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A TEXTBOOK OF THEORETICAL BOTANY 



eventually break down and form a nourishing liquid for the developing 

 archesporium. The sporangium when mature possesses a single-layered 

 wall composed of spirally thickened cells. It dehisces by a longitudinal split. 

 As the archesporial tissue develops, about one-third of its cells break 

 down to vield their food supply to the cells near them. The surviving cells 

 are the spore mother cells, each of which finally divides into four cells, 



PRIMARY 



PROTHALLUS CELL 



EXTINE 



NTINE 



Fig. 637. — Eqiiisetum arveuse. Spores and germination. A, Spore showing 

 uncoiling of epispore. B, Spore in dry air showing the four arms of the 

 epispore uncoiled. C to F, Stages in the germination of the spore. {Modified 

 from Sadebeck.) 



during which reduction division takes place, each four cells being arranged 

 in a tetrahedron. These are the spores. 



The young spore when first formed has a thin cellulose wall, but as 

 development continues it becomes more complicated. The membrane of 

 the mature spore consists of four layers. The outermost layer or epispore 

 splits along spiral lines into two long bands with flattened ends, called elaters, 

 which, until the spore is mature, remain closely wrapped around it. When 

 the spores are dried the two elaters stretch themselves out, crosswise, remain- 

 ing attached only by their central region, and forming four distinct appendages 

 (Fig. 637). These structures are hygroscopic, as they are cuticularized on 



