42 



ORGANOGEAPHY. 



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 ob- 

 served between them ; in other cases, however, they are more 

 or less separated by portions of membrane {fig. 78). The latter 

 appearance is probably 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 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. 78), and frequently the membrane 

 between their cavities then becomes absorbed so that they com- 

 municate with each other. Spiral vessels sometimes present 

 a branched appearance, which is generally occasioned by the 

 imion of separate vessels in a more or less oblique manner {fig. 

 81); or occasionally, it is said, as in the Gourd and some other 

 plants, by a division of the fibres of distinct vessels {fig. 80). 

 Spiral vessels occur in the sheath surrounding the pith of the 

 stems of Dicotyledons {fig. 163, d), in the vascular bundles of 

 Monocotyledons {fig. 159, sv), and in some of the higher Acoty- 

 ledons. They also exist in the petiole and veins of leaves, and 

 of all other organs which are modifications of them, as bracts, 

 sepals, petals, &c. They may be also frequently found in roots. 



J^ig. 80. J^ig. 81. __i_ ^^ ^^^ -^^j^ j^^ diameter. The 



RA'erage size is about tlie 77-?^. 

 Spiral vessels are sometimes called 

 TrachecB or Irachenchyma, from 

 their resemblance to the tracheae 

 or air-tubes of insects. 



c. Annular Vessels. — In these 

 vessels the fibre is arranged in 

 the form of rings upon their inner 

 sui'face {figs. 82 and 83). Some- 

 times the whole of the vessel pre- 

 sents this ringed appearance {figs. 

 82 and 83); while in others, we 

 find two rings connected by one 

 or more turns of a spiral, the two 

 forms irregularly alternating with 

 each other {fig. 84). These vessels are of rarer occurrence than 

 the other modifications of spiral vessels. In size they vary 

 from about j^ to gijj of an inch in diameter. Annular vessels 

 occur especially in the vascular bundles of the stems of soft 

 rapidly growing herbaceous plants among Dicotyledons ; also 

 in those of Monocotvledons ; and in Flowerless Plants. In 



Fig. 80. Branclied spiral vessel. 

 Fig. 81. Union of siiirai vessels 

 ill an oblique manner. 



