CHAP. I. 
VASCULAR TISSUE. 
27 
Sect. IV. Of Vascular Tissue^ or Traclienchyma. 
This consists of simple membranous tubes tapering to each 
end, but often ending abruptly, either having a fibre gener- 
ated spirally in the inside, or having their walls marked by 
transverse bars arranged more or less 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 
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 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. 3. b. 9. II.), and ducts (Plate II. fig. 12. 
c.f 15, 16. 18. 20.) 
Spiral vessels or Trachea are membranous tubes 
with conical extremities; their inside being occupied by a 
fibre twisted spirally, and capable of unrolling with elas- 
ticity. 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 (com- 
mon 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 spirally, without any 
connecting membrane ; or to have their coils connected by an 
extremely thin membrane, which is destroyed w'hen 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 
