12 STRUCTURE AND PROPERTIES 



&c. ; but are usually wanting in wood and bark, 

 and are never seen in any of the lower tribes of 

 plants. Their diameter varies from the three 

 hundredth to the three thousandth of an inch. 



B. Ducts are transparent tubes, the sides of 

 which are marked with rings, bars, or transverse 

 streaks. They differ essentially from Tracheae 

 by being inelastic, and incapable of unrolling. 

 They are found in the wood of phaenogamous 

 plants, and of Ferns and Lycopodia?. 



4. " The office of all the ducts is the same 

 that of conveying fluid. It is only in the true 

 spiral vessel that we find air."* And even here 

 at certain periods of the existence of a plant, 

 fluid has also been found by recent observers ; 

 though if a branch be cut asunder whilst in a 



* Carpenter's Elem. Veg. Pliys. p. 66. 



"The functions ' of the Ducts' have not been accu- 

 rately determined. It is probable that they act as spiral 

 vessels when young ; but it is certain that they become 

 filled with fluid as soon as their spires are separated." 

 (Lindley's Elements of Botany, p. 6.) 



"There are some large Ducts which appear to have 

 originated from cells, which have been placed together 

 end to end, and whose partitions have been so broken 

 down as to form one continuous tube. These are the 

 largest vessels (if they may be truly so considered) in 

 the whole vegetable fabric, and are of the class called 

 ' dotted ducts:' through them the sap principally rises." 

 See Dr. Carpenter's Vegetable Physiology 84, et 



