A I R 



A I R 



liir^rs of animals ; the latter to tlicir larteals and l)lo<i<.i- 

 vcfli-h. 



Dr. Grew, in an enquiry into the mot!i)ii and e'aule of 

 the air in vegetable:;, lhe«s, that it enters them various 

 ways, not only by the trunk, leaves, and other parts above 

 groiuid, but at the root. For the reception, as well as 

 txpult'ion of air, the pores arc fo very larjje in the trunks of 

 fume j;lants, as in the better fort of thick walking canes, 

 that they are vifible to a good eye witliuut a glals ; but 

 with a glafs, the cane feems as if it were ihick full of large 

 pin-holes refenibling the pores of the ll<in in the ends ot the 

 tingers, and ball of the hand. In the leaves of the pine, 

 through a gl^fs, they make an elegant Aiow, Handing 

 ahnoll exaftly in rank and file throughout the length of the 

 leaves. 



But though the air enters in partly at the trunk and alfoat 

 other parts, efpecially in foinc plants, yet its chief admidion is 

 at the root : much as in animals, lomc part of the air may 

 continually pafs into the body and blood by the pores of the 

 (kin ; but the chief draught is at the mouth. If the chief 

 entrance of the air were at the trunk, before it could be 

 mixed with the fap in the root, it mull defcend ; and fo 

 move not only contrary to its own nature, but in a contrary 

 courfe to the fap : whereas by its reception at the root, and 

 its tranlition from thence, it has a more natural and eafy 

 motion of afccnt. See Circulation of /aj>. 



The fame fact is farther deduced, from the linenefs andfmall- 

 nefs of the diametral apertures in the trunk, in comparlfon 

 of thofe in the ro<it ; which nature has plainly defigne'd for 

 the feparation of the air from the fap, after they. are both 

 together received into them. Grew, Anat. of Root, c'lap. 

 iii. p. 127. 



Air-veffels are found in the leaves of all plants, and are 

 even difcoverable in many without the help of gkifl'es ; for 

 upon breaking the ftalk or chief iibres of a leaf, the llke- 

 nei's el a fine \\ holly fubftance, or rather of curious fmall 

 cob-wcbs, may be feen to hang at both the broken ends. 

 This is taken notice of only in fome few plants, as In 

 f,:abious, where it is more viiible : but may alfo be feen 

 more or lefs in moft others, if the leaves be veiy tenderly 

 broken. This wool is really a flvcin of air-veffels, or ra- 

 ther of the fibres of the air-veffels, loofed from their fpiral 

 pofition, and fo drawn out in length. Id. ibid. chap. iv. 



That air is infpired by vegetables, has been fully proved 

 by Dr. Hales, in his Statical Effays, (vol. i. p. 155, &c.) 

 and he has in many inllanccs fhewn, that air freely enters 

 the veffels of trees, and that it is in great abundance wrought 

 into their fubllance. But as to particular air-veffels in 

 plants, he feems to fpeak doubtfully. He fays, by way of 

 queftlon, may not the ufe of thofe fpiral v^-reaths, that are 

 coiled round the infides of thofe veffels, which are fuppofed 

 to be air-veffels, and which are manifellly to be feen in feve- 

 ra! trees, and alfo in the leaves of the vine and fcablous, 

 may not thefc be defigned by nature to promote the quicker 

 afcent of air, by being in fome meafure conformed to its 

 elaftic contortions ? For fuch fpiral wreaths feem to be alto- 

 gether ufelefi;, for promoting the afcent of any liquor, as 

 the fap, which afcends raoll freely through innumerable 

 other capillary veffels, having no fuch fpiral coik in them : 

 not that we are to fuppofe the air in its elallic ilate actually 

 to touch, and thereby to be determined In the courfe of 

 thefe fpirals, as any liquor would be. But as the rays of 

 light, when they are reflefted from a folld body, are found 

 to be reflected, without . actually touching the reflecling 

 body in the point of rcfleftion ; fo it is not unreafonable to 



fnppofe, that elaftic air niay, like light, be diverted from 

 one courfe, and fo be determined to another, by the folid 

 bodies it approaches, without touching them, but rebound- 

 ing like light from thofe folid bodies near the point of 

 contaft. 



Dr. Hales has obferved, that thefe fpirals are coiled in a 

 courfe oppolite to the courfe of the fun, thiit Is, from well 

 to eall. Vide Static. Eff. vol. 11. p. 265, 266. 



Dr. Darv.ln, in his Phytologia, obferves, that the veffels 

 wlilch Malpighi, Grew, and many others, have denomi- 

 nated bronchia, and erroneoufly thought to be air-veffels, 

 and to ferve the pnrpofe of refplratoiy organs, are abfor- 

 bent veffels, dcftined to imbibe the luitriment of plants, and 

 that they arc the genuine lungs of vegetables. Thefe 

 abforbcnt veffels, he fays, which refemblc the lafteals of 

 animal bodies, are found in the roots of plants for imbibing 

 nourldiment from the moHl earth, on the external furfaces of 

 the bark and leaves, for abforbing the humidity of the at- 

 mofphere, and alfo in the internal lurfaces of the cells and 

 cavities of tlie vegetable fyllem, where they abforb the fe- 

 creted fluids, after they have perfonned the offices to which 

 thev are adapted. The exillence of the hrft fort of abfor- 

 bents, is evmced by the growth of plants, whlllt moilhire 

 is apphed to their roots, and by tlurlr withering when it is 

 withdrawn. Thofe of the fccond fort are manifefted by 

 plucking off a leaf and laying it in water, which is found 

 not to wither fo foon as if it were left expofed to the dry 

 air. The third clafs of veffels of this kind will be perceivtid 

 to perform its office by molftenlng the alburnum or fap-wood, 

 and the inner furface of the bark of a branch fevered from 

 a tree, which are thus preferved, whilff the fame parts left 

 unmollteneei in the dry air arc obferved to wither. Befidcs, 

 if vegetables be infertcd in glafs-tubes or narrow veffels, filled 

 with water, the furface of the water will be feen to fublide 

 much fooner than b)' evaporation alone in iimilar circum- 

 ftances. Dr. Darwin alfo contvived to evnnce thefe abfor- 

 bent veffels to the eye, by rjipping twigs of a fig-tree in a 

 decoction of madder and of logwood, which after fome 

 time, upon cutting off about an inch of the ftalk near the 

 bottom, exhibited a circle of red points, believed by him 

 to be the coloured ends of the abforbents, that exHied in 

 the newly formed alburnum. This ingenious writer expref- ' 

 fes his alloniniment that any perfon Ihould have conceived 

 thefe veffels, that are found in the alburnum, and which con- 

 fift of a fpiral line, to be air-veffels or tubes. He farther 

 obferves, that the abforbent veffels of trees, in paffing down 

 their trunks, coniift of long hollow cylinders, of a fpiral 

 fonn, and of fuch large diameters in fome vegetables, e.g. 

 in cane, as to be viiible, when di-y and empty, to the naked 

 eye. Through thefe air will pafs rapidlv upward and down- 

 ward ; and hence Dr. Hales has been led to coincide with 

 Grew and others in opinion, that they are air-veffels or 

 lungs, defigned for refpiration, and receding atmofpherical 

 air in their natural Hate. But to their ufe as air-veffels he 

 objefts, becaufe they have no communication with the hori- 

 zontal air-veffels of plants, and they exilirin the roots as 

 well as in the trunks of plants, where, not being expofed 

 to the atmofphere, they cannot ferve the purpofe of refpi- 

 ration. Air, however, in its combined ftate, or diffolved 

 in water, may be abforbed by thefe veffels ; and may appear 

 when the preffure of the atmolphere is removed in the ex- 

 haulled receiver, or when it is expanded by heat, as is the 

 cafe in the froth obferved at one end of a green ftick, when 

 the other is burning in the fire. Dr. Darwin apprehend*, 

 that the ftruclure of thofe large vegetable abforbents, which 

 have been erroneoufly called air-veffels, confifts of a fpirjl 



line, 



