376 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Cryptogams. 



Pteridophyta. 



(By A. Gepp, M.A., F.L.S.) 



Evolution of the Filicinean Leaf-trace.* — E. W. Sinnott writes 

 about the evolution of the filicinean leaf-truce, a question round which 

 cent re some of the most fundamental problems of plant morphology, 

 such as those concerning the retention of primitive characters by the 

 leaf, the inter-relations of the foliar vascular supply and that of the 

 axis, and the general questions of stelar theory and the origin and dif- 

 ferentiation of plant tissues. The conclusions reached by the author 

 after the examination of much material are summed up as follows : 

 1. The base of the leaf-trace in living ferns presents three main types 

 of structure : the primitively monarch, with one group of protoxylem ; 

 the primitively diarch, with two ; and the primitively triarch, with 

 three. 2. The first type is characteristic of the Osmundaceae and the 

 Ophioglossaceae, where the base of the leaf-trace is a single monarch 

 strand, which is often mesarch as well. It is more strikingly so in the 

 fossil ancestors of the former family. This single strand becomes in 

 the petiole a broad arch, which is continuous in the Osmundacea? and 

 broken up in the Ophioglossaceaj. The primitive condition of this type 

 of trace is an elliptical concentric strand with one mesarch protoxylem. 

 3. The second type is characteristic of the Marattiaceae. In all the 

 members of this family, save Angiopteris, and in young plants of this 

 genus, the base of the leaf-trace consists of two bundles, each of which 

 has a single protoxylem. This is endarch in all except Dansea, where it 

 is often mesarch. A complicated arch of bundles in the petiole develops 

 from these two early ones. The primitive condition of this type of 

 trace is two circular concentric bundles, each with one mesarch proto- 

 xylem. 4. The third type is characteristic of all remaining ferns, and 

 its primitive condition is a single, roughly triangular, concentric bundle, 

 with its base towards the stem-axis, and with three mesarch protoxylems, 

 one near each corner. In the Schizeacere, (lleicheniaceaj, and primitive 

 Matonineae among the Simplices, the strand is single throughout, and 

 at its base is always triarch and often mesarch. Lygodium alone among 

 the Filicales shows a petiolar structure which is neither endarch nor 

 arched, and which is doubtless very primitive. In the simpler Gradatae, 

 the trace becomes broadened into a tetrarch, flat-topped arch, which 

 becomes separated into many strands in the Dicksoniea? and Cyatheaceaj. 

 The Hymenophyllaceaa form a reduction series from the Simplices. In 

 the lower Mixtaj, the undivided tetrarch trace persists, but in the bulk 

 of the smaller and simpler forms it becomes divided into two equal 

 diarch bundles. The complicated petiolar system of the higher Poly- 

 podiacea3 is always referable to this simpler type. 5. The monarch 

 trace may be considered as the persistence of a very primitive condition. 

 The diarch type has perhaps been derived from the constriction and 

 separation into two of such a primitive diarch bundle as that of Clepsy- 



* Annals of Botany, xxv. (1911) pp. 167-191 (1 pi. and figs.). 



