CHAP. I. VASCULAR TISSUE. 25 



end in conical spires, the point of which becomes A-ery acute; 

 but one would not suppose, judging from the figure given by 

 the latter writer, that he had seen the terminations very 

 clearly. It is, however, certain that the statement of Nees 

 von Esenbeck is correct, and that the spiral vessel generally 

 terminates in a cone, or in a rounded manner. If the point 

 of such a vessel in the Hyacinth (Plate II. fig. 9.) be examined, 

 it will be seen that the end of the spiral fibre lies just within 

 the acute point of the vessel, and that the spires become 

 gradually more and more relaxed as they approach the ex- 

 tremity, as if their power of extension gradually diminished, 

 and the membrane acquired its pointed figure by the diminu- 

 tion of elasticity and extensibility in the fibre. It is not, 

 however, always in a distinct membrane that the spiral vessel 

 ends. In Nepenthes the fibres terminate in a blunt cone, in 

 which no membrane is discoverable. (Plate II. fig. 11.)* 



A spiral vessel is formed by the convolutions either of a 

 single spire, or of many always turning in one direction, and 

 formino- a right-handed screw {Slack). In the first case it is 

 called simple, in the latter comj^ound. The simple is the most 

 common. (Plate II. fig. 9.) Kieser finds fi'om two to nine 

 fibres in the Banana. De la Chesnaye as many as twenty-two 

 in the same plant. There are four in Nepenthes. (Plate II. 

 fig. 11.) In general, compound spiral vessels are thought to 

 be almost confined to Endogenous plants, where they are very 

 common in certain families, especially Marantaceee, Scitami- 

 neae, and Musaceae ; but their existence in Nepenthes, and, 

 according to Rudolphi, in Heracleum speciosum, renders it 

 probable that future observations will show them to be not 

 imcommon among Exogens also. 



* A singular change occurs in the appearance of the spiral vessels of 

 Nepenthes, after long maceration in dilute nitric acid, or caustic potash : 

 the extremities cease to be conical and spually fibrous, but become little 

 transparent oblong sacs, in which the spires of the fibres gradually lose 

 themselves. This alteration, which is a very likely cause of deception, is 

 perhaps owing to the extremities of the vessels being more soluble than the 

 other part, the sac being the confluent dissolved fibres. This is in some 

 measure confirmed by the subsequent disappearance of all trace of fibres 

 in any part of the vessels, under the influence of those powerful solvents. 



