INTRODUCTION 5 



more complex have been evolved. Some groups have made more 

 progress than others. That is why existing groups are at different levels 

 of development. Along with the tendency toward ever-increasing 

 complexity, much retrogression has occurred and, as a result, some 

 modern groups are more or less degenerate. 



Paleobotany, the study of fossil plants, has made great progress because 

 methods have been developed making it possible to study thin sections 

 of petrified material under the microscope. Many of these sections 

 show such an amazing wealth of structural detail that almost as much can 

 be learned from them as from sections of living plants. Unlike petri- 

 factions, fossils in the form of casts or impressions, made when some part 

 of a plant falls into soft earth that later hardens into stone, show no 

 internal structvu-e but preserve many external features. Most fossils 

 are of this kind. 



Geologic time, whose total duration is about 2 billion years, is divided 

 into five great eras. The Archeozoic era came first. Then followed, in 

 order, the Proterozoic, Paleozoic, Mesozoic, and Cenozoic eras. Each 

 era is divided into periods. The Archeozoic and Proterozoic, with an 

 estimated duration of 800 million and 650 million years, respectively, 

 comprise nearly three-fourths of all geologic time. Most of the evidence 

 for the existence of Hfe during these two great eras is indirect, consisting 

 of extensive deposits of graphite, limestone, and iron ores, substances 

 that are formed, at least to some extent, by organisms. The earliest 

 plants may have been similar to certain existing bacteria and blue-green 

 algae. 



The fossil record of nonwoody plants is very fragmentary. Because 

 of their soft and perishable nature, few have left any direct evidence of 

 their existence. Remains are more numerous of such algae as diatoms, 

 which have siliceous shells, and of lime-secreting seaweeds. Bryophytes 

 have been poorly preserved and their remains are scanty. Vascular 

 plants, on the other hand, are represented by an abundance of well- 

 preserved fossil material, and much is known of the geologic history 

 of many groups. 



The fossil record really begins with the Paleozoic era, since so little 

 is known of the life of the Archeozoic and Proterozoic. The periods into 

 which the Paleozoic, Mesozoic, and Cenozoic eras are divided are given 

 in the table on page 6. The figures in the time scale denote millions of 

 years. 



Fossil algae furnish the only record of plant life during the Cambrian 

 and Ordovician, and the diversity of types which have been found indi- 

 cates that all four of the great algal groups were represented in both 

 periods. Silurian deposits have yielded remains of the oldest known land 

 plants, the psilophytes, but these are scanty. During the Devonian so 



