30 VEGETABLE ORGANOGRAPHY. 



Section II. 



Of Trachece, or Spiral and Elastic Vessels. 



The Spiral Vessels {vasa spiralia), or the Trachea 

 (trachecs), or, as Cassiiii calls them, the Helicules, 

 (PI. 2, Fig. 1, a a, 3, 4,) are organs of an entirely 

 peculiar kind, and about the structure of which there 

 has been much dispute. Henshaw discovered them in 

 the Hazel in 1661, that is to say, a year after the com- 

 pletion of the microscope by Hook. Malpighi, who 

 first examined them with care, compared them to the 

 tracheae of insects, which name he has retained: lie 

 regarded them as the respiratory organs of plants, and 

 described them as tubes formed of a band rolled spirally 

 upon itself, capable of unrolling with elasticity. The 

 unrolled tracheae can be easily seen in breaking a young 

 shoot of the Rose or Scabious. A trachea, seen by the 

 naked eye or through a lens, presents the appearance 

 of a brilliant silver band rolled spirally, as a spring in a 

 funnel. Duhamel compares it to a riband wliich has 

 been rolled round a cylinder, and which by its spi- 

 ral circumvolutions woidd form a continuous tube ; 

 Mirbel confirms this opinion of the structure of the 

 trachea, and only adds that the edge of the band is a 

 little thicker than the middle. Hedwig, on the contrary, 

 has described these same organs in a manner entirely 

 different ; he names them Vasa pneumatochymifera, and 

 believes them to be formed of two distinct organs : he 

 thinks that that which was considered to be a band 

 previous to his observations, is a real tube, which is 

 rolled spirally upon another straight and central tube : 



