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BOTANY 



along its lower margin, and sometimes a single larger cell may be seen, to which 

 possibly may be referred all the cells of the older sporangium, but this is not 

 always certainly the case. The primary cell divides into an inner and an outer 

 cell, the former giving rise to the greater part, at least, of the sporogenous 

 tissue. From the outer one is developed the tapetum and the outer sporangium 

 wall. 



The sporangium increases rapidly in size, and forms an oblong sac, pendent 

 from the lower margin of the peltate sporophyll. The sporogenous cells are very 

 numerous, but the limits of the sporogenous tissue are not readily determined, 

 and a large number of the cells become broken down without forming spores. 



m. 





FIG. 268. Equisetum telmateia. A, group of sporogenous cells, just before the final 

 division of the spores, imbedded in the nucleated protoplasm derived from the 

 disintegrated tapetum, and sterile archesporial cells (x 500). B, optical section of 

 young spore, showing the three membranes ; m, the middle lamella (X 500). C, an 

 older spore, showing the formation of the elaters (X500). D, surface view. 

 E, section of the wall of a ripe sporangium (x 250). 



The sporogenous cells, before their final division, separate into small groups, 

 which are surrounded by a mass of nucleated protoplasm, derived partly from 

 the tapetum, partly from the sterile sporogenous tissue. The sporogenous cells 

 finally separate completely, and each divides into four spores in the usual manner 

 (Fig. 268). The ripe spores have the outer spore-coat split into four strips, 

 elaters, which are exceedingly hygroscopic, and by their pressure, as the spo- 

 rangium dries, they help to force it open. The elaters, when moist, coil up tightly 

 about the globular spore, but spread out when they are dry, and by their move- 

 ments probably aid in distributing them. The ripe spore contains numerous 

 crowded chloroplasts. 



