PAST HISTORIES OF PLANT FAMILIES 87 



in Asia, were still growing with the other European types 

 in Europe so late as middle Tertiary times. 



As in the case of the Angiosperms, the fossils we 

 have of Cretaceous and Tertiary Gymnosperms are nearly 

 all impressions and casts, though some more or less iso- 

 lated stems have their structure preserved. Hence our 

 knowledge of these later Gymnosperms is far from com- 

 plete. From the older rocks, however, we have both 

 impressions and microscopically preserved material, and 

 are more fully acquainted with them than with those 

 which lived nearer our own time. Hard, resistant leaves, 

 which are so characteristic of most of the living genera 

 of Gymnosperms, seem to have been also developed in 

 the past members of the group, and these tend to leave 

 clear impressions in the rocks, so that we have reliable 

 data for reconstructing the external appearance of the 

 fossil forms from the Palaeozoic period. 



The resinous character of Gymnosperm wood prob- 

 ably greatly assisted its preservation, and fragments of 

 it are very common in rocks of all ages, generally pre- 

 served in silica so as to show microscopic structure. 

 The isolated wood of Gymnosperms, however, is not 

 very instructive, for from the wood alone (and usually 

 it is just fragments of the secondary wood which are 

 preserved) but little of either physiological or evolutional 

 value can be learned. When twigs with primary tissues 

 and bark and leaves attached are preserved, then the 

 specimens are of importance, for their true character 

 can be recognized. Fortunately among the coal balls 

 there are many such fragments, some of which are 

 accompanied by fruits and male cones, so that we know 

 much of the Palaeozoic Gymnosperms, and find that in 

 some respects they differ widely from those now living. 



There is, therefore, much more to be said about the 

 fossil Gymnosperms than about the Angiosperms, both 

 because of the better quality of their preservation and 

 because their history dates back to a very much earlier 

 period than does the Angiospermic record. Indeed, we 



