STRUCTURE.] SPIRALS IRRITABILITY SITUATION. 73 



According to the observations of Link, they may be found 

 of extremely different size in one and the same bundle of 

 tissue in the stem of Canna, the largest being -3-^0^ the middle 

 size -g-fo , and the smallest -a-oVo, of an inch in diameter. 



An irritability of a curious kind has been noticed by Mal- 

 pighi in the fibre of a spiral vessel. He says (Anat. p. 3.), 

 that in herbaceous plants, and some trees, especially in the 

 winter, a beautiful sight may be observed, by tearing gently 

 asunder a portion of a branch or stem still green, so as to 

 separate the coils of the spires. The fibre will be found to 

 have a peristaltic motion, which lasts for a considerable time. 

 An appearance of the same nature has been described by 

 David Don in the bark of Urtica nivea. These observations 

 are, however, not conformable to the experience of others. 

 De Candolle is of opinion that the motion seen by Malpighi 

 is due to a hygrometrical quality combined with elasticity; 

 and as spiral vessels do not exist in the bark of Urtica nivea, 

 there must be some inaccuracy in Don's remark. 



The situation of spiral vessels is in that part of the axis of 

 Exogens which surrounds the pith, and is called the medullary 

 sheath, and also in every part the tissue of which originates 

 from it ; such as the veins of leaves, and petals, and of all 

 other modifications of leaves. It has been supposed that 

 they are never found either in the bark, the wood, or the 

 root ; and this appears to be generally true. But there are 

 exceptions to this : Mirbel and Amici have noticed their 

 existence in roots; Mr. Valentine and Mr. Griffith have 

 both extracted them from the root of the Hyacinth; and 

 they certainly exist in the roots of many exogens; in the 

 Parsnep and the Beet they are large, and readily extracted 

 entire. I know of no instance of their existence in bark, 

 except in Nepenthes, where they are found in prodigious 

 quantities, not only between the alburnum and the liber, 

 embedded in cellular tissue, as was first pointed out to me by 

 Mr. Valentine, but also sparingly both in the bark and wood. 

 They 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 



