PTE RIDOPHYTA— FILICIN^ 



135 



which the arohesporial mass remains. Each mother cell then 

 divides by free-cell formation into four ceUs,usually but not always 

 arranged at the four angles of a tetrahedron, so that the result- 

 ing cells are pyramidal in form. Each undergoes rejuvenescence 

 and surrounds itself with a new cell-wall, forming a spore. The 

 walls of the original mother cells then disintegrate as did those 

 of the tapetum, and the spores, sixty-four in number, lie free in 

 the cavity of the sporangium. The development is shown in 

 fig. 893. 



In some oases the stalks of the sporangia are not developed, 



Fig. 893. 



Fig. 893. Development of the sporangium of the Fern. The figures indicate 

 the successive stages, a. Archesporium. t. Tapetum. ap. Sporogenous 

 tissue. After Kny. 



so that those organs are sessile ; in others the stalk gives rise to 

 a glandular hair {fig. 893, 7 7t). 



When the sporangium is ripe, the drying of the cells of the 

 annulus leads to great tension in the stomium, which ruptures, 

 and the annulus recoils with some force, scattering the spores. 



In the Tree ferns the annulus extends all round the sporan- 

 gium, so that it includes the stomium. 



Different genera show a certain variety in the position of 

 the annulus, and consequently in the direction of the rupture. 

 The dehiscence is, however, brought about in a similar manner 

 in all. 



The gametophyte in the Ferns is always a thaUus, having 



