STRUCTURE OF FOSSIL PLANTS 



75 



Very few types can be mentioned here, and to make 

 their relations clear a short series of diagrams with 

 explanations will be found more helpful than a detailed 

 account of the structures. 



Fig. 52. Spores 



Each spore a single cell which develops 

 with three others in tetrads (groups of four). 

 Very numerous tetrads enclosed in a spore 

 case or sporangium which develops on a leaf- 

 like segment called the sporophyll. Each 

 spore germinates independently of the others 

 after being scattered, all being of the same 

 size. Common in fossils and living Pterido- 

 phytes. 



sporangium, wall 



spores. 



Fig- S3- Spores 



Each a single cell like the preceding, but here 

 only one tetrad in a sporangium ripens, so that 

 each contains only four spores. Compared with 

 the preceding types these spores are very large. 

 Otherwise details similar to above. Some fossils 

 have such sporangia with eight spores, or some 

 other small number; living Selaginellas have 

 four. In the same cone sporangia with small 

 spores are developed and give rise to the male 

 organs. 



spOTOL 



scores 



sborobhy 



Fig. 54- "Spores" of Seedlike Structure 



Out of a tetrad in each sporangium only one spore 

 ripens, s in figure, the others, s, abort. The wall of 

 the sporangium, 10, is more massive than in the pre- 

 ceding cases, and from the sporophyll, flaps, spf, 

 grow up on each side and enclose and protect the 

 sporangium. The one big spore appears to germi- 

 nate inside these protective coats, and not to be scat- 

 tered separately from them. Only found in fossils, 

 one of the methods of reproduction in Lepidodendron. 

 Other sporangia with small spores were developed 

 -which gave rise to the male organs. 



