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; art, 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 geotropic (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 
