374 BENNETTITAIiES [CH. 



(fig. 507). On the conical terminal bud preserved in some stems 

 (fig. 536) the foliage-leaves are replaced by linear scales. The 

 leaf-bases often show the vascular bundles which form a simpler 

 system than in the majority of recent species, their arrangement 

 being, as Wieland says, more Fern-Uke; they form a series of 

 coUatieral strands following the outline of the petiole-base ^ with a 

 U-shaped invagination in the middle of the upper face (fig. 517, A). 

 Beyond the structure of the bundles at the base of the fronds and 

 in the cortex of the stem our knowledge of the anatomy of the 

 vascular supply of the rachises is very meagre. At the petiole- 

 base each bundle consists either entirely of radial rows of centri- 

 fugal scalariform tracheids and medullary rays usually one-cell 

 broad (fig. 519, B) or of centrifugal and centripetal xylem 

 in varying proportions. The ground-tissue is well supphed 

 with large secretory canals and in the larger leaf-bases there 

 is a considerable development of periderm at the surface 

 (fig. 517, B) as in recent Cycads. The ramenta formed from 

 the epidermal cells consist in most species of fairly broad scales 

 one-cell thick at the edges and broader in the middle ; in 

 Cycadeoidea nigra^ they are generally one-cell thick throughout 

 and similar ramenta are common in G. Cribsoniana (fig. 517, B). 

 In C. micromyela unicellular hairs replace the scaly ramenta, 

 but transitional forms occvir between hairs and scales. In 

 Williamsonia scotica (fig. 562) and in an Indian species of that 

 genus the ramenta are exclusively long hairs as in recent Cycads, 

 the scale-form of the ramenta in Cycadeoidea being a Fern-character. 

 The degree of development of the ramental tissue varies in different 

 species ; in C. StilwelU and C. excelsa it is feebly developed while in 

 Cycadeoidea micromyela the ramenta almost cover the exposed 

 leaf-base armour. The exceptional abundance of the ramental 

 scales is a striking characteristic of some American stems referred 

 to a separate genus, Cycadella^. In the abundance of the ramental 

 tissue, in the compact structure of the well protected cones, and 

 in the thickly cuticularised epidermis of the bracts and leaves 

 Cycadeoidea exhibits xerophilous characters in a very high degree. 



1 Wieland (06) p. 63, fig. 33. 



2 See Wieland (06) for additional facts and illustrations. 

 ' See page 417. 



