A SIDELIGHT FROM ANATOMY 139 



question whether originally the leaf was relatively large or small. Those 

 who hold that the large-leaved forms were the more primitive will be 

 naturally disposed towards the view of the original preponderance of the 

 leaf over the axis, and will favour some phytonic theory; those who hold 

 the smaller-leaved forms to be the more primitive will probably 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 naturally involves a widespread reduction, since small-leaved forms 

 are numerous now, and have been from the earliest times of which we 

 have any record. Convincing evidence of reduction of leaf-complexity in 

 an evolutionary sequence, supported on grounds of comparison of form 

 and structure, and in accordance with the palaeontological facts, 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 zoidio- 

 gamic, or amphibious, may be understood 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. The case for reduction 

 of leaf-complexity seems to be here fully made out, and somewhat similar 

 arguments will also apply for other types of Gymnosperms. It must there- 

 fore be admitted that extensive reduction of appendages has occurred in 

 certain very ancient phyla. 



But while we thus recognise a probability of reduction in certain phyla 

 producing relatively smaller-leaved forms, it does not follow that all small- 

 leaved Vascular Plants originated thus. On this point the anatomical 

 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 Lycopodiales, 

 the Equisetales, and the Sphenophyllales, which are all cladosiphonic in 

 the terminology of Dr. Jeffrey. The question will largely turn upon the 

 meaning of this anatomical feature. The cladosiphonic character may be 

 held as 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 disturbed by its insertion. This type is seen constantly in 

 certain small-leaved Pteridophytes. On the other hand, the condition, 

 styled by Dr. Jeffrey the phyllosiphonic, 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 vascular arrange- 

 ment in the axis, leading to an open communication between the cortex 

 and the central medulla at each leaf-insertion. It is characteristic of 

 certain large-leaved Pteridophytes, and is seen also generally in Seed-Plants. 

 There is thus a probability, supported on anatomical evidence, that the 

 seed-bearing plants at large were descended from a large-leaved ancestry, 

 and had undergone reduction of leaf-complexity in their descent. 



