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 vai'iable 

 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 thev 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 etraiujles^ 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 developem.ent. 

 Tliey 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's 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 II. 

 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. 



