xiv] 



THE CORDATE TYPE 



75 



favourable. It may be held that a natural juvenile form for Ferns is that of 



the filament. This filamentous state may be prolonged, or even its repetition 



induced by cultivation under suitable 



conditions, and especially so in prothalli 



that are still young. But in most cases 



it is only on some massive development 



of tissue, such as the cushion normall)- 



supplies, that the archegonia themselves 



are borne. 



Fertilisation of an archegonium 

 habitually arrests the growth of the 

 prothallus that bears it, the available 

 material being used in the nursing of the 

 embryo, and the prothallus is as a rule 

 exhausted. But if fertilisation be pre- 

 vented the prothallus may attain large 

 size, and continue its growth for even 

 five or six years. The absence of ex- 

 ternal water, without which fertilisation 

 cannot take place, or growth under light 

 of insufficient intensity for the forma- 

 tion of the archegonia, may bring this 

 result. But even then the actual size 

 rarely exceeds one or two inches, as seen 

 in Ferns of robust habit such as the 

 Marattiaceae and Osmundaceae. Von Goebel figures a prothallus of Os- 

 viiinda regalis about two inches in length, and such prothalli occasionally 

 branch (von Goebel, I.e. Fig. 918). They resemble in their size, fleshy texture, 

 dark green colour, and method of apical growth and of branching some of 

 the thalloid Liverworts, such as Pellia or Aneiira. Differences of character 

 may arise according to the degree of branching which prothalli show. In 

 filamentous forms branching is common, and may appear to follow an almost 

 regular scheme, as in Trichomanes, and Schizaea (see below. Fig. 269, A). 

 In flattened types it is less common. Gleichenia pectinata is one of the best 

 examples, in which adventitious shoots are formed in numbers upon the 

 margin or from the ventral surface, and may develop into perfectly normal 

 prothalli (Campbell, Mosses and Ferns, 2nd Edn. p. 367, Fig. 208). It has been 

 suggested that since certain characteristic features, usually absent in normally 

 fertilised prothalli, appear on those which have not been thus arrested in 

 their development by embryo-formation, the present-day prothallus ma\' 

 be a structure the ancestors of which showed a greater complexity. \n 

 example in point is seen in those scales and bristles which are borne on old 



18—2 



Fig. 266. Woodsia ilvensis, after Schlum- 

 berger. A separated fragment of a pro- 

 thallus (/), from which, under feeble light, 

 branched filaments have sprung, bearing 

 antheridia {a). 



