the difference existing between the size and length of the cellular 

 and vascular tissues. 



Another opinion as to the method of formation of the membranous 

 tube of a vessel has been adopted by Mirbel, Treviranus and Slack, 

 which is, that this part of a vessel is formed from a number of cells 

 being arranged end to end, which have their connections ultimately 

 absorbed, so that there is produced one continuous cylinder instead 

 of several separate cavities. I must state that in many instances I 

 have met with arrangements of cells in such a way as would lead one 

 to suspect that this was the true origin of a vessel ; and it is a curious 

 fact that cells adhere end to end much more strongly than they do 

 side by side ; therefore, when disturbed, they appear more frequently 

 in strings than in other forms : but I believe no one has ever seen the 

 fact farther than here described, or followed out the complete deve- 

 lopment of a vessel from this condition of cells ; and there is one fact 

 presently to be mentioned, connected with the development of a ves- 

 sel, which entirely disproves this theory. 



In order to watch the development of the membranous tube of a 

 vessel, some part of a plant should be taken that is in a nascent state, 

 being about to be developed into the organ it is intended to repre- 

 sent ; for instance, a bulb or other bud answers the purpose, examin- 

 ing the scales from without inwards. But of all the parts hitherto 

 found the most convenient is the young flower-stalk of the "long 

 leek" {Allium porrum), which, in the state in which this vegetable is 

 usually sent to the market, is found to be about an inch or more in 

 length and from a quarter to half an inch in diameter. This organ 

 occurs very low down amidst the sheathing bases of the leaves, and 

 from having to lengthen to two or three feet, and containing large 

 vessels, forms a very fit subject for ascertaining the early appearances 

 of the vascular tissue. 



To examine the development of vessels, it is necessary to be very 

 careful in making dissections of the recent part, in order to witness the 

 appearances to be detailed ; and I have found it often more suitable 

 for the purpose to macerate or boil the part used for a short time, un- 

 til the tissues allow of being easily separated from each other. 



When the examination is directed to discover the larger vessels, 

 it will be found that at this early stage they present merely the form 

 of very elongated cells arranged in distinct lines, amongst which, it 

 appears extraordinary that some vessels are already arrived at matu- 

 rity, especially the annular, even before the cytoblasts have disap- 

 peared from any of the cells of the surrounding tissue. The young 



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