CHAP. I. 
VASCULAR TISSUE. 
38 
Some have thought that the spiral vessels terminate in those 
little openings of the cuticle 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 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 and 
the Q~ of an inch in diameter. 
All 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 accurately, 
they, like spiral vessels, are formed of a spiral thread, but 
it is often broken, so as, in some parts, to separate 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 are particularly abundant. They are among the largest 
kinds of vessels. 
3. The Reticulated (Plate II. fig. \% f). 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. 
D 
