320 



BOTANY OF THE LIVING PLANT 



forcibly by pressure of the everted valves upon them (Fig. 261). The 

 spores fall upon the soil, and germinate there. Thus both the mega- 

 spores and microspores are shed from the parent plant. 



FIG. 262. 



A, B radial sections through young sporangia. C = transverse section of one 

 more advanced. D tangential section. E = radial section of an older sporangium, 

 with ligule : the tapetum is shaded, and the sporogenous cells lie within. (A. B, C, 

 0=350 ; =200.) F. O. B. 



The sporangium arises in the axil of the sporophyll, just within the ligule. 

 After the first segmentations are past it is found to consist of a short thick 

 stalk, bearing the slightly flattened sporangial head. This contains a group 



of sporogenous cells, from which later a sur- 

 rounding tapetal layer is cut off. Outside 

 this is a wall composed of two layers of cells 

 (Fig. 262, E). Later the spore-mother-cells 

 round themselves off, becoming isolated in a 

 fluid that fills the enlarging cavity. Up to 

 this point it is impossible to tell which type 

 of spore the sporangium will produce. This 

 fact indicates that the megasporangium and 

 microsporangium are differentiated from one 

 original type. In the case of a microspor- 

 angium all the spore-mother-cells undergo the 

 tetrad division, and a large number of micro- 

 spores is the result. But if it is to be a 

 megasporangium, only one (hi some species 

 two) of the spore-mother-cells develops further 

 (Fig. 263), the rest becoming disorganised. 

 The four megaspores, with rugged walls, occupy 

 the whole sporangium at maturity (Fig. 261). 



FIG. 263. 



Selaginella spinulosa. Section of 

 megasporangium, showing the single 

 fertile tetrad still very small, and the 

 rest of the sporogenous cells arrested. 

 ( x 100.) F. O. B. 



