SIGILLARIA ELEGANS. 131 
diagram, for it is impossible without the aid of colour to 
convey a faithful idea of the original. The student should 
observe, that when mineral matter has permeated the stems 
of plants, the vascular tissue is often so well preserved, that 
one such specimen affords more important information, than 
hundreds of examples in which the form alone remains. 
The external surface of this specimen possesses the cha- 
acteristic markings of the insertion of the leaf-stalks of 
Sigillaria elegans. The internal organization, as seen in 
the transverse section, is as follows :— 
a. The centre, filled with silex ; it exhibits no traces of 
structure. 
b. The zone which surrounds the interspace on which this 
letter is placed, is composed of bundles of vascular tissue. 
A portion of one of these bundles, highly magnified, is re- 
presented in Plate V. fig. 7. 
The «nner circle of this zone, indicated by the convex un- 
dulating line, is made up of medullary vascular tissue ; the 
external circle is divided by rays, and is composed of woody 
fibre, constituting a ligneous cylinder. One of the spiral 
vessels (jig. 3), and another showing a remarkable difference 
of structure in a short space (jig. 2), as seen in a longi- 
tudinal section of the medullary tissue, are figured in 
Lign. 35. 
The ligneous cylinder is surrounded by a band of cellular 
tissue, and the space between this and the cortical integu- 
ment is occupied by silex, in which there are but obscure 
traces of structure. 
The inner layer of bark, f, is composed of elongated cells, 
facts and inferences so admirably enunciated by the author ; not only 
for the illustration of the structure of the tribe of plants under con- 
sideration, but as a valuable exemplification of the manner in which 
all such inquiries should be conducted. See Archives du Muséum 
@ Histoire Naturelle, tom. i. Paris, 1839, 
