VASCULAR TISSUE. 437 



arrangement of the fibre has been considerably modified in its depo- 

 sition. Elongated tubes or ducts, with porous walls, come under the 

 head of vascular tissue ; they somewhat differ from the spiral varieties, 

 inasmuch as they cannot be unrolled without breaking. It is a curious 

 fact, that mostly the spiral coils from right to left ; and it has been 

 suggested that the direction of the fibre may determine that in which 

 the plant coils round an upright pole. The Hop % as left-handed spirals, 

 and is a left-handed climber, which would therefore appear to support 

 this theory. The nature of the fibre, and the development of the 

 tissue, have been frequently the subject of dispute between botanists. The 

 late Mr. Edwin Quekett gave much attention to the subject ; and in 

 an excellent paper published in the Microscopical Society's Transac- 

 tions, 184.0, gave the result of his observations. 



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

 vessel, no better example can be chosen than the young flower- stalk of 

 the long-leek (Allium pornwn), in the state in which this vegetable is 

 usually sent to market ; it is then most frequently 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 contain- 

 ing 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 ; 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 dis- 

 appeared 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 vegetable 

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

 at least the albuminous portion, become coagulated. From this circum- 

 stance, every cell appears to enclose another in a shrivelled condition ; 

 this state is sometimes so far extended, that a thick granular cord is 

 all that can be seen of the contents. When the granules have arranged 



