LYCOPODIALES 



517 



The sporangium arises in the axil of the Bporophyll, just within the lignle. 



After the first segmentations arc past it is found to consi I I a short tl 

 stalk, bearing the slightly flattened sporangial head. This contain up 



of sporogenous cells, from which later a surrounding tapetal lay< I is ( ut off. 

 Outside this is a wall composed of two layers of colls (Fig. .)<".. / . ,. Later the 

 spore-mother-cells round themselves off, becoming isolated in .1 liquid 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 microsporangium all the spore- 

 mother-cells undergo the tetrad division, and 

 a large number of microspores is the result. 

 But if it is to be a megasporangium, only one 

 (in some species two) of the spore-mother-cells 

 develops further (Fig. 410), the rest becoming 

 disorganised. The four megaspores, with 

 rugged walls, occupy the whole sporangium 

 at maturity (Fig. 408). 



Fig. 410. 



Selaginella spinulosa. Section of 

 megasporangium, showinc: the single 

 fertile tetrad still very small, and the 

 rest ot the sporogenous cells arrested. 

 ( x 100.) F. O. B. 



The germination of both types of 

 spores may begin before they are shed, 

 but it is continued on the moist soil. 

 The microspore first partitions off a lenti- 

 cular cell, which, as it appears to perform no function, and does not 

 develop further, may be held as vestigial, representing the vegetative 

 region of a male prothallus. The rest of the contents segment to 



A B 



Fig. 411. 



A , microspore of S. apus after germination. B the same M below extiwioo 



of spermatozoids. (After Mi- Lyon.) 



form a wall of eight sterile cells surrounding a numerous group of 

 spermatocytes (Fig. 411, 4 This is in fact the correlative of an 

 antheridium: the whole male prothallus thus consists of a 



