168 INVESTIGATION OF FOSSIL PLANTS. 



In recent plants, the botanist has the opportunity of 

 examining the organs both of fructification and vegetation, 

 and, from their structure, is enabled to judge of the class, 

 order, or genus to which the specimen belongs ; but in a fossil 

 plant, the only parts capable of being examined are the 

 internal structure of the stem, and its external surface, 

 together with the position, division, outline, and venation of 

 the leaves. 



Thus if the subject of inquiry be a fragment of the fossil 

 trunk of some unknown tree, if no trace can be ascertained 

 of its exact anatomical structure, it may be possible to 

 ascertain whether its wood was deposited in concentric zones, 

 or grew after the manner of the cane ; in the former case, it 

 would have been dicotyledonous, or exogenous ; in the latter, 

 monocotyledonous, or endogenous. If a transverse section 

 should show the remains of sinuous unconnected layers, 

 resembling arcs, or parts of a circle, with their ends out- 

 wards, of a solid homogeneous character, and embedded 

 among some softer substance, then it may be considered 

 certain that such a stem belonged to some tree-fern. If the 

 tissue is altogether cellular, and it can be satisfactorily 

 ascertained that no vascular tissue is combined with it, the 

 specimen, in all probability, belonged to that division of the 

 vegetable kingdom, termed cryptogamia; care being observed 

 to examine it rigorously, lest it prove a succulent portion of 

 some dicotyledonous tree, in which the vascular system is so 

 scattered among cellular substance as to be scarcely dis- 

 cernible. If the tissue consists of tubes placed parallel with 

 each other, without any trace of rays passing from the centre 

 to the circumference, it is endogenous, even if there be an 

 appearance of concentric circles in the wood ; but if any 

 trace whatever can be discovered of tissue crossing the longi- 

 tudinal tubes at right angles, from the centre to the circum- 

 ference, such a specimen is exogenous, whether concentric 

 circles can be made out or not ; for such an arrangement of 

 tissue would indicate the presence of medullary rays, which 

 are the most certain sign of a dicotyledonous stem. If, in 

 a specimen having these medullary rays, the longitudinal 

 tubes are all of the same size, a circumstance obvious upon 

 the inspection of a transverse section, the plant is either 

 coniferous or cycadeous; but if among the smaller tubes, which 



