1 40 THEORY OF THE STROBILUS 



It is a fact of importance that, in the individual life, the one or the other 

 anatomical type is usually constant ; but in certain Ferns the progression 

 may be traced from the cladosi phonic in the young plant to the phyllosi- 

 phonic in the mature, thus suggesting a similar progression in descent, 

 viz. that the large-leaved phyllosiphonic Ferns were derived from a smaller- 

 leaved cladosiphonic stock. Of the converse, viz. the progression from 

 the phyllosiphonic to the cladosiphonic state in the individual life, I know 

 of no example among the Pteridophytes, though it is true that there is 

 some approach to it in the Marsiliaceae. Thus the anatomical evidence 

 indicates a probability that, even in large-leaved Ferns, the cladosiphonic 

 was the primitive type ; but that the phyllosiphonic, once initiated, is as 

 a rule maintained : this is shown by its persistence in the Seed-Plants, 

 even where the leaf has been reduced in size. 



Having thus gained a valuable sidelight from anatomy, indicating 

 that small-leaved types were probably primitive, we may now return to 

 our central question of the initial relation of leaf to axis. Of the three 

 theories already noted, the theory of overtopping as applied to the origin 

 of the leaf may, in my opinion, be dismissed, as it is not based upon com- 

 parison of nearly related forms, while the facts of embryogeny and of 

 leaf-origin do not support it : and further, the sympodial development of 

 a dichotomous system, on which it is founded, is a general phenomenon 

 of branching, restricted neither to leaves nor to the sporophyte generation. 

 As to the other two, the facts, whether of external form or of internal 

 structure, seem to me to indicate this conclusion : that the strobiloid 

 condition, was primitive for certain types, such as the Equisetales, Lyco- 

 podiales, and Sphenophyllales : that in them the leaf was from the first 

 a minor appendage upon the dominating axis, and anatomically they have 

 never broken away from the cladosiphonic structure which is the internal 

 expression of their microphyllous, strobiloid state. That the Filicales and 

 also the Ophioglossales were probably derived from a microphyllous 

 strobiloid ancestry, and achieved the phyllosiphonic structure as a conse- 

 quence of leaf-enlargement, this being the derivative rather than the 

 primitive condition ; its derivation is even illustrated in the individual 

 life of some Ferns. From the Filicales the phyllosiphonic structure was 

 probably handed on to the Seed-Plants, and by them retained notwith- 

 standing the subsequent leaf-reduction which followed on their adaptation 

 to an exposed land-habitat. Thus a strobiloid origin may be attributed 

 to all the main types of Vascular Plants. It seems to harmonise more 

 readily with the facts than any phytonic theory does. 



A prototype, which was probably a prevalent, though perhaps not a 

 general one for the Pteridophytes, may then be sketched as an upright, 

 radial, strobiloid structure, consisting of a predominant axis, showing con- 

 tinued apical growth, and bearing relatively small and simple appendages. 

 On our theory the origin of these appendages in descent would be the 

 same as it is to-day in the individual development, viz. by the outgrowth 



