22 
ORGANOGRAPHY. 
BOOK I. 
a fibre generated spirally in their inside, or having their walls 
marked by transverse bars arranged in a spiral direction. 
Such appears to me to be the most accurate mode of de- 
scribing this kind of tissue, upon the exact nature of which 
anatomists are, however, much divided in opinion ; some be- 
lieving that the fibre coheres independently of any membrane, 
others doubting or denying the mode in which the vessels ter- 
minate ; 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 and woody tissue. It will 
be most convenient to consider all these points separately, 
along with the varieties into which vascular tissue passes. 
There are two principal kinds of vascular tissue ; viz. spiral 
vessels (Plate II. fig. 9. 11.), and ducts (Plate II. fig. 13. 15, 
16. 18. 20.) 
Spiral VESSELS (^y. 6, 7.) (Vasa spiralia, Lat. ; Trachece of 
many ; Fistulce spirales of Malpighi ; Spiralgefdsse or Schrau- 
hengefdsse^ Germ.;) are membranous tubes with conical extre- 
mities ; their inside being occupied by a fibre twisted spirally, 
and capable of unrolling with elasticity. To the eye they, when 
at rest, look like a wire twisted round a cylinder that is after- 
wards removed. For the purpose of finding them for examin- 
ation, 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 
exact structure of spiral vessels. They have been considered 
to be composed of a fibre only, twisted spira'ly, without any 
connecting membrane ; or to have their coils connected by an 
extremely thin membrane, which is destroyed when the vessel 
unrolls; or to consist of a fibre rolled round a membranous 
cylinder ; or even, and this was Malpighi's idea, to be formed 
by a spiral fibre kept together as a tube by interlaced fibres. 
Again, the fibre itself has been by some thought to be a flat 
strap, by others a tube, and by a third class of observers a 
kind of gutter formed by a strap having its edges turned a 
little inwards. Finally, the mode in which they terminate, 
