200 REINHARDT THIESSEN 



wall have disappeared completely, or almost so. In the photo- 

 graphs at 200 diameters prepared from horizontal sections (Fig. 22), 

 the spore-exines are shown on their broad side, and appear as 

 circular to oval or sHghtly triangular disks. At a higher magni- 

 fication, say at 1,000 diameters (Figs. 24 and 25), the characters, 

 such as form, sculpturing, and tetrasporic marks remaining on the 

 exines, are clearly shown. 



An excellent way to study the spore-exines is to macerate the 

 coal by means of Schulze's reagent and digest it with ammonia. 

 The spore-exines and cuticles are left undissolved and apparently 

 unchanged. Figures 27-41 show some of the spore exines thus 

 isolated. 



From a collective study of all the spores, it is evident that a 

 considerable number of species and genera of plants contributed to 

 the spore matter in coal. Two distinct types of exines are dis- 

 tinguishable. The one is always in the shape of a circular disk, in 

 some cases tending to be triangular and less often tending to be 

 oval or ovoid, and all bear the familiar tetrasporic mark. These 

 are the exines of Paleozoic Pteridophytes. The other is always 

 oval or ovoid, but does not bear the tetrasporic mark and has a 

 long slit parallel to the long axis of the oval; often with a second 

 short sKt at, or toward, one of the extremities. The surface is 

 apparently smooth and unsculptured (Figs. 25 and 30). These 

 are undoubtedly the exines of the pollens of certain Paleozoic 

 Gymnosperms. 



The exines of true spores. — On the whole, the true spore-exines 

 are much more abundlant in coal than are the exines of pollen 

 grains. There is a large range of sizes among them, from that of 

 only about 10 microns (Figs. 22 and 24) or yoo ^^ ^ millimeter, to 

 that of 2 and 3 millimeters in diameter (Fig. 38). There are to be 

 recognized two kinds: megaspores and microspores. From a 

 biological standpoint, three kinds should be distinguished: mega- 

 spores (Figs. 35 and 38), microspores, and neutral spores. This is 

 not only a distinction of size, but also of function. Megaspores in 

 living plants are always large and on germination produce male 

 gametophytes ; microspores are always relatively small and produce 

 fem.ale gametophytes; while certain other spores, classed among 



