462 THE ANATOMY OF WOODY PLANTS 



utilization of botanical technique in the study of coal has appar- 

 ently made it necessary to revise the accepted views as to the origin 

 of this invaluable mineral. The hypothesis of the derivation of 

 coals from peat formed in place, as in the northern bogs of our 

 epoch, must be given up in favor of the conception of an accumu- 

 lation in open water by transport either aerial or aquatic. Coal 

 from its internal organization obviously is comparable to the muck 

 in the bottoms of modern lakes rather than to the surface accumu- 

 lations of peat which often surround them. The technique of 

 mounting sections of coal is the same as for imbedded material in 

 which the matrix of nitrocellulose is retained. No staining is 

 necessary. 



PHOTOMICROGilAPHIC METHODS 



Photography with the microscope has become in recent years 

 an extremely important aid in microscopical investigations of the 

 structure of plants. To such perfection have photographic lenses 

 and photographic plates attained that it is possible to reproduce 

 photographically most of the structural features of tissues and cells 

 in plants. The advantage of photomicrographic reproduction is 

 that it is at once less laborious and more accurate than represen- 

 tation by drawings. 



The lenses used for photomicrographic purposes do not differ, 

 except in special cases, from those employed in general microscopic 

 investigation. For very low magnifications such as are ordinarily 

 ^not available with the compound microscope special lenses of great 

 perfection are now manufactured by the leading makers of micro- 

 scopical appliances. These are used without eyepieces and can 

 be applied as a rule only to special photomicrographic stands. 

 For moderate and higher magnifications, however, the same optical 

 outfit is used as for observation with the eye. The better grades of 

 lenses naturally give better photographic results than those which 

 are of inferior quality. In the case of very high magnifications 

 special condensers, insuring a great degree of concentration of 

 light and freedom from chromatic aberration, are employed fcr 

 illumination. 



Microscopic lenses have certain general defects which militate 

 against their employment for photographic purposes. The most 



