BITUMINOUS COALS 201 



the smaller ones, may produce gametophytes that may reproduce 

 either male or female gametophytes, there being apparently no pre- 

 determination. Among the spores of the coals, no such distinction 

 can be made, and all that can safely be said is that some are large 

 and others are small, and assume that the larger ones are mega- 

 spores; but between the spores that functioned as microspores 

 and those that are neutral, no distinction can be made, and the 

 term microspores must apply to both kinds, if used at all. This 

 affords a convenient, if not exact, distinction, between the very 

 large spore-exines and the smaller ones. It should be stated 

 that the range in sizes is gradual from the smallest to the largest, 

 and that no fast line, in regard to size, can be drawn between 

 the two. 



The thickness of the exine walls varies very greatly with the 

 kind of spore from which derived, and ranges from the tinest 

 film of only a few microns in thickness to such where it is a huge 

 mass of a hundred microns or more, as in the large megaspores 

 as shown in Figure 38. But the size of the spore is not always 

 commensurate with its thickness. Very large spores are observed 

 with but very thin walls, and again comparatively small exines are 

 met with which have walls equal in thickness to half their diameter. 



Almost all spore-exines are sculptured, and only comparatively 

 few are smooth, and each kind has a definite t)rpe of sculpturing, 

 which affords a ready means of distinction between them. The 

 sculpturing may take a variety of forms and may consist of ser- 

 pentine ridges, irregular elevations, echinate protuberances, sharp, 

 slender spines, and short hair like coverings. In many cases, 

 these are arranged in definite order, as in spirals or rows (Figs. 27- 

 41). Some exines are covered all over with a ramentum; others 

 bear a long tuft of ramentum on a small area only; others have a 

 number of long slender wings; and still others have three large 

 air sacks. 



Exines of pollen grains. — A large number of exines present in 

 coals are apparently those of pollen grains (Figs. 25 and 30). 

 In a very large number, especially noticeable when they have 

 been isolated from the coal by means of Schulze's reagent, there 

 is a long slit running parallel to the long axis of the oval; and 



