74 



THE GAMETOPHYTE, AND SEXUAL ORGANS 



[CH. 



lobes. This is the customary type of prothallus described in textbooks, 

 and in them it is often the only one mentioned for Eerns. But variants from 

 this type are common in Dryopteris itself, as well as in other Ferns. If the 

 prothalli are crowded during development, and consequently half-starved, 

 they assume an attenuated filamentous form, with frequent branching, and 

 they bear antheridia only (compare Fig. 20, i ; also Fig. 266). A partial sepa- 

 ration of the sexes is the result. Prothalli of Dryopteris grown in deficient 

 light may persist for years, with a very attenuated form, and bear antheridia 

 only. But if transferred to normal conditions they will then develop normally, 



Fig. 265. Mature prothallus of Dryopteris FiUx-nias, as seen irom below, bearing antheridia 

 among its rhizoids, and archegonia near to the apical indentation. (After Kny.) 



bearing functional archegonia (Phillips, Ann. of Bot. 1919, p. 265). Such facts 

 suggest a high degree of plasticity of ordinary Leptosporangiate prothalli. 

 They appear to be more directly the creatures of circumstance than the 

 sporophyte generation. The behaviour on germination of the spore may be 

 held to give facts of value. Usually a filament composed of one or more 

 cells is formed first, the length of which depends upon the conditions. Under 

 insufficient light the filamentous stage is prolonged. On the other hand, 

 occasionally in the Polypodiaceous Ferns, and normally in some Marat- 

 tiaceae, the filamentous stage may be omitted where the light is specially 



