VEGETABLE PHYSIOLOGY: ELEMENTARY ORGANS. 11 
fibre, which latter may be compared to hair of inconceivable 
fineness, whereas membrane is an extremely thin substance, 
which does not cohere more strongly in one direction than an¬ 
other. Vasiform cellular tissue, consists of tubes or cylindrical 
cells, whose sides are marked with numerous dots, arranged 
more or less in a spiral manner. These tubes are internally di¬ 
vided by transverse partitions, formed by the uniting of the ends 
of the individual cells ; they may be readily observed by the 
naked eye in a transverse section of the cane, the bamboo, 
the vine, and also in monocotyledonous plants generally ; the 
openings which are seen on examining the sections being the 
mouths of the tubes, or what would in vulgar terms be called 
their “ porosity.” 
Vascular Tissue consists of simple membranous tapering 
tubes, and exists in two principal forms, viz. as spiral vessels and 
as ducts. Spiral vessels , or trachece , are membranous tubes with 
conical terminations, which are internally occupied by a spirally 
twisted fibre, capable of unrolling with elasticity, and presenting 
to the eye an appearance resembling a twisted wire. It is not 
clearly ascertained whether the fibre composing these spirals 
is in itself hollow or solid, both opinions being entertained by 
men of science. It would seem most natural to regard it as a 
solid body, were it not that Hedwighas discovered that coloured 
fluids, when rising in these vessels, follow the same direction as 
the spiral; this may, however, be an illusion, from the colouring 
matter rising in the channels formed by the approximation of 
the cylindrical fibres, and not actually in the fibres themselves. 
Their extremities are, without doubt, of a conical form, and in 
this way we may suppose them to elongate during the increas¬ 
ing developement of the part in which they occur. They are 
formed, sometimes by the convolution ofa single fibre, and in other 
cases by the union of several fibres turning in the same direc¬ 
tion, and forming a right-handed screw ; the former, which is its 
most common character, is called a simple spiral ; the latter, a 
compound one. They vary considerably in size, being met with 
from - 37 Lq to -3 0V0 P art °f an inch in diameter; in the floral 
parts they are exceedingly minute. They are situated in the 
medullary sheath, that is, in that part of the axis which sur¬ 
rounds the pith ; they are also found in every part of the 
ascending axis, the tissue of which emanates from thence, such 
as the veins of leaves, and all the modifications of leaves. They 
