SECT. H. 



TUBES. 363 



perhaps formed of the true spiral tubes, by means 

 of a partial union of the spires. 



ARTICLE 5. Mixed Tubes. The mixed tubes are Descrip- 

 tubes combining in one individual two or more of 

 the foregoing varieties. Mirbel exemplifies them 

 in the case of the But omits iimbellatus or Flowering 

 Hush, in which the porous tubes, spiral tubes, and 

 false spiral tubes, are often to be met with united 

 in one. He seems however to be of opinion that 

 the appearance is to be regarded as being merely 

 an indication of the incipient stage of the process, 

 of the union of the contiguous rings of the spiral 

 tubes, by which they are to be converted into a 

 new form. 



SUBSECTION II. 



Small Tubes. The small tubes are tubes com- Formed of 

 posed of a succession of elongated cells united 

 like those of the cellular tissue. Individually they 

 may be compared to the stem of the grasses, which 

 is formed of several internodia separated by trans- 

 verse diaphragms ; and collectively to a united as- 

 semblage of parallel and collateral reeds. The 

 membrane of which they are formed is often pierced 

 with a great number of pores ; but is at the same 

 time thick and strong,, being cut with difficulty in 

 a transverse direction, though in a longitudinal di- 

 rection it is divisible without effort. M. Mirbel 

 says the solidity of the vegetable depends upon the 

 number of interrupted or cellular tubes. They are 



