THE VASCULAR TISSUE. 



25 



The position of woody fibre is readily determined, 

 stems of wooded trees, but is found in single bundles 

 in the stems of delicate herbaceous plants, and may be 

 readily seen there when the stem is torn across. In 

 a similar manner it occupies the thin cuticle of herbs, 

 and may be readily observed in the ridges, or veins, 

 which run from the root upwards. It is also met with 

 in the bark of all trees, in the veins of leaves and 

 flowers, and even accompanying the spiral vessels 

 into the fruit of plants. 



Vascular Tissue or Trachenchym. The tis- 

 sues which we have already described are chiefly de- 

 voted to the circulation of fluids, or to the inclosure 

 of solid substances. Those, under this head, are in 

 great part associated with the transmission of air within 

 the plant. They arc divided into two classes viz., spiral 

 vessels and ducts, and are, perhaps, the most beautiful 

 microscopic objects in plants. It is not at all times 

 easy to distinguish between these two classes of struc- 

 tures, since both consists of thin'membrane in a tubular 

 form, and inclosing a fibre wound in a spiral direc- 

 tion. The theoretical distinction is, that the fibre of 

 the spiral vessel may be unrolled without' breaking, 



It constitutes not only the 

 A 



Fig. 64. Spiral Vessels. 



A, a simple spiral vessel, that is, 

 having but one fibre. The lines 

 bounding tbe pointed extremity 

 represent the inclosing mem- 

 brane. 



B, a compound spiral, or a vessel 

 composed of many fibres, wound 

 in a spiral manner. 



C, a compound spiral from the 

 Canna bicolor, with five spiral 

 fibres : more highly magnified. 



r hilst that of the duct is inseparably connected with 

 the membrane, and cannot be unrolled in its integrity. This general distinction is 

 doubtless correct ; but an unrolled spiral vessel, and a duct, in which the membrane con- 

 necting the spiral fibre has been destroyed, have a very close resemblance to each other. 

 It is highly probable that the distinction is less one of nature than one established by 

 botanists as a matter of convenience. 



The SPIRAL VESSEL is a cylindrical tube with conical extremities, and having one 

 or more fibres wound as a right or left-handed screw, which may unroll without 

 breaking. It has been disputed whether the fibre is placed within or without the 

 membrane, and whether it is solid or hollow ; but we are of opinion that it is inclosed 

 by the membrane, and that it is always solid. These vessels are not individually 



of great length, but are con- 

 nected together by their coni- 

 cal extremities; and it is not 

 unusual to find the intervening 

 partition ruptured. When but 

 one fibre is inclosed the vessel is 

 termed a simple spiral vessel (Fig. 

 64 A) ; but when two or more 

 exist, it receives the appellation of 

 compound (Fig. 64 B & C). In 

 some instances, upwards of twenty 

 fibres have been counted in a 

 compound spiral vessel. The 

 spiral vessels are very numerous in 



Fig. 65. A bundle of spiral vessels from the veins of the 

 hazel nut (Corylus avellana), showing their great num- 

 ber and very minute size. They are embedded in a mass 

 of hexagonal cellular tissue, as represented at a. Mag- 

 nified 200 diameters. 



