164 



THE VASCULAR SYSTEM OF THE LEAF 



[CH. 



;/z>rz^w^(Fig. I57,rt),and many others. Often, however, the subdivision is carried 

 further, and a number of small oval or circular strands appear arranged in a 

 curve, while two adaxial strands, which 

 are usually larger than the rest, corre- 

 spond to the free margins of the horse- 

 shoe. Dryopteris Filix-uias is a common 

 example of this last state, which holds 

 for most of the advanced Polypodiaceous 

 Ferns (compare Fig. 1 6 1 , 4, 6). A specially 

 high degree of subdivision of the leaf- 

 trace, accompanied by further complica- 

 tions arising from the highly divided 

 polycyclic structure of their stems, is 

 found in all of the living Marattiaceae. 

 It may appear strange that Ferns so 

 antique in other characters should have 

 this advanced type of leaf-trace. But 

 they are modern survivors, and in the 

 ancient Psaronms and Megaphyton, to 

 which they are probably related, the 

 leaf-trace consisted of a continuous, 

 highly-curved sheet of vascular tissue. 



The protoxylem-groups in the adult leaf- trace are mesarch at the extreme 

 base, but as the trace flattens in its course outwards they pass towards the 

 adaxial face of the xylem (Fig. 155). Their number varies, and it often in- 

 creases as the meristele passes upwards into the petiole. The attempt has 

 been made to establish definite types of leaf-trace in living Ferns, according 

 to the number of the protoxylem-groups ; the primitively monarch with only 

 one, the primitively diarch with two, and the primitively triarch with three 

 (Sinnott, Ann. of Bot. 191 1, p. 167). To the first the Osmundaceae and 

 Ophioglossaceae are assigned ; to the second the Marattiaceae ; to the third 

 all remaining Ferns. It is true that speaking generally, and from examina- 

 tion of the adult leaves, there is a larger number of protoxylem-groups at 

 the leaf-base in the advanced than in the more primitive Ferns, and that 

 the single protoxylem is the most primitive of all. But all Ferns show this 

 character in their first sporeling-leaves. In fact Ferns generally start their 

 ontogeny from the archaic state with a single protoxylem in the leaf. Some 

 primitive Ferns retain this throughout life, but most of them pass on 

 quickly to larger numbers (Fig. 155, 7, 8, 9). The Marattiaceae will serve 

 as an example. Sinnott assigns two protoxylems to these Ferns. But 

 Campbell {Eusporangiatae, 191 1) has shown that if sections are made of the 

 cotyledons there is only one vascular strand in each, with a very minute 



Fig. 157. Modifications of meristele in Fern- 

 leaves. a,f>, Alhyrium Filix-foemiiia, « = at 

 base, (^ — higher up. r, d, e, Asplenhim 

 adiantiim-nigriivi, f = at base, d, e, at 

 points higher up. (After Luerssen from 

 Rabenhorst.) 



