18 



FORM AND ARRANGEMENT OF VESSELS. 



where they overlap, it is sometimes absorbed, so as to allow direct com- 

 munication between the vessels. The fibre or spiral filament is 

 generally sbgle, forming simple trachem (fig. 52); but sometimes 

 numerous fibres, varying from two to more than twenty, are united 

 together, as in the banana, assuming the aspect of a broad ribband 

 (fig. 53), and constituting Pkiotrachece (ntXi'im, more). The fibre is 

 elastic, and can be unrolled. This can be seen by taking the leaf of 

 a Pelargonium, and after making a superficial cut round the stalk, 

 pulling the parts gently asunder, when the fibres will appear like the 

 threads of a cobweb. 



Spiral vessels were first noticed as early as 1661, by Henshaw. 

 They occur principally in the higher classes of plants, and are well 

 seen in annual shoots, as in Asparagus ; in 

 the stems of Bananas and Plantains, where 

 the fibres may be pulled out in handfuls, 

 and used as tinder ; in many aquatics, as 

 Nelumbium and Nymphsea; and in Lili- 

 aceous plants. In hard woody stems they 

 are principally found in the sheath sur- 

 rounding the pith, and they are traced 

 from it into the leaves. They are rarely 

 found in the wood, bark, or pith. Spiral 

 vessels occasionally exhibit a branched ap- 

 pearance. 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. 54, u, a), or it may depend on a 

 regular division of the fibres, as is seen in the Mistleto, House-leek, 

 and Gourd (fig. 55). 



The fibre is on the inside of the membrane. Quekett has shown 

 this in sUicified spiral vessels, where the mark of the 

 spil-al 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. 

 56), 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 microscope there is often the appear- 

 ance of the crossing of fibres (fig. 56), in consequence of 

 . 57. 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 



Fig. 54 Spiral vessels, united so as to have a 1)13110116(1 appearance. Pig. 55. Branch- 

 ing fibre, from spiral vessels of Gourd (CwitrMto Pepo). Fig. 66. Spiral vessels. Coils 

 seen on both sides. Fig. 57. Coils of fibre, much separated in trachea of Gourd. 



Fig. 64. 



Fig. 65. 



