90 OPHIOGLOSSACEAE [CH, 



true: but the existence of a general trend in the evolution of a Class as 

 a whole cannot be held to preclude the recognition of an opposite trend in 

 individual families. Nature is not built on lines of logic, and is not through- 

 out consistent. Opportunism is the ruling influence. In the present instance 

 the biological conditions met by Ophioglossum as a whole, and hy \Ophioderma 

 and \Cheiroglossa in particular, are different from those of most Ferns. They 

 are of xerophytic structure, with a strong tendency to mycorhizic nutrition. In 

 the ordinary ground-species the habitat is as a rule exposed. In Ophiodervia 

 and Cheiroglossa the habit is even epiphytic and partially saprophytic, 

 conditions which may well be expected to produce that leathery texture 

 which their leaves show; also a deeply sunk state of their sporangia, these 

 being otherwise without any protection. On the other hand, the very large 

 spore-output may be recognised as a biological offset against the risks 

 involved in establishing a progeny that is both partially saprophytic and 

 also epiphytic. 



A very special biological interest attaches to that curious degradation 

 illustrated by O. intermeduim and simplex. These species are clearly related 

 to O. penduliiin, of which they are probably ground-growing derivatives. 

 The two last-named species show their advanced position anatomically by 

 their divided leaf-traces. They are strongly mycorhizic. It has been sug- 

 gested that the saprophytic nutrition in the wet and dark forest in which 

 they grow has superseded their photosynthetic nutrition, and that abortion 

 of the sterile blade has been the result. If this be true then these simplest- 

 leaved Ophioglossaceae will be phyletically the most advanced. 



The more complicated arrangements of spikes in the genus OpJiioglossum, 

 shown in Figs. 343, 344, have elsewhere been the subject of theoretical 

 comparison with a view to the morphological elucidation of the spike. But 

 it may be questioned whether such comparisons are enlightening, or even 

 permissible. It cannot properly be assumed that these specimens have 

 retrograded along the lines of their previous evolution. If Ophioglossum is 

 really a late and derivative type of the family it would appear more probable 

 that its spike, condensed and standardised, would behave as a morphological 

 unit, subject rather to new fissions and interpolations than to normal 

 retrogression. In fact that the spike may, like the stamen of an Angiospermic 

 flower, show pleiomery. But such pleiomery cannot be read as elucidating 

 the general morphological history. This is the interpretation which appears 

 justifiable in such examples as are shown in Figs. 343 and 344. They may 

 be compared with the condition of the sporangiophores of the Spheno- 

 phylleae as regards their position, and varying number. It is significant 

 that these anomalies in Ophioglossum are characteristic of those sections of 

 the genus which have the most advanced vascular structure, and in particular 

 the disintegrated leaf-trace. 



