CHAP, II. 
EXOGENOUS STEMS. 
77 
Coniferae ; their place being supplied by such woody tissue as 
has been already described at p. 19. 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. This 
structure is represented by Adolphe Brongniart, in the 16 th 
volume of the Annales des Sciences. 
My friend Mr. Griffith has beautifully illustrated the struc- 
ture of a plant called Phytocrene i^Jig, 34.),in Wallich's Plantae 
Asiaticae, vol. iii. t. 216. _In this curious production the wood 
consists of plates containing vessels and woody tissue, having 
no connection with each other, and separated at very consi- 
derable intervals by a large mass of prosenchymatous cellular 
tissue filled with vasiform tissue, and representing medullary 
rays. * When the stem is dry, the woody plates separate from 
the other tissue, in which they finally lie loose. 
34 
In Nepenthes distillatoria the pith contains a great quantity 
of spiral vessels ; the place of the medullary sheath is occu- 
pied by a deep and dense layer of woody tissue, in which no 
vessels, or scarcely any, are discoverable ; there are no me- 
dullary rays ; the wood has no concentric zones ; between the 
bark and the wood is interposed a thick layer of cellular tissue, 
in which an immense quantity of very large spiral vessels is 
formed; on the outside of this layer is a thinner coating of 
* It will be seen that thie view I now take of the analogies of the parts 
in the trunk of Phytocrene is very different from that in the first edition of 
this work. 
