INTRODUCTION 



that serial peels can be made with com- 

 paratively little loss, for it is possible 

 to get five, or even ten, peels to the 

 millimeter. When sections are cut with 

 a saw, they are generally a millimeter 

 in thickness; and the saw, with the 

 polishing for the next section, takes an- 

 other milHmeter. Consequently, serial 

 sections are seldom less than two 

 millimeters apart. 



However, it must be remembered 

 that in the peel method all material 

 which is soluble in the acid is etched 

 away, leaving the insoluble material 

 standing erect. The celloidin surrounds 

 the standing material and hardens. 

 When the peel is pulled off the stand- 

 ing material is torn loose from the sur- 

 face and, being held in place by the 

 hardened celloidin, can be mounted 

 and studied. While the method is very 

 popular, its limitations must be recog- 

 nized. 



Instead of celloidin, many pal eobot- 

 anists are now using gelatin in making 

 peels. 



From impressions, and from sec- 

 tions and peels of petrified material, 

 plants which lived hundreds of mil- 

 lions of years ago have been studied 

 and described until paleobotany has 

 become a major subject of critical 

 importance in any study of evolu- 

 tion or phylogeny. Besides, plant 

 remains have been so thoroughly 

 identified that they serve to identify 

 geological horizons, and are of great 



MILUOWSoF YEARS 



0itMT£R?5AKYT]U^ ^ 

 TERTIARY ^"" OZ 



CRET/1CE005 



JKJR/KSSIC 



TR/A55/C 



PERMiAIV 



30 



HO 



CARBONIFiROUS 80 



DEVONIAN 



50 



SILURIAN 



30 



ORDOVICIAN 



70 



CAHBRIAN 



m 



PROTEROZOJC 



soo 



MCHEOZOIC 



1000 



Fig. I. — Diagram of geological 

 time. — Compiled by Dr. A. C. NoE 

 from various sources. 



