TRACHEA AND TYLOSES. 161 



more approaching the ducts (D) ; finally the ducts themselves, 

 either separated into segments (F), or else forming long tubes. 

 The very long and narrow-cavitied bast-fibres (G) are very notice- 

 able in the preparation. 



Length of the Trachea. If for an example a twig of the Alder, 

 Alnus glutinosa, eight or ten years old, be plunged under water, 

 and, from its middle portion, a piece of about four inches in 

 length be cut while still immersed, this piece can be used for the 

 purpose of studying the length and continuity of the vessels. 

 To the upper end of this piece of twig, i.e., the end which was 

 nearer the leaves, can be attached a glass-tube, about two and a 

 half inches long, by means of a piece of india-rubber tubing, the 

 other end of the glass-tube being connected with an air-pump 

 or an aspirator. The lower end of the piece of twig is now 

 immersed in a fluid composed of one part commercial " dialysed 

 iron" (officinal liquor ferri oxychlorati) in three parts water. 

 By the gentle action of the pump, or by the aspirator, this brown 

 fluid can be sucked into the piece of twig, and it will be seen that 

 the fluid which flows out into the glass-tube is colourless. The 

 action may be continued gently for about an hour. Oxychloride 

 of iron is a colloid body, and therefore diffusible neither through 

 membranes nor through the closing membrane of bordered pits ; 

 hence it penetrates into each vessel only so far as the first closing, 

 diaphragm-like wall ; and so long as the brown fluid does not 

 appear in the glass-tube it is clear that it has not passed through 

 the length of the twig. 



When injection with the iron is judged to be sufficiently com- 

 plete, the lower end of the twig can be removed to a solution of 

 ammonia (one part official spirits of sal ammoniac, three parts 

 water), and the suction again resumed until the fluid which is 

 drawn out at the upper end strongly smells of ammonia. All the 

 iron salt will then have been deposited as a reddish- brown pre- 

 cipitate ; and both with a lens and with microscopic sections, 

 transverse and longitudinal, the distribution of this precipitate in 

 the piece of twig can be studied. 



Precisely similar experiments can be performed with Tilia. 



Tyloses, or Tracheal Plugs. In the vessels of especially the 

 older wood of many stems and roots are to be found, sometimes 

 occasionally, sometimes systematically, plugs of intruded cells, 

 completely stopping the cavity, known as Tyloses, or, as we may 

 call them, Tracheal plugs. We will study them in the wood of 



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