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SCIENCE. 



[N. S. Vol. XX. No. 512. 



over the axis, and will favor some phytonic 

 theory; those who hold the smaller-leaved 

 forms to be the more primitive will prob- 

 ably adopt a strobiloid theory of origin of 

 the leafy sporophyte. I propose to offer 

 some remarks on the relative probability of 

 these alternative views. 



If large-leaved prototypes be assumed 

 generally for vascular plants, this nat- 

 urally involves a widespread reduction, 

 since small-leaved forms are numerous now, 

 and have been from the earliest times of 

 which we have any record. Reduction is 

 a ready weapon in the hands of the specu- 

 lative morphologist, and it has often been 

 used with more freedom than discretion. 

 But reduction should never be assumed in 

 order to meet the demands of convenience 

 of comparison, nor as a cover for doubt. 

 The justification of a view involving re- 

 duction must be found in its physiological 

 probability in the case in question, and this 

 should be backed by comparisons of form 

 and of anatomical structure ; the conclusion 

 should also be in accordance with the 

 paleontological record. All suggested cases 

 of reduction where such justification is ab- 

 sent should be looked upon as doubtful. 



Convincing evidence of reduction of 

 leaf complexity in an evolutionary se- 

 quence, supported on all these grounds, has 

 been adduced in the progression from ferns, 

 through cycado-filicinean forms, to the 

 cycads ; and it applies with special force in 

 the case of their sporophylls. Ferns, which 

 are essentially shade-loving and typically 

 zoidiogamic, or amphibious, may be under- 

 stood to have given rise to the cycado-filices, 

 and cycads, which are more xerophytic, 

 and show that essential character of land 

 plants— the seed habit. Not only is such a 

 progression physiologically probable, but it 

 is supported by paleontological evidence, 

 as well as by detailed facts of anatomy, and 

 of reproductive morphology. The ease 

 for reduction of leaf complexity seems to 



be here fully made out, and somewhat simi- 

 lar arguments will also apply for other 

 types of gymnosperms. 



The facts relating to the vascular sys-. 

 tern of the shoot have also their bearing 

 on the question of the relative size of 

 primitive leaves. The origin of the leaf 

 trace from the axial stele in conifers, and 

 also in angiosperms, has been shown by 

 Dr. Jeffrey to be of the type styled by him 

 phyllosiphonic. This is specially char- 

 acteristic of those plants where the leaf is 

 essentially the dominating influence in the 

 shoot. In this I see a probability, which 

 their physiological position as land-growing 

 plants would justify, that the seed-bearing 

 plants at large were descended from a 

 large-leaved ancestry, and had undergone 

 reduction of leaf complexity in their de- 

 scent. But while we thus recognize a prob- 

 ability of widespread reduction producing 

 relatively smaller-leaved forms, it does not 

 follow that all small-leaved vascular plants 

 originated thus. On this point the anatom- 

 ical evidence is of importance, as bearing 

 on the origin of the small-leaved strobiloid 

 pteridophytes. Of these (putting aside the 

 hydropterids as being a special reduction 

 problem in themselves), there remain the 

 Lyeopodiales, the Equisetales and the 

 Sphenophyllales, which are all cladosi- 

 phonic in the terminology of Dr. Jeffrey. 

 The question will largely turn upon the 

 meaning of this anatomical feature. I 

 take it to be as follows : The cladosiphonic 

 character is the anatomical expression of 

 the dominance of the axis in the shoot. 

 Here the leaf trace is merely an external 

 appendage on the stele, which is hardly dis- 

 turbed by its insertion. This type is seen 

 in certain small-leaved pteridophytes. The 

 phyllosiphonic character, on the other 

 hand, is the anatomical expression of the 

 dominance of the leaf over the axis in the 

 shoot. Here the insertion of the vascular 

 supply of the leaf profoundly disturbs the 



