34 CYCADALES [CH. XXVIII 



of the leaf-trace bundles towards a concentric arrangement and 

 similar vascular strands are recorded in the peduncle of Dioon 

 edule, in various sporophylls^ and in other cases. It is possible, 

 as Worsdell believes, that the fairly frequent occurrence of con- 

 centric bundles in plants characterised by collateral bundles may 

 have a phylogenetic significance. 



The pinnae are dorsiventral and the veins exarch or pseudo- 

 mesarch: secretory canals occur between (Encephalartos), above, 

 or below the veins. The mesophyll of Cycas is characterised by 

 the presence of isolated xylem-elements passing from the midrib 

 to the edge of the lamina and, as Lignier^ suggests, these may be 

 regarded as a reduced system of lateral conducting strands. 



The epidermal cells of the leaflets have straight or slightly 

 curved walls except in Stangeria where they are undulate and fern- 

 like*. The stomata, with few exceptions confined to the lower 

 epidermis, are larger than in other gymnosperms (on the average 

 •075 X -034 mm.) and are more or less depressed below the surface ; 

 the guard-cells are usually surrounded by 4 — 6 subsidiary cells. 



The roots exhibit no feature to which attention need be called : 

 the pericycle is several cells broad and as in the stem there may 

 be extrafascicidar cylinders of xylem and phloem. 



I WorsdeU (98). 2 Lignier (92). 



s Nestler (95); Porsoh (05); Thomas and Bancroft (13); Dusdnek (13). 



