08 SPIRALS DIRECTION OF THE SPIRE. [BOOK i. 



some believing that the fibre coheres independently of any 

 membrane, others doubting or denying the mode in which the 

 vessels terminate ; some describing the vessels as ramifying ; 

 and a fourth class ascribing to them pores and fissures, as we 

 have already seen has been done in cellular, pitted, and woody 

 tissue. This has probably arisen from observers having con- 

 founded modifications of bothrenchym under the name of 

 vascular tissue, which should be strictly confined to tubes 

 lined with a spiral coating more or less complete. 



The spiral fibre is observed to be generally wound to the 

 right ; that is to say, the turn of the spiral appears to an 

 observer placed in the interior of the cylinder to mount from 

 left to right. Mohl maintains that this is the usual direction ; 

 Schleiden, on the other hand, asserts that left-handed spirals 

 are very common in connection with right-handed. Mohl 

 nevertheless holds to his opinion, and adds that " the volu- 

 tion to the right or left is quite independent of the organi- 

 sations of the surrounding parts, as is proved by the fact that, 

 in certain cases, not only the spirals of two superposed 

 utricles of the same vessel are wound in opposite directions, 

 but sometimes even in the same vessel, (as in the Gourd), the 

 parts of a spiral fibre separated from each other by rings, are 

 wound in an opposite direction. (Annals of Natural History, 

 vol. viii., p. 20.) 



There are two principal kinds of vascular tissue ; viz. spiral 

 vessels (Plate II. fig. 3. b. 9. 11.), and ducts (Plate II. fig. 12. 

 c.f. 15. 16. 18. 20.) 



SPIRAL VESSELS or TRACHEAE are membranous tubes with 

 conical extremities ; their inside being occupied by a fibre 

 developed spirally, and capable of unrolling with elasticity. 

 To the eye they, when at rest, look like a wire twisted round 

 a cylinder that is afterwards removed. For the purpose of 

 finding them for examination, the stalk of a strawberry leaf, 

 or a young shoot of the Cornus alba (common dogwood) may 

 be conveniently used ; in these they may be readily detected 

 by gently pulling the specimen asunder, when they unroll, 

 and appear to the naked eye like a fine cobweb. 



Very different opinions have been entertained as to the 



