CHAP. II. 
EXOGENOUS STEMS. 
97 
In Coniferous wood [Jig, 35.) there is 
scarcely any mixture of bothrenchyma 
among woody tissue, as in other exogenous 
plants ; in consequence of which a cross 
section exhibits none of those open mouths 
which give what is vulgarly called porosity 
to wood. Instead of this, the wood gene- 
nerally consists exclusively of that kind of 
tissue which has been described at p. 25., 
under the name of glandular, with the 
exception of themedullary sheath, in which 
spiral vessels are present in small num- 
bers. The Yew and Abies Douglasii are the principal excep- 
tions : in the former the woody tissue is the same as that of 
other Coniferae; but many tubes have a great quantity of 
little fibres lying obliquely across them at nearly equal dis- 
tances, — sometimes arranged with considerable regularity, — 
sometimes disturbed as it were, — so that the transverse fibres, 
although they retain their obliquity, are not parallel, — and 
sometimes, but more rarely, so regular as to give to the tubes 
of woody tissue the appearance of spiral vessels, the coils of 
which are separated by considerable intervals. The latter 
only is represented by Kieser, at his tab. xxi. fig. 103, 104.; 
but the former is by far the most common appearance. 
In Cycadaceae the vascular system is destitute of vessels, as in 
Coniferae ; their place being supplied by such bothrenchyma as 
has been already described at p. 22. But the zones of wood 
are separated by a layer of cellular substance resembling that 
of the pith, and often as thick as the zones themselves, while 
the pith itself is filled with bundles of fibro- vascular tissue. 
This structure is represented by Adolphe Brongniart, in the 
16th volume of the Annales des Sciences. 
Mr. Griffith has beautifully illustrated the structure of a 
plant called Phytocrene (^^.36.), in Wallich’s Plan toe Asiaticae, 
vol. iii. t. 216. In this curious production the wood consists 
of plates containing vessels and woody tissue, having no con- 
nection with each other, and separated at very considerable 
intervals by a large mass of prosenchymatous cellular tissue 
