16 



FORM AND ARRANGEMENT OF VESSELS. 



surrounding the pith, and they are traced from it into the leaves. 

 They are rarely found in the wood, bark, 

 or pith. Spiral vessels occasionally exhi- 

 bit a branched appearance. This may 

 arise from the union of separate vessels in 

 an angular or jointed manner, as where a 

 leaf or branch is given off (fig. 52 a a), or 

 it may depend on a regular division of the 

 fibres, as is seen in the Misletoe, Long- 

 leek, and Gourd (fig. 53). 



34. The fibre is on the inside of the mem- 

 brane. Quekett has shown this in silicified 

 spiral vessels, where the mark of the spiral 

 was on the outside of the mineral matter 

 filling the tube. The fibre usually turns 

 from left to right, if we suppose the observer placed in the axis of 

 the tube (fig. 54), or from right to left, if we suppose him looking 

 at the vessel in its natural position. The fibre retains its direction 

 throughout the length of the vessel. When examined under the micro- 

 scope, there is often the appearance of the crossing of fibres (fig. 54), in 

 consequence of the transparency of the membrane, and the 

 observer seeing the fibre on each side of the vessel at the 

 same time. In twining plants, the direction of the fibre 

 does not always correspond with that of the stem. The 

 coils of the spiral fibre may be close together (fig. 50), or 

 be separated (fig. 55). Sometimes they become united 

 together, and to the membrane of the tube, so that they 

 cannot be unrolled. Such vessels are called closed trachea?, 

 or closed ducts. 



35. False or Spurious Tracheae, the ducts of some authors, are ves- 

 sels in which the internal fibre does not form a complete spiral coil. 

 The chief varieties are annular, reticulated, and scalariform vessels, or 

 ducts. In annular vessels (annulus, a ring), the fibres form complete 

 rings round the tubes (fig. 56). They resemble the trachea? of animals 

 more than spiral vessels do. The rings are by no means regular ; 

 they may be horizontal or inclined, simple, or forked (fig. 57), placed 

 near to each other or separated by considerable intervals, the inter- 

 mediate spaces being sometimes occupied by a fibre of an elongated 

 spiral form, which is continuous with the rings or distinct from them 

 (fig. 58). All these forms are easily recognized in the common Bal- 

 sam. Occasionally, the ring becomes very much thickened in a direc- 

 tion perpendicular to the walls of the vessel, so as to leave only a 



Fig. 52. Spiral vessels, united so as to have a branched appearance. 

 Fig. 53. Branching fibre, from spiral vessels of Gourd (Cucurbita fepo). 

 Fig. 54. Spiral vessels. Coils seen on both side.s. 

 Fig. 55. Coils of fibre, much separated in trachea of Gourd. 



