CHAP. I. 
VASCULAR TISSUE. 
29 
brane growing more rapidly than the enclosed fibre, which is 
consequently broken into pieces which 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 men- 
tioned, 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 etranglcs, corpuscula vermiformia) . These are rightly de- 
termined by Bischoff to be mere accidental forms, caused by 
their irregular compression, when growing in knots or parts 
that are subject to an interrupted kind of developement. 
They may be found figured in Mirbel's Elemens, tab. x. 
fig. 15.; and in Kieser, fig. 56. and 57.; but the best 
view of their origin and true nature is in Slack'g plate, 
fig. 33., in the Transactions of the Society of Arts, before 
referred to. 
Vascular tissue always consists of tubes that are unbranched. 
They have been represented by Mirbel as ramifying in some 
cases; but this opinion has undoubtedly arisen from imperfect 
observation. When forming a series of vessels, the ends of 
the tubes overlay each other, as represented in Plate 11. 
fig. 18. 
Slack states that the membrane is often obliterated at 
the place where two vessels touch each other, and that trans- 
verse bars only remain under the form of a grating ; this 
appearance is produced by the remains of the spiral fibre, se- 
veral of whose convolutions are partially uncovered by the 
absorption of the enveloping membrane. It would hence ap- 
pear that ducts open into each other at their points of contact. 
