TISSUE OF VESSELS. 73 



Mirbel and De Candolle seem to support the notion 

 of Du Ham el, that the spiral vessels are air tubes. 



Treviranus and Bernhardi distinguish several sorts 

 of these spiral vessels, which may probably serve dif- 

 ferent purposes. 



Kieser, most originally and ingeniously following 

 the suggestions of Grew, attempts to demonstrate, 

 from numerous observations on the gourd, that the 

 variations in the form of the straight and the spiral 

 vessels, arise in the different progressive stages of 

 growth. 



In the first stage they are of two forms : simple, 

 constructed of one or more contiguous fibres, twisted 

 round an empty space; and ringed, or consisting of 

 a series of rings, disposed in a perpendicular line. 

 " The turns," says, Link, " of the tube become mutually 

 detached; the rounds become more slender; and in- 

 tervals 1 are formed which are never seen in young 

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

 scribed by authors, nor are there pores either in the 

 vessels or the cells. Often what are said to be pores 

 are nothing more than the remains of a spiral tube ; nor 

 are there pores either in the vessels or the cells. Fre- 

 quently what are alleged to be pores are nothing more 

 than the remains of a spiral tube, or of small bulgings 

 thereof, remaining permanent, when the spiral cha- 

 racter has disappeared." 



(1) In Latin, Lacunae; objectionably, Pseudo-tracheae. 



