SPIRAL VESSELS. 
97 
by which a communication is established with a neigh- 
bouring vessel, as in Fig. 84, c, from the petiole of 
the garden Rhubarb. A similar perforation in a dotted 
duct is represented by Fig. 84, D. 
In most spiral vessels the fibre is single, (Fig. 86, a,) 
it is then called a simple spiral ; in others, two or more 
fibres running in the same direction form a band, which 
for distinction is termed a compound spiral vessel 
(Fig. 86, b) ; sometimes the coils of the fibre separate, 
and the extremities of each coil uniting form a series of 
rings; such vessels are called annular (Fig. 84, a b). 
In other cases, the turns of the spiral may be connected 
together here and there by the branching of a fibre, or 
the development of lateral processes, forming what is 
termed reticular tissue (Fig. 84, e). 
Occasionally the spiral fibre bifurcates, or short 
longitudinal fibres are developed, which connect the 
spiral coils, and give the vessel a reticulated appearance ; 
such vessels represented by E and F, in Fig. 84, occur 
in sections from the Balsam. In other cases, the 
spiral thread is not so evident, but the interior of 
the vessel (a, Fig. 91) is marked by long slit-like 
pores, which are placed regularly one above the other, 
resembling the rounds of a ladder ; these are termed 
scalariform vessels, are almost peculiar to Ferns , and 
will be alluded to hereafter. That these are really 
all modifications of the spiral type, is known by the 
frequent occurrence of several varieties in one and 
the same plant, and even section, as in the Balsam , 
