X] DISINTEGRATION OF THE STELE 191 



NepJirolepis dilates into a tuber the same features of stelar expansion appear 

 as in the conically enlarging axis of many Leptosporangiate Ferns. This 

 suggests that the increase in size in both cases is at least one factor deter- 

 mining the structural change, while the reversal of that change which follows 

 on the apical contraction of the tuber strongly supports that conclusion. 



The triumph of the Leptosporangiate Ferns, which thus show disinte- 

 gration of the stele in stem and leaf in all their more advanced types, is 

 witnessed by their 6000 living species spread widely over the face of the 

 earth. They are essentially the Ferns of the present day. That triumph 

 has been won structurally by a compromise, effected without cambial increase 

 in the enlarging stem. Their conducting stele has enlarged with the conically 

 enlarging shoot. It has maintained its endodermal barriers complete, and 

 has met the difficulty of physiological interchange consequent on that en- 

 largement by various steps of moulding and disintegration of the stele. This 

 has given the necessary proportion of surface to bulk of those conducting 

 tracts even for stems as large as those of the Tree Ferns. 



The structure seen in those Ferns which both comparison and the fossil 

 record stamp as primitive, viz. the Ophioglossaceae, Osmundaceae, and 

 Marattiaceae, is susceptible of elucidation along similar lines. Their success 

 as measured by genera and species, and by geographical spread, is only 

 partial. Their less perfect structural adaptation to the demands of increasing 

 size may well have been one among the factors which have led to this result. Its 

 most marked feature is the opening of the endodermal sheath as greater 

 size is attained. This imperfection of their conducting tracts has probably 

 imposed upon them that semi-xerophytic character which their foliage 

 habitually shows. From the comparative point of view these Ferns may best 

 be regarded as evolutionary indicators. They point the way towards, but never 

 fully attain, that more perfect adjustment of structure to the requirements of 

 increasing size without cambial activity, which is realised with such successful 

 results in the Leptosporangiate Ferns. 



Actual size may thus be held to be an important factor in determining 

 the appearance of features which have been habitually used in comparison. 

 The effect of this should be to impose caution in drawing conclusions from 

 characters so pliable. At once the door is opened for frequent homoplasy in 

 stocks phyletically quite distinct from one another, in any one of which 

 increase in size of the individual or the race is possible. The appearance of 

 polycycly in the Matonineae, in Thyrsoptej'is, Saccoloma, and in various 

 Pterids, may well be held as homoplastic; that is, as isolated consequences 

 of increase in size rather than as any index of affinity of Ferns otherwise 

 isolated and diverse. More cogent still is the case for dictyostely, or for 

 perforation, the appearance of which seems quite general in stocks distinct 

 one from another. From this point of view the recognition of the Principle 



