34 



ORGANOGRAPHY. 



coiled when the vessel is pulled asunder, in which case 

 the wall is ruptured between the coils. This may be com- 

 monly seen by the naked eye by partially breaking the young 

 shoots or leaf-stalks of almost any plant, or the leaves of the 

 Hyacinth, Banana, and others, and gently pulling asunder 

 the two ends, when the uncoiled fibres appear like a fine 

 cobweb. In most cases the coils of the fibre are close together, 

 so that the primary membrane cannot be observed between them; 

 in other cases, however, they ai'e more or less separated by 

 portions of membrane {fig. 62). The latter appearance is prob- 

 ably caused by the growth of the primary membrane after the 

 fibre has been deposited, by which the coils become extended and 

 separated from each other. The fibre is generally turned to the 

 right as in spiral cells, although instances occur in which it is 

 wound in the opposite direction. Balfour says that in the garden 

 Lettuce, spiral vessels are found, some of which have the fibre 

 turned to the right, while in others it turns to the left. When spiral 

 vessels come in contact they overlap more or less at their ends 

 {fig. 62), and frequently the membrane between their cavities then 

 becomes absorbed so tliat they communicate with each other. Spi- 

 ral vessels sometimes present a branched appearance, which is 

 generally occasioned by the union of separate vessels in a more 

 or less oblique manner {fig. 65); 

 or occasionally, it is said, as in the 

 Gourd and some other plants, by a 

 division of the fibres of distinct 

 vessels {fig. 64). Spiral vessels 

 occur in the sheath surrounding the 

 pith of the stems of Dicotyledons 

 {fig. 171, d), in the vascular bun- 

 dles of Monocotyledons {fig. 167, 

 sv), and in some of the higher Aco- 

 tyledons. They also exist in the 

 petiole and veins of leaves, and of 

 all other organs which are modifi- 

 cations of them, as bracts, sepals, 

 ^. oA -o V. , ^- , , petals. &c. They may be also fre- 



Fig. 64. Branched spiral vessel. ^ ■, n -, • t • 



Fig. e,r,. Union of spiral vessels quently found m roots. In size 



in an oblique manner. tjicy Vary from tllC ^^77 tO 5^ of an 



inch in diameter. The average size is about the — ^. Spiral 

 vessels are sometimes called Trachece or Trachenchyma, from 

 their resemblance to the tracheal or air-tubes of insects. 



Annular, lieticulaUd, and Scalarifurm Vessels. — These con- 

 stitute the si)urious traclic.-c of some authors. 



c. Annular Vessels. — These consist of vessels in which the 

 spiral fil)rc ajjpcars in tlie fcjrm of rings {figs. 66 and 67). Some- 

 times tlie wliolc of tlie vessel j)resents tliis ringed ajipearance 

 {figs. 66 and 67); while in others, we find two rings connected by 



Fig. 64. Fig. 65. 



