THE TERNS 127 



are usually sterile, and seem to be the result of exces- 

 sive vegetative activity. They not infrequently multi- 

 ply by means of special buds, or gemmae, by which the 

 number of the gametophytes may be rapidly increased 

 exactly as in the liverworts. 



In some species of Trichomanes (Fig. 35, E), a genus 

 of the filmy-ferns, the gametophyte may have the form 

 of an extensively branched filament, closely resembling 

 an alga ; and it has been suggested that this may be the 

 primitive type of the gametophyte. However, as many 

 closely allied species produce the usual flat thallus, and 

 all of the forms, when exposed to excessive moisture, 

 show a tendency to assume a filamentous stage, it 

 is quite as likely that this is an adaptation to a moist 

 environment, rather than being the primary condition. 



Another group of ferns, the so-called Eusporangiatae, 

 which includes the adder-tongue (Ophioglossum) (Fig. 

 34, A) and its allies, as well as certain interesting trop- 

 ical forms, the Marattiacese (Figs. 81, 34), show a long- 

 lived gametophyte of a somewhat different type. In all 

 of these, so far as they are known, the gametophyte is 

 massive and quite different from the thin, delicate 

 thallus of the filmy-ferns and Vittaria, but like these the 

 gametophyte may live for a long time, often for several 

 years, and not infrequently remains alive long after the 

 young sporophyte is quite independent. The gameto- 

 phyte in the Marattiacese, especially (Fig. 32, A, B), 

 is extraordinarily like a simple thallose liverwort, both 

 as regards the thallus itself and the sexual organs devel- 

 oped upon it. In the adder-tongues the gametophyte, 

 so far as at present known, is subterranean and quite 

 destitute of chlorophyll ; but whether this is originally 



