426 STRUCTURE AND LIFE HISTORIES 
horsetails, and other cryptogams (Figs. 315 and 316). It 
is instructive in this connection to recall recent careful 
studies of the structure of coal as seen in transparent 
sections by the aid of the microscope. These sections, 
prepared by Professor Jeffrey, reveal the remarkable fact 
that the soft, or bituminous, coals contain carbonized 
remains of innumerable spores of the plants which con- 
stituted the dominant vegetation during the geological 
FIG. 317. Photomicrograph of a thin section of cannel coal from 
Kentucky, formed under open-water conditions, i.e., of the muck at the 
bottom of ancient lakes or lagoons. The light, roundish bodies are spores. 
(Cf. Figs. 315 and 316.) (After E. C. Jeffrey.) 
period (Carboniferous), when coal was being formed (Fig. 
317). Such studies necessitate a radical change in our 
earlier conception as to the conditions and method of 
coal-formation. 
These facts also illustrate how the study of what might, 
at first thought, seem insignificant, impractical, or unim- 
portant, and not closely related to our daily lives, may, 
at any time, furnish the key to unlock the mystery of 
