48 



THE HABIT AND THE HABITAT OF FERNS 



[CH. 



first to appear above ground, and the young plant is thus holo-saprophytic 

 up to its eighth or ninth year. In Helminthostachys. the fungus is present 

 in the first three or four roots of the young plant, but 

 it is absent from the later roots. Fungal invasion has 

 been observed in the adult plant of Ophioglossum V2il- 

 gatum, though it is stated to be absent from the young 

 plant. It is present in the adult plants of O. pendulum, 

 and it is specially prevalent in the precocious mycorhizic 

 root of its embryo. The most peculiar case of all is that 

 of Ophioglossum simplex, in which a pronounced state 

 of mycorhiza goes along with the apparently complete 

 absence of a sterile lamina (Fig. 55). Here it would 

 seem that the mycorhiza makes the nutrition of the 

 large spike still possible in the dense wet forest in which 

 the plant grows, notwithstanding that the usual photo- 

 synthetic organ is functionally absent. Reduction is 

 not, however, apparent in the spike itself, for provided 

 nutrition be kept up from whatever source it would still 

 retain its character, being essentially a spore-bearing 

 organ. Mycorhiza has also been found in the roots of 

 the Marattiaceae, and in Cyathea, but without any 

 consequent reduction. Strangely enough it is absent 

 in Asplenium nidus. Thus its occurrence in Ferns is 

 sporadic, and seemingly arbitrary : it may be associated 

 in extreme cases with morphological reduction, but this 

 is not a general feature, and no Fern is known in which 

 the holo-saprophytic nutrition continues throughout the 

 whole life-cycle. Irregular nutrition has never secured 

 a complete hold among the Filicales. 



The hygrophilous habit so prevalent among Ferns 

 has in some few been extended to a definite swamp-life, 

 or even to a floating habit. Dryopteris Thelypteris grows 

 habitually in fen-land : Marsilia and Pilularia live in Fig. 55- 

 swampy ground, and grow quite well with their rhizomes 

 floating in water, but they do not appear to fruit except 

 on firm ground. Ceratopteris thalictroides is a semi- 

 aquatic Fern, and it is particularly at home among 

 crops of rice, which are grown in artificially irrigated, 

 muddy ground. Azolla floats with its roots pendent into the water, and 

 Salvinia with a like habit has no proper roots, but root-like leaves. Thus 

 various steps are seen leading to a completely aquatic life. Some few 

 Ferns affect salt water. Asplenium marinum lives on maritime rocks within 



Ophioglossum 

 si7iiplex, Ridley, slightly 

 reduced. Three leaves 

 are seen inserted on a 

 short stock. But the 

 leaves appear to consist 

 each of a fertile spike, 

 with no sterile lamina. 



