222 ANATOMY AND PHYSIOLOGY OF VI.AM-. 



drawn asunder, the spiral coats of their vessels will unroll, exhibit- 

 ing a curious spectacle even to the naked eye. 



13. Considerable difference in opinion, has always 

 been entertained concerning the real nature, form and 

 structure of the common vessels. From what we can 

 collect upon the subject, it would appear, the specific 

 varieties of spiral, annular, heeded and so on, frequently 

 mentioned in modern authors, are nothing more than 

 common sap-vessels under different states of trans- 

 formation. 



It appears, that the common vessels, though they may originally 

 have a spiral character, do not continue the same. By the action of 

 the v ital principle on their structure, they not only sometimes acquire 

 the annular or other specific quality, but are in their regular turn, 

 completely transformed to proper vessels, in the same manner that 

 the liber is annually converted into outer bark. 



14. The proper vessels are simple tubes formed of 

 imperforated membrane. They are cylindrical, firmer 

 and larger than the central, and are generally disposed 

 in the cellular part of the bark. 



Entire vessels may be found in the young shoots of almost every 

 kind of plant ; and in the fasciculated state, may be readily detected 

 and examined by the aid of magnifying glasses, in the leaf-stalks ot 

 the common fern, in the arrow-head, and in the common hop. 



15. The proper vessels are intended to convey the 

 proper juices of the plant. 



They are always found to contain the peculiar or proper >uices of 

 the plant. Thus, the juice with which they are filled i milky in 

 the various species of spurge, in the common dandelion, and several 

 oilier kinds of syngenesious plants. It is yellow or orange-coloured 



