570 fiOTANY. 



several Divisions of the Vegetable Kingdom are very un- 

 equally distributed in geologic time. Thus no traces of 

 Protophyta have yet been discovered earlier than the Terti- 

 ary (Miocene), while the Zygosporeae appear to extend back 

 to the Secondary (Jurassic), and the Oosporese and Carpospo- 

 reae to the Silurian. Bryophyta have not been detected in 

 strata earlier than the Eocene (Tertiary), while Pteridophyta 

 extend back to the Devonian. Of the Phanerogamia the 

 Gymnosperms originated in the Devonian, the Monocotyle- 

 dons in the Triassic, and the Dicotyledons in the Cretaceous. 

 These facts may be more clearly shown by the table on the 

 preceding page. 



It must be borne in mind that our knowledge of fossil 

 plants is as yet extremely limited, a comparatively small 

 portion only of the earth's strata having hitherto been care- 

 fully examined. It is very probable that as we come to 

 know more of the fossil remains of plants some or all of the 

 lines in the table will be extended downward. On the other 

 hand, we need not expect to find many remains of the ex- 

 ceedingly simple organisms which constitute the Protophy- 

 ta, although they probably have existed in abundance 

 since pre-Silurian times. So, too, few Zygosporeae have a 

 sufficiently durable plant-body to allow them to be preserved 

 in a fossil state. The softness of texture and easy perisha- 

 bility of the tissues of the Bryophyta, especially in the lower 

 orders, probably accounts for the few fossil remains hitherto 

 discovered. Doubtless we must in the same way account for 

 the fact that most of the species of fossil Phanerogams are 

 trees and shrubs; the softer tissues of the herbaceous spe- 

 cies have yielded but few fossils as compared with the harder 

 arid denser ones of the ligneous species. 



