LINK ON THE FLUIDS OF PLANTS. 



107 



but I found in some experiments, particularly in those made upon 

 Vinca herbacea, that it was the vessels (trachees) alone which 

 took a blue colour. The resorption then appears to be made by 

 the same vessels as the absorption of fluids. 



It is the entire tube which conducts the sap and not the spiral 

 tube, as Hedwig alleges, (see B, in the figure above,) though his 

 opinion is partly true, for the spiral tube is in reality a vessel • 

 as I have often seen it coloured interiorly. I have represented this 



in a slice from the root of the date (Phoenix dactylifera,) cut length- 

 wise, and seen by an excellent microscope by Frauenhofer of Munich. 

 At b are seen the modified vessels (trochees) having the rest of the 

 spiral tube coloured as in the entire tube at a. At c there is a 

 straight vessel (vaisseau) coloured in the same manner. At d there 

 is not the least colour, which proves that the colour observed is 

 not an optical illusion. 



There is then no real difference among the vessels of plants, 

 whether they be termed tracheae pseudo-tracheae, porous tubes, or 

 annular vessels. All these are organs which conduct the sap and 

 distribute it throughout the plant. They can be all coloured blue, 

 as I have above explained ; and the changes are wholly produced 

 by the changes of the spiral tube. The turns of this tube become 

 mutually detached, the rounds become more slender, and intervals 

 (lacuna) termed pseudo-tracheae are formed, which are never 

 seen in young plants. There are no porous tubes, such as are de- 



