72 SPIRALS NUMBER OF FIBRES - SIZE. [BOOK i. 



their power of extension gradually diminished, and the mem- 

 brane acquired its pointed figure by the diminution of elas- 

 ticity 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 IT. fig. 11.)* 



A spiral vessel is formed by the convolutions either of a 

 single spire, or of many, always turning in the same direction. 

 In the first case it is called simple, in the latter compound. 

 The simple is the more common. (Plate II. fig. 9.) Kieser 

 finds from two to nine fibres in the Plantain (Musa sapientum.) 

 De la Chesnaye as many as twenty-two in the same plant. 

 There are four in Nepenthes (Plate II. fig. 11.), five in 

 Liparis pendula. In general, very compound spiral vessels 

 are thought to be almost confined to Endogenous plants, 

 where they are very common in certain families, especially 

 Marants, Gingerworts (Zingiberacese), and Musads ; but their 

 existence in Nepenthes, and, according to Rudolphi, in He- 

 racleum speciosum, renders it possible that future observa- 

 tions may show them to be not uncommon among Exogens 

 also. 



In Conifers the spiral vessels have in some cases their 

 spires very remote, and even have glands upon their mem- 

 brane between the spires. Link speaks of a peculiar kind of 

 vessel found in Coniferous plants " fibris tenuissimis distincta," 

 and calls such spirals vasa spiralia fibrosa. 



In size, spiral vessels, like other kinds of tissue, are vari- 

 able ; they are generally very small in the petals and filaments. 

 Mirbel states them to be sometimes as much as the -^-^ of 

 an inch in diameter; Hedwig finds them, in some cases, 

 not exceeding the -^oVo ; a ver y common size is the 10 1 00 . 



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

 thes, after long maceration in dilute nitric acid, or caustic potash : the extremi- 

 ties cease to be conical and spirally 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 fibres being more soluble than the other part, the sac being 

 composed of the confluent dissolved fibres. This is in some measure confirmed 

 by the subsequent disappearance of all trace of fibres in every part of the vessels, 

 under the influence of those powerful solvents. 



