374 



STRUCTURE AND LIFE HISTORIES 



isolation of single spores is prevented, and the advantage 

 of this is recognized at once when we recall that the 

 prothallia are dioecious. 



335. The Gametophytes. Under suitable conditions of 

 moisture and temperature the spores begin to germinate, 

 and by successive cell-divisions produce the lobed pro- 

 thallia. The male prothallia are one cell in thickness, 

 and bear the antheridia at the tips of the lobes or on the 

 margins (Fig. 269). 



FIG. 270. Female prothallus of Equisetum arvense L. ar\, young 

 archegonium; ar 2 , archegoium before fertilization; st, sterile lobe of pro- 

 thallus; hw, rhizoids. Several lobes were removed in order to show 

 the cushion and the archegonia. Enlarged about 20 times. (After 

 Sadebeck.) 



The female prothallia form a cushion of relatively 

 thick, spongy tissue (the meristem), and on this cushion 

 (as in all Pteridophytes) are borne the archegonia. In 

 contrast to the true ferns, the archegonia are borne on 

 the upper surface of the prothallus, and point upward, 

 in consequence of being negatively geo tropic (Fig. 270). 

 From the edges of the cushion numerous thin flaps of 

 green tissue form; these fold over the cushion, enclosing 

 the archegonia, and thus retaining the moisture of dew 



