44 



THE HABIT AND THE HABITAT OF FERNS 



[CH. 



able, and as in Dischidia the cavities are penetrated by roots, which spring 

 from neighbouring points on the same plant. Physiologically in both cases 

 they appear to be a means of securing a supply of water during rains, to be 

 absorbed at leisure into the tissues. But whereas the urns are specialised 

 leaves in Dischidia, they appear in these Ferns to be highly modified branches 

 of the rhizome. 



Fig. 51. Polypodium bifrons, after E. Ule, from Christ; showing the epiphytic 

 habit, and sac-shaped urns, into which roots penetrate, as in Dischidia. 



An alternative adaptation seen in epiphytic Ferns is the so-called "nest- 

 habit," where the leaves are aggregated in a dense tuft, with roots below, 

 the whole serving as a means of holding together a considerable mass of 

 humus, which collects within the leaves, and retains water. A very successful 

 example of this is seen in Aspkniiim nidus, a species whose leathery lanceo- 

 late leaves often attain large size, but are not specialised in form (Fig. 52). 

 But the nest-habit is more effectively developed in Platycerium and in 



