302 



Mr Seivard, Notes on Cycads : 



usual Cycadean features. A complete section of the axis of a 

 pinna exhibits a dorsiventral differentiation : the upper half is 

 distinguished by a more compact band of hypodermal fibres and 

 by the occurrence of a few layers of chlorophyll-containing cells 

 internal to the stereome. The hypoderm next the lower epi- 

 dermis forms a less clearly defined band and passes gradually into 

 scattered fibres interspersed with the parenchymatous ground- 

 tissue. No secretory canals were observed. 



Figs. 1 — 4* illustrate the gradual change in the form of a 

 vascular strand in the region of forking. The single protoxylem 



PX 



group (poc) becomes double (Fig. 2), and the transverse elongation 

 of the centripetal xylem and separation of the two protoxylem 

 points are followed by an equal division into two strands. The 

 forking of the pinna is thus seen to be accompanied by a dichotomy 

 of the vascular bundle. 



The occurrence of forked pinnae of Cycadean fronds was first 



* The sections were prepared by Miss Sykes of Girton College. 



