SPIRAL VESSELS.—ANNULAR VESSELS. 53 
Hyacinth, Banana, and others, and gently pulling asunder the 
two ends, when the uncoiled fibres appear likea fine cobweb. In 
most cases the coils of the fibre are close together, so that the en- 
closing membrane cannot be observed between them ; but in other 
instances they are more or less separated by portions of membrane 
(fig. 104). The latter appearance is probably caused by the growth 
of the cell-wall after the thickening which forms the fibre has 
taken place, by which the coils become extended and separated 
from one another. The fibre is generally turned to the right as 
in the ordinary spiral cells, although instances occur in which it 
is wound in the opposite direction. When spiral vessels come 
in contact they overlap more or less at their ends (fig. 104), and 
frequently the membrane between their cavities then becomes 
absorbed so that they communicate with each other. Spiral 
vessels sometimes present a branched appearance; this is gene- 
rally occasioned by the union of separate vessels in a more or less 
Fre, 104. Fre: 105. Fie. 106. Fic. 107. 
Fig.104. Simple spiral vessels.—Fig. 105. Compound spiral vessel.—— 
Fig. 106. Branched spiral vessel.——ig. 107, Union of spiral vessels in an 
oblique manner, 
oblique manner (fig. 107), or occasionally, it is said, as in the 
Gourd and some other plants, by a division of the fibres of dis- 
tinct vessels (fig. 106). 
Spiral vessels occur in the sheath surrounding the pith of 
Dicotyledons (figs. 179, s, s’, and 185 B, d), in the fibro-vascular 
bundles of Monocotyledons (jig. 181, sv), and in some of the 
Cormophytes, as the Lycopodiacee. They also exist in the 
petiole and veins of leaves, and in those of all other organs 
which are modifications of leaves, as bracts, sepals, petals, and 
other parts of the flower. They may be also frequently found 
in roots. In size they vary from the 54; to 5,45 of an inch in 
diameter. The average size is about the 54,5. Spiral vessels 
are sometimes called Trachee or Trachenchyma, from their re- 
semblance to the trachee or air-tubes of insects. 
c. Annular Vessels.—In these vessels the fibre is arranged in 
the form of rings more or less regularly arranged upon their 
