68 



ANALYSIS OF THE SHOOT-SYSTEM OF FERNS [CH. 



Period. Here the two-fifths divergence is clearly indicated by the stelar 

 anatomy (Fig. 59). In the adult plant of Dryopteris Filix-mas the leaf- 



Fig- 59- Ankyropteris{Zygopteris) Grayi. Transverse section of stele, 

 showing wood and remains of phloem, i — 5 = the five angles of the 

 wood, from which leaf-traces are given off, in order of the phyllo- 

 taxis, no. 5 belonging to the lowest of the series, x — principal ring 

 of xylem; x/= small tracheides of internal xylem ; xe? = small tra- 

 cheides at periphery; /,^ = phloem; ;' = base of adventitious root. 

 ( X 14.) Will. Coll. 1919B. (From Scott's Studies in Fossil Botany.) 



arrangement is more complicated, giving the basket-like head of leaves so 

 usual in the Ferns of ascending or upright habit (Fig. 3 1 ). The Osmundaceae, 

 both fossil and modern, give good examples 



of more elaborate spiral arrangements. In 



large Tree Ferns the phyllotaxy may be 

 very complex. In such cases, as Schoute 

 has shown for the adult stem of AlsopJiila 

 glauca, Sm. var. settdosa, Hassk, a tendency 

 to a whorled arrangement may be found. 

 This appears also in the apical region of 

 Ainphicosmia Walkerae, where the leaf- 

 primordia stand in apparent alternating 

 whorls of three (Fig. 60). Again, in the 

 minute floating plant Salvinia the leaf- 

 arrangement has been described as whorled 

 and comparisons have even been drawn 

 on this ground between Salvinia and the 

 Sphenophylleae, where the leaves are 

 typically whorled. Nevertheless, even in Salvinia, the ontogeny opens by 



Fig. 60. Apex of stem of a large plant of 

 Ainphicosmia Walkerae, showing the ar- 

 rangement of the leaves apparently in 

 alternating tri-merous whorls. 



