Stephenson. — Young Stages of Dicksonia and Cyathea. 5 



The Young Sporophyte. 



The first leaf, guided perhaps by its positive heliotropism, 

 soon appears between the prothallial lobes, and in Dicksonia, 

 if the embryo is far from the apex, the leaf may break through 

 the thallus. 



The blade of the first leaf of Dicksonia consists nearly always 

 of two equal lobes (fig. 77), but sometimes a simple spathulate 

 form occurs similar to that of Cyathea dealbata (fig. 78). In 

 C. Cunninghamii a more complex form is found. The attainment 

 of the more complex form by Dicksonia and C. dealbata is de- 

 pendent on the conditions of growth. For instance, under un- 

 favourable conditions C. dealbata may form as many as five 

 spathulate leaves. 



The first leaf, except at the veins, consists of two layers of 

 •cells, with well-developed intercellular spaces (fig. 1). 



" Rodlets " projecting into the air-spaces are not yet pre- 

 sent in Dicksonia, but occur in the first leaf in Cyathea. These 

 cuticular threads or rodlets are found in many different kinds 

 of ferns, and probably point to some similar metabolic pro- 

 cess. 



The young leaves of Dicksonia are marked out from the 

 others by the presence of hairs. These are sparsely scattered 

 over the leaves along the line of the veins, and consist of eight 

 or nine cells united into a slender filament, the terminal cell 

 being somewhat larger and rounder. The cells of the filament 

 become larger, with brown thick walls, as the plant grows, and 

 finally we reach the long brown robust hairs of the mature 

 plant, which protect the growing point and developing leaves, 

 and later serve to retain moisture on the stem for the aerial 

 roots. 



In the young Cyatheas (plants of four or five leaves) short 

 ramenta are present on the petiole, and especially at its base; 

 but C. dealbata remains glabrous for some time. C. medvllaris 

 is more nearly similar to Dicksonia. Fig. 73 shows a long sec- 

 tion of apex of Dicksonia (six leaves), and fig. 76 a similar stage 

 of Cyathea, showing the developing ramenta (r). 



Petiolar Wings. 



The first few leaves have a bulky green thin-walled cortex 

 in the petiole. But as the leaves become more robust the as- 

 similating tissue is found only in lateral wings, and later still 

 in clusters of thin-walled cells forming discontinuous streaks on 

 each side of the petiole. These groups are cut off and die; a 

 lignification of the f.v. bundles begins. They are probably 

 for aeration of the developing leaf. 



