CHAP. I. 
VASCULAR TISSUE. 
31 
plants, where they are very common in certain families, 
especially Marantaceae, Zingiberaceae, and Musaceae; but 
their existence in Nepenthes, and, according to Rudolph!, 
in Heracleum speciosum, renders it probable that future 
observations will show them to be not uncommon among: 
Exogens also. 
In Coniferae 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 spiral found in Coniferous plants “ fibris tenuissimis dis- 
tincta,” and calls them 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 288th of 
an inch in diameter ; Hedwig finds them, in some cases, not 
exceeding the 3000th; a very common size is the 1000th. 
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 the 
middle size and the smallest 2 ouo 
meter. 
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 
Don in the bark of Urtica nivea. These observations are, 
how'ever, 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, 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 
