28 ORGANOGRAPHY. BOOK I. 



because they occupy a station iii plants in which true spiral 

 vessels are not often found ; and it is therefore probable that 

 their functions are different. They vary between the j^q and 

 the g^j, of an inch in diameter. 



All the forms of the duct seem reducible to the followinir 

 varieties : — 



L The Closed (Plate 11. fig. IS.), which are absolutely 

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



2. The Annular {firj. 10., and Plate II. fig. 13. h.). 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 is broken at every coil, so as to separate into a number 

 of distinct rings. These rings are included within a membran- 

 ous tube, by which they are held together. Annular ducts 

 are conunon in the soft parts of plants, especially in such as 

 grow with much rapidity ; in the Garden Balsam they are 

 particularly abundant. 



3. The Reticulated [Jig. 8. 11., and Plate II. fig. 13. a.). 

 In these the spiral fibre, instead of separating into a niunber 

 of distinct rings, is continuous in some places, and anastomoses 

 in others, so as to form a sort of netted appearance. Vessels 

 of this kind, like the last, are found in the stem of some 

 herbaceous plants; as, for example, the Garden Balsam, in 

 which they may be seen in a great variety of states. 



4. The Scalarifor?n, which are extremely abundant in Ferns. 

 These are angular tubes, whose sides are marked by transverse 

 bars which scarcely reach the angles, but have such an appear- 

 ance as is exliibited by the cellular tissue represented at fig. 3. 

 page 7. These bars are unquestionably formed of short 

 lengtlis of solid fibre, as is exceedingly obvious in the wood of 

 Tree Ferns. 



In all probability the spiral vessel is the type of all these; 

 and the differences we perceive in them are owing to the vari- 

 ous modes in which tliey are subjected to the developing 

 forces. Tlius the closed duct may be considered to be abso- 

 lutely a spiral vessel, with little or no power of unrolling ; the 

 annular to be the same thing, but with the enveloping mem- 



