II] THE CREEPING HABIT 33 



adult state would have been secondarily acquired (Fig. 39). But on the other 



hand the creeping habit is not necessarily a mere continuation of the prone 



axis of the sporeling. This is shown by the inversion of the axis in Poly- 



podimn vulgare, where by hyponastic growth 



the creeping axis arches backwards over the 



parent prothallus (Fig. 39). While the upright 



habit and basket-form are effective in growth 



in the open, or under forest-canopy, and in 



restricted space, the creeping habit is suitable 



for Ferns living as dense undergrowth, or 



where the space is not restricted. Moreover 



it brings many advantages. Rooting in the 



soil is not localised at a given centre, but 



adventitious roots spread over the whole area "'fiXn C'rn^TouTlSp^.i.aUul 



of the soil covered by the creeping stem. and embryo, partly diagrammatic: 

 -,, , , r J 1 -J J.I J showing one series only of the disti- 



Each leaf can develop mdependently, and chous leaves, A, 4, etc. : ^= roots: 



without competition for exposure to light. «/ = apex of axis. The hyponastic 

 . . shoot becomes completely inverted, 



Branchmg is common and even prevalent m growing backwards over the pro- 

 creeping Ferns (Fig. 32): while the original Phallus. 



individual may be multiplied to many by the simple process of decay of 

 the older parts progressing beyond a branching. This is conspicuous in the 

 Bracken, and it accounts for a large proportion of the individuals of that 

 species that we see. These advantages, together with the opportunity 

 offered for a climbing habit, sufficiently account for the prevalence of a 

 creeping form in Ferns at large. 



Like other Pteridophytes the Ferns are perennials, and have been so 

 from the time of their early fossil progenitors. Occasional exceptions are 

 seen, such as Anograuinie leptop]iylla, which perennates by its storage- 

 prothallus, while the sporophyte dies off each autumn (see Chapter Xiv). 

 Ceratopteris tJialictroides is short-lived, but frequently perennates by means 

 of its sporophytic buds. The Salviniaceae are also mostly annual plants, 

 though they may perennate by their apical buds. Putting such special cases 

 aside, it may be said of modern Ferns in general that they are perennials. 

 Frequently their active vegetation may continue without any marked seasonal 

 break, as evergreens. This is the case with most tropical Ferns, and especially 

 with those of dendroid habit. Successive series of leaves may be added to 

 the terminal tuft, and the older leaves may successively lose their firmness, 

 shrivel, and fall, sometimes with a clean detachment as in Cyathea Schansiii 

 (Fig. 40); sometimes they leave ragged and persistent stumps, as in Dicksonia 

 squarrosa. On the other hand, many of the Ferns of temperate climates 

 show a regular seasonal leaf-fall, as in the Male Shield Fern, and the Bracken. 

 Sometimes the shedding of the leaves is carried out piecemeal, the separate 



B. 3 



