determining Fossil Plani'i. 225 



the leaves tliemselves, yet they indicate, with precision, their jx)- 

 sition, the form of their base, and sometimes also their probable 

 direction. We can tell whether they were opposite or verticillate, 

 alternate or spirally disposed, deciduous or persistent, and im- 

 bricated or remote ; all characters of great use as means of dis- 

 crimination, and as often affording important negative evidence 

 upon doubtful points. The geologist will, however, be careful 

 not to ascribe too much value to modifications in the origin of 

 leaves, and, in particular, to the spiral mode, which forms so 

 striking a feature in many fossil remains : he will bear in mind, 

 that the latter is theoretically the normal mode in which all 

 leaves originate, and that other modes are more or less obvious 

 modifications of it ; and, finally, he will consider, that if he is 

 not familiar with instances of it in recent plants, it is because 

 the lines of spires are broken by the leaves that are interposed 

 between them and the eye. He will, possibly, only remember 

 that the leaves of firs, the fruit of the pine-apple, and the foli- 

 age of the screw pine (Pandanus), are arranged upon this plan ; 

 but, if he draw a lino from base to base of the leaves of any al- 

 ternate-leaved plant, always proceeding in the same direction, 

 he will find, that that line will describe a spire round the axis 

 from which the leaves originate ; so that a spiral appearance will 

 be apparent in proportion as leaves are approximated. 



In judging of the indentity of fossil stems that are character- 

 ized by their external appearance, care must be taken not to 

 distinguish, as different species, those stems that have still their 

 cortical integument upon them from such as have lost it. In 

 these two cases, the appearance of scars will be different ; those 

 of the former being more rounded, broader, and probably more 

 deeply furrowed, than the latter ; for the one is a real scar shew- 

 ing the outline of the base of the leaf, while the latter is solely 

 caused by the passage of bundles of vessels out of the stem into 

 the petiole of the leaf. 



The manner in which stems branch is sometimes well deserv- 

 ing consideration. Where no trace of leaves can be found, their 

 position may possibly be indicated by the origin of branches, for 

 the latter being always axillary to the leaves can only originate 

 as they do : but, unfortunately, the value of this fact is often 

 reduced to nothing by the appearance of branches from the axillae 



VOL. XIH. NO. XXVI.— OCTOBER 1832. P 



