CHAP. I. VASCULAR TISSUE. 27 



Tliey have been described by myself as forming part of the 

 testa of the seed of Collomia, and Brown has described 

 them as existing abundantly in that of Casuarina. In the 

 former case, the tissue was rather the fibro-cellular, as has 

 been already explained (p. 11.) ; in the latter, they are appa- 

 rently of an intermediate nature between the fibro-cellular 

 and the vascular ; agreeing with the former in size, situation, 

 and general appearance, but differing in being capable of un- 

 rolling. In the stem of Endogens, spiral vessels occur in the 

 bundles of woody tissue that lie among its cellular s\ibstance ; 

 in the leaves of some plants of this description they are found 

 in such abundance, that, according to De la Chesnaye, as 

 quoted by De Candolle, they are collected in handfuls in some 

 islands of the West Indies for tinder. The same author in- 

 forms us that about a draclmi and a half is yielded by every 

 plantain, and that the fibres may be employed either in the 

 manufacture of a sort of down, or may be spun into thread. 

 In Coniferous plants they are few and very small, and in 

 Flowerless plants they are for the most part altogether absent; 

 the only exceptions being in Ferns and Lycopodiacece, orders 

 occupying a sort of middle place between flowering and 

 flowerless plants : in these they no doubt exist. My friend 

 Mr. Griffith has succeeded in unrolling them in the young 

 shoots of Lycopodium denticulatum. 



Some have thought that the spiral vessels terminate in those 

 little openings of the cuticle called stomates ; but there does 

 not seem to be any foundation for this opinion. 



Ducts (^fig. 8, 9, 10, 11.) {Fansses trochees, Fr. ; Lym- 

 phcediicts, or Sap-vessels of Grew and others ; Vaisseaux lym- 

 phatiques of De Candolle, Vaisseaux pneumatiqiies of others) 

 are membranous tubes, with conical or rounded extremities ; 

 their sides being marked with transverse lines, or rings, or 

 bars, and being incapable of unrolling without breaking. 



These approach so nearly to the spiral vessel that it is im- 

 possible to doubt their being a mere modification of it. Some 

 writers confound all the forms under the common name of 

 spiral vessels, but it is more convenient to consider them as 

 distinct, not only on account of their peculiar appearances, but 



