74 PLANT MORPHOLOGY 



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 comparison of nearly related forms, while 

 the sympodial development of a dichotomous system, on which it is 

 founded, is a general phenomenon of branching, neither restricted 

 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, lycopodiales, and spheno- 

 phyllales, 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 expres- 

 sion 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 

 consequence 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, notwithstanding 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 me to harmonize 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, bearing relatively small and simple appendages. On our theory 

 the origin of those appendages in descent would be the same as it 

 is to-day in the individual development: viz., by the outgrowth of 

 regions of the superficial tissue of the axis to form them: the axis 

 would preexist in descent, as it actually does in the normal, develop- 

 ing shoot. The origin of these appendages may have occurred inde- 

 pendently along divers lines of descent, and the appendages would 

 in that case be not homogenous in the strict sense. Thus there would 

 be no common prototype of the leaf, no morphological abstraction, 

 or archetypic form of that part. More than one category of append- 

 ages might even be produced on the same individual shoot, differing 

 in their function on their first appearance: such has perhaps been 

 the ease in the calamarian strobilus, where the leaf-tooth cannot 

 be readily homologized with the sporangiophore. These suggestions 

 will suffice to indicate how elastic a strobiloid theory is, and how 

 its application will cover various types of construction, and even 

 such as are shown by the most complex cones of pteridophytes. 



From the comparison of living species there is good reason for 



