determining' Fossil Plants. 223 



light is likely to be thrown upon the extinct Flora than could 

 be otherwise anticipated. If the tissue of a stem should be 

 found entirely cellular, and it could be satisfactory made out 

 that no vascular tissue whatever was combined with it, the speci- 

 men would, in all probability, have belonged to that division of 

 the vegetable kingdom, which, being propagated without the 

 agency of sexes, is by botanists called Cryptogamia. A specimen 

 of this kind should, however, be examined with the most rigorous 

 accuracy ; because it might have been a succulent portion of 

 some Dicotyledonous tree, in which the vascular system was so 

 scattered among cellular substances as to be scarcely discernible. 

 If the tissue should have consisted of tubes placed parallel with 

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

 the circumference, it would have been Monocotyledonous or En- 

 dogenous, even if there should be an appearance of concentric cir- 

 cles in the wood ; but if any trace whatever can be discovered of 

 tissue crossing the longitudinal tubes at right-angles, from the 

 centre to the circumference, then such a specimen would have 

 been Dicotyledonous or Exogenous, whether concentric circles can 

 be made out or not ; for such an arrangement of tissue would in- 

 dicate the presence of medullary rays, which are the most certain 

 sign of a Dicotyledonous plant. If, in a specimen having these 

 rays, the longitudinal tubes are all of the same size, a circum- 

 stance obvious upon the inspection of a transverse section, the 

 plant will have been either Coniferous or Cycadeous ; but if, 

 among the smaller tubes, which in fact are woody fibres, some 

 larger ones are interspersed in a definite manner; it would, in 

 that case, have belonged to some other tribe of Dicotyledons. 

 It is indispensable that the arrangementof the larger tubes should 

 have been definite, for appearances of the same kind exist in 

 much Coniferous wood ; but, in the latter, they are scattered in 

 an indefinite manner among the smaller tubes, and are not ves- 

 sels but cylindrical cavities, for the collection of the resinous se- 

 cretion peculiar to the Fir tribe. Again, if the walls of the lon- 

 gitudinal tubes of any fossil specimen are found to exhibit ap- 

 pearances of little warts, growing from their sides, such a speci- 

 men had certainly belonged to some Coniferous or Cycadeous 

 plant, no other tribes whatever possessing such a structure at 

 the present day. Finally, if a trace of pith can be discovered. 



