Stephenson. — Young Stages of Dicksonia and Cyathea. 7 



the mature form the breaking-up into separate bundles takes 

 place very early. The separate bundles (fourteen or fifteen) 

 take their origin almost directly from the protruding lips of the 

 leaf-gap. But the bundles always show a single protoxylem 

 group, and always fuse into a continuous arc before the first 

 pinna is given off. 



The protoxylem of the first few petioles is persistent, but 

 later, when the petiole is marked by very rapid growth, the 

 protoxylem cells are destroyed. Provision is made for this in 

 a single layer of small dense cells that surround the protoxylem. 

 These grow into the spaces that are left by the destruction of 

 the spiral cells (fig. 8, c.p. ; fig. 9, d.L). 



The phloem tissue, hardly distinguishable in the first few 

 petioles, later contains very large sieve-tubes. These occur 

 at first only on the convex side of the arc, but they finally form 

 a ring. In the mature petiole the sieve-tubes are numerous, 

 but each tube is in contact with at least one parenchyma cell 

 (fig- 7). 



Petioles of other ferns were examined — Gleichenia flabettata 

 and Cunninghamii, Aspidium aculeatum, and Hypolepis distans — 

 and though the sieve-tubes were numerous each bordered on a 

 parenchyma cell. 



Cyathea. 



The first bundle is marked by collateral (fig. 2), and the 

 cell-layer inside the endodermis is densely granular. 



In very young leaves the petiolar arc breaks up into three, 

 and then there is no fusion, as in Dicksonia, before the pinnse 

 are given off. Smaller differences from Dicksonia are in the 

 large size of the last-formed metaxylem and the variation in 

 position of the protoxylem group. 



Pinna prom Petiole. 

 Dicksonia. 

 In the first leaf of Dicksonia the venation is generally dicho- 

 tomous. In later leaves the successive pinnae arise by segments, 

 being given off from the free ends of the bundle arc. * But when 

 the bundle has three groups of protoxylem elements only the 

 two lateral groups provide for the pinna?. 



rocoxy/em 

 group 



pinna, bundle 



12 3 4 



A Series of Sections showing the Derivation of the Pinn.e 

 Bundle from the Petiolar Bundle. 



Later leaves show a similar process. 



