October 21, 1904.] 



SCIENCE. 



531 



vascular arrangement in the axis. It is 

 characteristic of certain large-leaved pteri- 

 dophytes, and is seen also generally in seed 

 plants. 



It is a fact of importance that, in the 

 individual life, the one or the other type is 

 usually constant; but in certain ferns the 

 progression may be traced from the cladosi- 

 phonie 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 cladosiphonie 

 stock. Of the converse, viz., the progres- 

 sion from the phyllosiphonic to the cladosi- 

 phonie state in the individual life, I know 

 of no example among the pteridophytes, 

 though it is true that there is some ap- 

 proach to it in the Marsileaceje. Thus the 

 anatomical evidence indicates a probability 

 that, even in large-leaved ferns, the cladosi- 

 phonie 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, 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 ap- 

 plied 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, 

 restricted neither to leaves not to the 

 sporophyte generation. As to the other 

 two, the facts whether of external form or 

 of internal structure seem to me to indi- 

 cate this conclusion: that the strobiloid 

 condition was primitive for certain types, 

 such as the Equisetales, Lycopodiales 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 

 cladosiphonie structure, which is the in- 

 ternal expression of their microphyllous, 

 strobiloid state. That the Filicales and 

 also the Ophioglossales were probably de- 

 rived from a microphyllous strobiloid an- 

 cestry, and achieved the phyllosiphonic 

 structure as a consequence of leaf enlarg- 

 ment, this being the derivative rather than 

 the primitive condition; its derivation is 

 even illustrated in the individual life of 

 some ferns. Prom the Filicales the phyl- 

 losiphonic structure was probably handed 

 on to the seed plants, and by them retained, 

 notwithstanding the subsequent leaf re- 

 duction 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 phytonie theory does. 



A prototype, which was probably a prev- 

 alent, 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 tis- 

 sue of the axis to form them. The axis 

 would preexist in descent, as it actually 

 does in the normal developing shoot. The 

 oi'igin of these appendages may have oc- 

 curred independently along divers lines of 

 descent, and the appendages would in that 

 case be not homogeneous in the strict sense. 

 Thus there would be no common prototype 

 of the leaf, no morphological abstraction, 

 or archetypie form of that part. More 

 than one category of appendages might 

 even be produced on the same individual 

 shoot, differing in their function on their 

 first appearance. Such has perhaps been 



