76 DUCTS. [BOOK i. 



pointed extremities ; 2. containing air ; 3. being composed 

 of a fibre or fibres and a membrane ; 4. uncoiling elastically. 

 So that, although not actually spiral vessels, in consequence 

 of the edges of the fibres not being free but adherent, 

 Dr. Griffith regards them as being undoubtedly formed from 

 them, and performing precisely the same physiological 

 functions. (See Annals of Natural History, vol. 10, p. 169, 

 where these appearances are figured.) 



Some have thought that the spiral vessels terminate in 

 those little openings of the epidermis called stomates; but 

 there does not seem to be any foundation for this opinion. 



DUCTS (Plate II. fig. 12. and 15.) are membranous tubes, 

 with conical or rounded extremities ; their sides being marked 

 with transverse lines, or rings, or bars, and being incapable 

 of unrolling without breaking. 



These approach so nearly to the spiral vessel that it is im- 

 possible to doubt their being a mere modification of it. Some 

 writers confound all the forms of ducts, and even bothrenchym 

 itself, under the common name of spiral vessels, but it is more 

 convenient to consider them as distinct, not only on account 

 of their peculiar appearances, but because they occupy a 

 station in plants in which true spiral vessels are not often 

 found; and it is therefore probable that their functions are 

 different. They vary between the 4 -^ and the -^ of an inch 

 in diameter. 



The forms of the duct seem reducible to the following 

 varieties : 



1. The Closed (Plate II. fig. 15.), which are absolutely the 



same as spiral vessels, except that they will not unroll. 



2. The Annular (Plate II. fig. 12. d). These are well 



described by Bischoff as being formed of fibrous rings, 

 placed at uncertain intervals ; or, to speak more accu- 

 rately, they, like spiral vessels, are formed of a spiral 

 thread, but it often separates into a number of distinct 

 rings. These rings are included within a membranous 

 tube, by which they are held together. Annular ducts 

 are common in the soft parts of plants, especially in such 

 as grow with much rapidity ; in the Garden Balsam they 



