THE MAIDENHAIR TREE SEWARD 447 



but that in itself is not proof of specific identity. Fortunately, 

 many fossils are not mere impressions on shale and sandstone — 

 ghosts without substance; they are often covered with a black or 

 brown film which is all that remains of the tissues. This film repre- 

 sents the altered or carbonized remains of the leaf substance: its 

 surface is the resistant cuticular skin which covered the epidermal 

 layer of the living leaf. The film can often be detached from the 

 rock, or it may peel off naturally. By treatment with appropriate 

 reagents it is rendered less opaque and becomes amenable to micro- 

 scopical examination, revealing the cell-pattern of the epidermal 

 layer, including the minute and vitally important stomata. The 

 stomatal apparatus, which regulates gaseous exchange between the 

 plant and the atmosphere, serving also as an exit for water vapor, 

 has a characteristic structure of great value in distinguishing one 

 genus or family from another. Focusing below the surface of the 

 film one can sometimes see the remains of underlying tissue, crushed 

 and contracted, and readily detect the presence of secretory cells 

 still containing dark patches of the original products of secretion. 

 Thus valuable evidence is obtained enabling us to compare in detail 

 the surface layer and some of the deeper tissues with the correspond- 

 ing structures in fresh leaves. By the use of modern methods of 

 technique it has been possible to correct conclusions based solely on 

 external form. Botanists concerned only with living plants natur- 

 ally and very properly attach the greatest importance to reproduc- 

 tive organs as criteria of relationship. The paleobotanist is in a 

 much less favorable position; he hardly ever finds fossil leaves and 

 flowers preserved together, and indeed flowers are seldom available; 

 he has, therefore, been compelled to make the best use he can of 

 leaves, and has studied them intensively. In rare instances it is 

 possible to examine not only the surface layer of fossil leaves but 

 the structure of all the tissues: leaves and other parts of plants 

 occasionally occur as petrifactions, preserved in amazing perfection. 

 Petrified leaves are abundant in the calcareous nodules of coal seams 

 (Carboniferous period), but in rocks belonging to periods in which 

 the Ginkgo family flourished they are exceedingly rare. One of the 

 few examples of Ginkgoalean foliage preserved in this way is men- 

 tioned on a later page. 



Nearly 30 years ago the writer suggested the employment of the 

 generic name Ginkgoites in place of Ginkgo for fossil leaves which 

 through lack of evidence cannot be regarded as generically identical 

 with Ginkgo biloba. Fossil leaves closely resembling the foliage of 

 the maidenhair tree may have belonged to a tree having male and 

 female flowers which differed in some important respects from those 

 of the surviving species. Paleobotany is not an exact science: the 

 documents which the student endeavors to decipher provide only 



