26 ORGANOGRAPHY. BOOK I. 



In Coniferas the spiral vessels have in some eases their 

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

 brane between the spires. (Plate II. fig. 6.) 



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 288th of 

 an inch in diameter ; Hedwig finds them, in some cases, not 

 exceeding the 3000th; a very common size is the 1000th. 



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

 pighi in the fibre of a spiral vessel. He says {Ariat, 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 

 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 hygroscopic quality combined with elasticity ; and as 

 spiral vessels do not exist in the bark of Urtica nivea, it seems 

 that there is some inaccuracy in Don's remark. 



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

 the stem surrounding the pith, and 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; and Mr. Valentine and Mr. Griffith have both ex- 

 tracted them from the root of the Hyacinth ; they do not, 

 however, appear to have been hitherto seen in the roots of 

 Exogeus. 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, em- 

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

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



