34 
ORGANOGRAPHY. 
BOOK I. 
4. The Scalariform, which are extremely abundant in Ferns. 
These are angular tubes, whose sides are marked by transverse 
bars that scarcely reach the angles. {Fig. 9.) 
Fig. 9. 
In all probability the spiral vessel is the type of all these ; 
and the differences we perceive in them are owing to the va- 
rious modes in which they are subjected to the force of 
developement. Thus the closed duct may be considered to 
be absolutely a spiral vessel, with little or no power of unroll- 
ing ; the annular to be the same thing, but with the envelop- 
ing membrane growing more rapidly than the enclosed fibre, 
which is consequently broken into pieces that contract into 
rings. Reticulated ducts may in like manner be considered 
as spiral vessels, whose internal spire, instead of snapping 
into short lengths as the membrane extends, accommodates 
itself to the growth of the latter by separating its coils, which 
thus gain an irregular direction, and grow together at points 
of variable distance. I think this view of the nature of ducts 
was first taken by Mr. Solly. It is well illustrated by Slack 
in the paper already referred to, and it derives additional 
strength from the fact, which, I believe, has never before been 
mentioned, that ducts, common as they are in the Garden 
Balsam when full grown, are scarcely to be found in that 
plant in a young state. 
Some anatomists have added to the varieties above enumer- 
ated, what they call strangulated vessels {vaisseaux en chapelet 
or etrangles^ coi'puscula vermiformia^ vasa moniliformia). These 
