- 

 pears no rcafon to fiifjicci that both do 

 not carry a funilar fluid, and that the 

 courfe oi' this fluid is, in the firft inftance, 

 alviays towards the roots. 



The afrending fap, on the contrary, 

 rifes wiioUy through the alburnum and 

 central vell'els ; for the deftruMion of a 

 portion of bark, in a circle round the 

 tree, does not iiinnediately, in the flight- 

 ell degree, check the growth of its leaves 

 and branches; but the alburnous veffels 

 appear, as well from former experi- 

 ments as from thofe now related, to be 

 capable of an inverted aftion, when 

 "that becomes necefVary to preferve the 

 exilience of the plant. We cannot fol- 

 low Mr. Knight in all his curious and 

 interefting experiments. In tuberofe- 

 rooted plants, he obferves, the roots and 

 ftems which collect and convey the fap 

 in one fcafon, and thofe in wliich it is 

 depofited and rcferved for the fucceed- 

 iiig feafjii, are perfectly dillintt organs : 

 •and from one of thefe, viz. the potatoe, 

 he obtained fbme interefting and decifive 

 rcfults. The principal objeft was to 

 prove, that a tiuid defcends from the 

 leaves and flcm to form the tuberous 

 roots of tiiis plant ; and that this fluid 

 will in part efcape down the alburnous 

 fubflance of the item, when the conti- 

 .nuity of the cortical vefl'els is interrupted. 

 1'he early potatoe, it is well known, 

 .never allWds citlier bloffoms or Iccda, a 



peculiarity which Mr. Knight mttributed 

 to a privation of nutriment, owing to 

 the tubers being formed prttcruaturall/ 

 early, and thence drawing oif that por- 

 tion of the true fap, which, in the ordi- ^ 

 nary courfe of nature, is employed in 

 the formation and nutrition of biolfoms 

 and feeds. To aic;crtain this, he plante4 

 fomc cuttings of a very early potatoe in 

 garden-pots, heating the mould as high 

 as poffible, and planting the root nearlj 

 at the top. When the plants had growa 

 a few inches high, they were fecured by 

 Itrong flicks, and the mould waflied awajf 

 from the bate of tlie ftenis by a flrong 

 current of water. Each plant was now 

 fufpended in air, and had no connnuui- 

 cation with the foil, except by its fibrous 

 roots; and as thefe are j)erfe6tly diliiuct 

 organs from the runners which generate 

 and feed the tuberous roots, the forma- 

 tion of them was eaiily prevented. Ef- 

 forts were foon made by every plant to 

 generate runners and tuberous roots; 

 thefe were deilroyed as fo(^n as they be- 

 came perceptible, and an increafed luxu- 

 riance of growth became vifible in every 

 plant, numerous bloiroms were emitted, 

 and every blolVum aft'ordcd fruit. Con- 

 ceiving that a linall part only of tiie true 

 fap would be expended in this way, Mr. 

 K. was anxious to know what ufe nature 

 would make of that which remained; 

 he prevented, therefore, the formation 

 of tubers on any part of the plants, ex- 

 cept the extremities of the lateral 

 branches; thofe being the points moll 

 dillant from the earth, in which the 

 tubers are naturally depofited. After 

 fome flruggle, the plants became per- 

 fectly obedient to Mr. K.'s wifhes, and 

 formed their tubers precifely in the 

 places he had affigned them. Many of 

 the joints of the plants, during the ex- 

 periment, became enlarged and turgid ; 

 and lie thinks, if he had totally pre- 

 vented the formation of regular tubers, 

 thefe joints vvould have acquired an or- 

 ganization capable of retaining, life and 

 of affording plants in the fucceeding; 

 fpring. Another experiment we Ihall 

 gi^"e in his own words : 



" I had another variety of the pota- 

 toe, which grew with great luxuriance, 

 ind afl'orded many lateral branches; and 

 jufl at the period when I had nfcertained 

 tiie firfl commencing formation of the 

 tubers beneath the Ibil, I nearly detached 

 many of thefe lateral branches from the 

 principal flcms, letting them remain fuf- 

 pended by fuch portioiT only of alburr 

 uous aud cortical fibres aud veffels. as 



weie 



