90 
MORPHOLOGY OF TISSUES, 
with the parenchymatous cells which adjoin them by such lattice plate?, which, 
although they do not bring the cells into communication, must nevertheless facilitate 
the interchange of certain constituents by diffusion. 
An illustration of the coalescence of cells is furnished by the vessels of the wood 
(mostly filled with air) especially those which have bordered pits, as well as the wood- 
cells with bordered pits {Trac beides), of which sufficient has already been said (Figs. 2^,- 
27, pp. 25-27). In the vessels of the secondary wood of Angiosperms formed from short 
cells with large cavities, the transverse or oblique septa commonly disappear altogether, 
so that the entire row of cells forms a completely continuous tube. But frequently, 
as in Helianthus, Sojtcbiis, Cirsium, &c}, the 
septa are only partially absorbed, thick ridges 
remaining, which have a reticulate or lat- 
ticed form, or, when the septa are very ob- 
lique, even scalariform. When true wood-vessels 
form air-conducting tubes in this manner, the 
separate parts having previously been closed 
cells, the tubes are also in commiunication with 
one another laterally through the open bordered 
pits already described. In tracheides which are 
arranged in a prosenchymatous manner, as those 
of Conifers (Fig. 23, p. 25) and Ferns (Fig. 27, 
p. 27), this lateral communication is the only one, 
since in the cells pointed at both ends there are 
no true septa which could be broken through. It 
is, on the other hand, doubtful whether the cells 
out of which annular and spiral vessels are formed 
(see Fig. 18, p. 23) are always in communica- 
tion with one another, especially when the spiral 
cells remain short, as in the ultimate branches 
of the vascular bundles in the veins of leaves, 
where they are often considerably enlarged. 
Preparations in which the structure can be very 
easily observed may be obtained by boiling very 
young leaves for some time in potash solution 
and then placing them in glycerin. Where, 
however, the spiral vessels are formed at an 
early period in these organs, attaining subse- 
quently a considerable length, so that the coils 
of the spiral thread which were at first very 
close become widely separated (Fig. 75), it may 
be assumed that the thin membrane which sepa- 
rates the contiguous ends of the spiral prosen- 
chymatous cells becomes ruptured, and thus the 
cells are placed in communication with one another for considerable lengths. For these 
and other reasons, it is convenient not to limit the definition of a vessel to cases 
in which the component cells actually coalesce into a tube. Here, as elsewhere in 
Fig. 75.— From a very young- nbro-vascular bundle 
of a young petiole of Scrophiilaria aqiiatica; part 
of a spiral vessel surrounded by procambium ; two 
spirally thickened cells are in prosenchymatous appo- 
sition ; by the elongation of the petiole the coils of 
the spiral band, now lying close to one another, are 
drawn apart ; the spiral band becomes detached from 
the thin wall which is common to the vessel and to 
the adjoining cells, and in this v^'ay a spiral band is 
formed capable of unwinding. 
must not be founded on a single characteristic selected arbitrarily, but on a general 
consideration of all the morphological and physiological characters. 
The forms of coalescence now described possess the common physiological function of 
providing a means for the transport of food-materials, and of promoting and accelerating 
^ See E. Tangl in Sitzungsber. der kais. Akad. der Wiss. Vienna, May 187 1. 
