PLANT MORPHOLOGY 73 



The facts relating to the vascular system 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 characteristic 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 under- 

 gone reduction of leaf-complexity in their descent. But while we thus 

 recognize a probability of widespread reduction producing relatively 

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

 cular plants originated thus: on this point the anatomical evidence 

 is of importance, as bearing on the origin of the small-leaved stro- 

 biloid 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. 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 

 disturbed 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 vascular arrangement in the axis; it is 

 characteristic of certain large-leaved pteridophytes, 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 pro- 

 gression may be traced from the cladosiphonic in the young plant 

 to the phyllosiphonic in the mature, thus suggesting a similar pro- 

 gression in descent, viz., that the large-leaved phyllosiphonic ferns 

 were derived from a smaller-leaved cladosiphonic stock. Of the con- 

 verse, viz., the progression from the phyllosiphonic to the cladosi- 

 phonic 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 Marsileacece. Thus the anatomical evidence indicates a probabil- 

 ity that, even in large-leaved ferns, the cladosiphonic was the prim- 

 itive 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 side-light from anatomy, we may 

 now return to our central question of the initial relation of leaf to 



