136 THEORY OF THE STROBILUS 



higher plants have been derived in the course of generations from parts 

 of an Algal thallus like that of Fucus, or at least from Alga-like plants, 

 by means of the overtopping of dichotomous branches, and the develop- 

 ment as leaves of the branches, which thus become lateral." Dr. Hallier, 

 who adopts Potonie's position, "prefers to draw the comparison with Liver- 

 worts, which show a similar sympodial development of a dichotomous 

 branch-system. 1 



It seems not improbable that the condition of many branched Fern-leaves 

 may have been derived through a process of " overtopping " in an indifferent 

 branch-system of the leaf itself. But it lies with Potonie' to show, on a 

 basis of comparison of forms more nearly related to them than the Fucoids, 

 that the relation of axis to leaf in the Ferns was so derived ; and, further, 

 that such an origin is in any way applicable to other foliar developments in 

 Vascular Plants, especially in Pteridophytes such as the Lycopods, Equiseta 

 and Sphenophylls. I am not aware that this has yet been done. But 

 granting that this can be done, the question still remains whether similarity 

 of method of branching is any criterion of comparison as to descent. 

 For sympodial development of a dichotomous system (and this is all that 

 such "overtopping" actually is) has occurred in cases where it cannot 

 be held to have resulted in a branching which is foliar ; and of this instances 

 can be found without going so far afield as the Fucaceae. If this be so, 

 then little value need be attached to the comparison of such branchings 

 in plants not nearly allied to one another ; these may be held to be quite 

 distinct examples of a general phenomenon of branch-development, without 

 the one being in any sense the prototype of the other Such reflections 

 as these indicate that the comparison in mode of branching between the 

 leaves of Ferns and the thallus of Fucoids, which forms the groundwork of 

 the view of Potonie (or between the Ferns and the Thalloid Liverworts, 

 as may be preferred by others), are not to be held as more than distant 

 analogies ; consequently they are no demonstration of the origin of the leaf 

 by a process of "overtopping." 



The view recently advanced by Professor Lignier (" Equisetales et 

 Sphenophyllales : leur origine filicineenne commune," Bull Soc. Linn, de 

 Normandie, Caen, 1904, p. 93) is analogous to that of Potonie, though 

 differing from it in detail. It involves ranking the Lycopod leaf as a 

 "phylloid," of the nature of a flattened hair, and comparable to the 

 amphigastrium of a Liverwort. The leaf of the Fern, however, is held 

 to be a true leaf, or phyllome, derived by differentiation from an indifferent 

 system of "cauloids," on which the "phylloids" have become abortive. 

 All such hypotheses have critical points in their application ; in the present 

 case it lies in the comparison of the Psilotaceae and Sphenophylleae. For 

 Lignier regards the leaf-lobes of Tmesipteris as only " phylloids," whereas 

 the leaves of the Sphenophylls, and also of the Equisetales, are "phyllomes," 

 reduced from the larger-leaved type of the Ferns. The argument is chiefly 

 1 Beitriigc z. JHorph. d. Sporophylle n. d. Trophophylls t Hamburg, 1902. 





