28 
ORGANOGRAPHY. 
BOOK I. 
because they occupy a station in plants in wliicli true spiral 
vessels are not often found ; and it is therefore probable that 
their functions are different. They vary between the and 
the of an inch in diameter. 
All the forms of the duct seem reducible to the following 
varieties : — 
1. The Closed (Plate II. fig. 18.), which are absolutely 
the same as spiral vessels, except that they will not unroll. 
2. The Annular {fig. 10., and Plate II. fig. 13. b.). 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 thready 
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 common 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 (fig. 8. 11., and Plate II. fig. 13. a.). 
In these the spiral fibre, instead of separating into a number 
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 Scalariform^ which are extremely abundant in Ferns. 
These are angular tubes, whose sides are marked by trans vei*se 
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 
lengths of solid fibre, as is exceedingly obvious in the wood of 
Tree Ferns. • 
In all probability the spiral vessel is the t}^e of all these; 
and the differences we perceive in them are owing to the vari- 
ous modes in which they 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 wdth the enveloping mem- 
