54* 



Expei'iinents en the Respiralion of Vegetables. [Nov. I , 



to determine tlie precife fituation oi 

 thefe tracheiF. In order to accomplifti 

 llijs, it is neceffary to fele6t vegetables 

 v/hoie pith is very abundant ; fiich, for 

 example, as the elder. 



1. I took, at the end of fprinsj, new 

 fiiQOts of the elder, and divided them 

 nearly half through the middle with a 

 pen-knife, and afterwards bent them 

 hack, in fuch a manner as not wholly to 

 fcparate the two extremities. In this 

 fi.tuation numerous trachea; are readily 

 thfreiniblc befvvocn the medulla and the 

 wood'. 



Tliis medullary part of the flinot, 

 which is replete with a very abundant 

 g;reeui(h juice, may juftly be compared to 

 the inner fubllance of fomc fruits, fuch 

 as an apple, pear, peach, &c. But in 

 jjro|)ortion to tl»e augmentation of the 

 Ihoot, this juice becomes lets in quan- 

 tity, and towards the end of funmier it is 

 fo much diminilhed, as fcarcely to tinge 

 the pith, which at this period is almofc 

 eomplotely white. 



2. After divichng with care all the lig- 

 neous parts of a young brancli of elder, 

 in the medulla of which the grcenllh 

 juice was no longer vifiblc, I jjlainiy 

 perceived the tracliex forming a zone 

 round the medullary fubilance, and, on 

 gently feparating the two extremities, I 

 beheld the elongation of the air-velVels, 

 or trachea-. 



3. I fplit one of tJie elder branches 

 em[)loycd in the preceding ex])eriment, 

 and raifed the medullary part with cau- 

 tion ; the trachcaj had not been divided, 

 and were perceived adhering lo the lig- 

 neous part contiguous to the medulla, 

 forming a concentric (iratum. They 

 ■were divided mto fmall fafciculi, to the 

 number of thirty or forty, all in contact 

 with each other. In order to examine 

 them accurately, it is necclVary to em- 

 ploy a microfcope. 



Each of thefe fafciculi may be about a 

 fourth of a line in diameter, and con- 

 tains a great number of air-vcffels, or 

 trachea?. Tliefe combined tracheae form 

 a kind of iJieath, which furrounds the 

 pith on every iide. 



-]. (Jn railing the pith from tliisliranch 

 of elder, I could readily diftingiiidi a 

 jii-eat number (jf reddiJb veficls, ibrming 

 in general a concentric zone. They arc 

 firuated within the medulla, at the <lif- 

 tance of nearly a quarter of a line from 

 the vvood. When thefe red veilels are 

 ii:.i.:icKcd from the pith, and viewed with 



a microfcojje, they appear feini-tranfpa- 

 rent, and compofed, like the lymphatic 

 veffels in animals, of conglobate maiVes. 

 They completely diH'er from the trachea, 

 or air-veffels ; and it appears highly pro- 

 bable, that, through them, a circulation 

 is carried on in tiie medullary fubilance, 

 in like manner as in fruits. 



From thefe obfervations it fliould 

 feem that the tracheae are neither fituat- 

 cd in the ligneous nor medullary fub- 

 ilance, but that they form an intermedi- 

 ate layer between the two, fo as to fervc 

 as a kind of ihealh or covering to the 

 pith in the elder, inc. 



It is extremely probable that they ac- 

 company the elongations of the medulla 

 int/) the wowly fubftance, as far as the 

 bark itfelf, for plants appear to inlpire 

 and expire through every part of their 

 furface. From analogy we are alfo led 

 to conclude that the fame organization 

 takes place in all vegetables. 



The inferences deducible from thefe 

 obfervations are, that the trachea; ferve 

 the pm'pofe of refpiratory organs in vege- 

 tables whilft the circulation is carried on 

 through the red vclfcls 



Atmofpheric air is abforbed or inhaled 

 by the leaves, and the whole furfacc of 

 vegetables. Plants placed under glafl'es 

 clofed by mercury abforb air. This air 

 is afterwards expelled or expired by the 

 fame organs, for it may be I'een to efcape 

 from the leaves placed in xirater, and ex.- 

 pofed to the influence of ihe fun. 



Hence it Ihould feem, that the air in- 

 fpired by the furface of the vegetable 

 ])enetrates into the trachea;, by which it 

 is carried into every part of the medul- 

 lary fubllance. This air connnunicates 

 with the large fucciferous vclfels, chieily 

 with the veins, thus vivifying and oxy- 

 genizing the whole mafs of fluids. 



Trachea-, or air-velVols, are dillributed 

 through every part of the body of in- 

 fects, and ferve to circulate the air which 

 vivifies and oxygenizes all their fluids ; 

 and from the, above experiineuls it has "^ 

 been, rendered futliciently evident that a 

 fimilar mcchanilin prevails throughout 

 the vegetable kingdom. 



From this fmnlarity of organization, 

 fiii;iil;ir dil'eales frequently prevail in ani- 

 niak and vegetables. 



The importance of this truth, now fo 

 univerfidly acknowledged, has induced 

 1 he Academy of VVilua lo propofe the 

 following <iueft,ion : — " What is t/ic Cauj'e 

 ofliic DiJiuiJ'es of Ve^rlubks f 



For 



