VASCULAR TISSUE. 



357 



young flower-stalk of the long-leek (Allium porrum), in 

 the state in which this vegetable is usually sent to market ; 

 it is then most frequently found to be about an inch or 



Fig. 201. 



1, Portion ot transverse section of stem of Cedar, showing pith, wood, and hark. 



2, Portion of transverse section of stem of Clematis, shoving medullary ray>. 



more in length, and from a quarter to half an inch in 

 diameter. This membrane 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, it 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 

 plant; and it will be found useful to macerate the specimen 

 for a time in boiling water, which will render the tissues 

 more easily separable. When the examination is directed 

 in search of 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 some 

 vessels, especially the annular, will be found matured, 

 even before the cytoblasts have disappeared from the cells 

 of the surrounding tissue. 



As development proceeds, the vessels rapidly increase 

 in length, till they arrive at perfection. No increase in 

 diameter is perceptible after their first formation. At 

 this period, in the living plant the young vessels appear 

 full of fluid, which is apparently, as remarked by Schleiden, 

 of a thick character, and which he has designated vege- 

 table jelly ; by boiling which, or by the addition of alcohol, 

 the contents, or at least the albuminous portion, become 

 coagulated. From this circumstance, every cell appears 



