THE VESSELS. 39 



then, from Striped Vessels, in having these dots separated 

 from one another, and not joined in continuous lines ; 

 and from Strangulated Vessels, (of which we shall speak 

 presently,) in their tube being cylindrical and not con- 

 tracted at intervals. 



These vessels are found abundantly among Dicoty- 

 ledons, in the woody layers both of the root and of the 

 stem and branches; among Monocotyledons, in the 

 woody bundles : when they make part of a bmidle of 

 vessels, they are commonly situated in the side nearest 

 the circmnference of the stem. We are assured that they 

 have also been found in the bark of Dicotyledons, but 

 this fact is contested by more recent anatomists. The 

 diameter of dotted vessels generally exceeds that of 

 tracheae and striped vessels ; but this rule is liable to 

 frequent exceptions. 



Kieser considers dotted vessels to be formed of a 

 trachea or annular vessel, of which the spires or rings 

 are joined by a dotted membrane. 



Mirbel, who first discovered these organs, does not 

 admit the evidence of spires or rings in the formation of 

 these vessels ; but regards them as simple membranous 

 tubes, marked with pores which are surromided by a 

 rim, giving them a dotted appearance. 



Dutrochet also considers them as simple membra- 

 nous tubes, marked, not with pores, but with dots 

 caused by projecting vesicles. 



As for my own observations, I have neither seen 

 the spires nor the rings which Kieser admits in the 

 structure of these vessels ; but as I am of opinion that a 

 negative observation cannot invahdate a positive one, 

 when it is confirmed by all observers, I hesitate be- 

 fore I confirm their absence. At present I am disposed 

 to consider these vessels as membranous tubes, marked 

 with glandular doti. 



