362 SELAGINELLA. 



(b) The formation of the female prothallus in the megaspore 

 begins while the spore is in the sporangium and before it has 

 reached its full size ; the nucleus of the young spore (which lies at 

 the apex the point where the three radiating lines meet) divides 

 repeatedly, and the apical lens-like portion of the prothallus tissue 

 is formed first this tissue is easily seen on warming some mega- 

 spores with potash, dissecting off the coat or pressing the spore 

 under the cover-glass. 



(c) In free germinated megaspores, the prothallus tissue enlarges, 

 filling the cavity of the spore, ruptures the outer coat of the spore 

 along the three radiating lines, protrudes from between the flaps of 

 the burst coat, and produces several archegonia resembling those 

 of the Male Fern but with much shorter neck ; the marginal portion 

 of the exposed prothallus tissue may become green and produce a 

 few rhizoids from the superficial cells. 



{d) In older prothalli treated with potash, note the contrast 

 between the so-called apical tissue (that first formed) and the large- 

 celled lower tissue formed later. 



