2-2 
ORGANOGRAPHY. 
BOOK I. 
comes a tube, open from end to end when the partitions be- 
tween the cells are absorbed. The cjdinders in some plants, 
as in Phytocrene, are regular in size, and easily separate from 
each other, as has been observed by Griffith ; in other cases, 
and this is the most common case, their ends are oblique and 
produce the appearance of bands when they come in contact 
{Jig. 6 . Z>.) : hence they have been looked upon as a modi- 
fication of the spiral vessel. 
2. Continuous Botlirenchjma. — This {Jig. 6. a a a) forms the 
Vasa spiroidea porosa of Link, and is still less of the nature of 
vascular tissue than the last. It consists of long, slender, 
uninterrupted, pitted tubes, resembling Pleurenchyma in 
form, but not tough, nor collected into solid bundles. This 
is common in the roots of plants, and is often found in con- 
nection with spiral vessels. 
It has been said by Bischoff that tissue of this kind is an 
alteration of a spiral vessel, whose fibre is broken into short 
pieces, which stick to the sides of the tube and cause the pitted 
appearance. Mr. Slack adopted this idea: he considered 
them to be transparent spaces in the sides of the cells, and 
caused by the separation, at intervals, of a spiral fibre whose 
convolutions are partially and firmly united in the spaces 
between the dots ; and he represents a case of vasiform tissue 
from Hippuris in illustration of his position. But I have 
sought in vain for any proof of the correctness of these views. 
On the contrary, it is probable that the functions of Taphren- 
cliyma are to convey fluid, which is an additional reason for 
regarding it entirely distinct from vascular tissue. 
The granular woodv tissue of former editions of this work 
I now regard as a form of continuous Bothrenchyma. Re- 
newed observation with better instruments satisfy me that 
the marks on the sides of the tubes of Cycadaceae, taken by 
Adolphe Brongniart for pores, and by myself for granules, 
are neither the one nor the other, but short oblique furrows 
in the lining of the tubes. 
Sect. III. Of Woody Tissue^ or Pleurenchyma. 
This, which Meyen calls Pleurenchyma^ consists of very 
slender, tough, transparent, membranous tubes, tapering 
