204 HEREDITY AND EVOLUTION IN PLANTS 



to the stem of [the fern] Osmunda, but when more mature 



certain cycadean characters appear to predominate." 1 



Its foliage and other characters closely resemble some 



of our modern tree-ferns (Cf. Figs. 87 and 88), but more 



FIG. 88. Leaf of a fern (Angiopteris evecta). (Cf. Fig. 87.) 



careful study of the calcined specimens of much beauty, 

 found in calcareous nodules (the so-called English 

 "coal balls" 2 ), has disclosed both the microsporophylls, 



1 Knowlton, F. H. American Fern Journal, 5: 85. 1915. 



2 Coal balls are "concretions of the carbonates of lime and magnesia 

 which formed around certain masses of the peaty vegetation as centers 

 and, through inclosing and interpenetrating them, preserved them from 

 the peculiar processes of decay which converted the rest of the vegetation 

 into coal. In them the mineral matter slowly replaced the vegetable 

 matter, molecule by molecule, thus preserving the cellular structure to a 

 remarkable degree. Such balls are especially frequent in the coal of 

 certain parts of England (Lancashire and Yorkshire)." Shimer, H. W. 

 " An introduction to the study of fossils," p. 53. London, 1914. 



