THE VESSELS. 31 



he imagines that these spiral tubes are destined for 

 carrying the sap, and he calls them for this reason 

 Vasa adducentia spiralia, Vasa chymifera hydrogera; 

 on the contrary the central tubes may always be full 

 of air, whence he has given them the name of Vasa 

 pneumatophora. Schrader and Link differ from the 

 opinion of Hedwig in this, — that instead of admitting 

 that the spiral is formed by a tube, they think that it 

 is composed of a band hollowed out into a gutter in its 

 internal part. Bernhardi, on the contrary, admits 

 that there is an upright membranous tube, continuous 

 and transparent, in the interior of which is rolled a 

 spiral band which serves to keep it open ; and which, 

 proceeding to imroll itself with elasticity when the 

 exterior tube is broken, only appears to our eyes under 

 the name of trachea. He supposes that this same band 

 exists in the tubes of all vessels ; that when it is con- 

 tinuous and spiral, it forms the trachea ; when it is divided 

 into interrupted lines, the striped vessel, (PL 2, Fig. 2, a ;) 

 and more interrupted still, the dotted vessel, (PL 2, 

 Fig. 2, h.) Kaeser calls what we name trachea, a simple 

 spiral vessel; and he believes that membrane exists 

 neither on the inside, nor on the outside, nor among the 

 spires. Lastly, Dutrochet admits that the spires of 

 the tracheae are united by an intermediate transparent 

 membrane, which is torn when the spiral thread is 

 unrolled ; he believes that in their natural state they 

 have not any openings, but that they form a continuous 

 tube. 



To attempt to choose our opinion in the midst of so 

 many contradictions, it is necessary to discuss separately 

 each of the assertions of these authors, and to divest 

 them as much as possible of all systematic idea. 



1st. Does there exist in the interior of the spire a 

 peculiar tube, as Hedwig was the first to assert ? — Let 



