8 CYCABALES [CH. 



with the Palaeozoic Medulloseae^. The occurrence of concentric 

 cauhne strands in the cortex of Cycas is also a peculiarity worthy 

 of notice. Successive bands of periderm, and occasionally a 

 considerable amount of phelloderm^, are formed in the peripheral 

 region of the stem. 



The leaf-traces in an adult stem exhibit a striking feature in 

 their indirect or girdle-Hke course to the leaves (fig. 396, H, g) 

 and in the gradual change from an endarch (fig. 396, 0) to an 

 apparently mesarch structure (fig. 400) as they pass from the 

 perimeduUary zone to the petiole : except at the base of the 

 petiole the vascular bundles of the frond-axis consist of (i) centri- 

 petally developed xylem with a median protoxylem and a much 

 smaller amount of centrifugal xylem (fig. 400) separated by a few 

 parenchymatous elements from the centripetal xylem, (ii) an 

 external arc of protophloem and within this metaphloem and 

 parenchyma* In the slender petiole of Bowenia there are a few 

 collateral bundles arranged in the form of a circle or ellipse* ; in 

 Cycas and some other genera the more numerous bundles form a 

 pattern like an inverted U, and in some species of Encephalarios 

 the number is greater and the strands more irregularly scattered^. 

 In the vegetative stems there is no centripetal xylem in the stele, 

 but scattered centripetal tracheids occasionally occur internal to 

 the protoxylem in the steles of the peduncles®. 



Cycadeae. Megasporophylls each bearing 2 — 8 ovules, borne separately 

 like foh'age-leaves and not in strobili. Pinnae have a midrib but no lateral 

 veins (figs. 384, 387, A). Gycas (fig. 377). 



Zamieae. Both kinds of sporophylls form strobib. Pinnae have several 

 dichotomously branched, more or less parallel veins. Zamia (figs. 388 — 390), 

 Macrommia, Encephalarios (figs. 379, 386, C), Ceratozamia, Dioon (fig. 386, B), 

 Microcycas. 



Stangerieae. Strobili as in Zamieae. Pinnae fern-like, numerous dicho- 

 tomously branched lateral veins given off from a midrib. Stangeria. 



Bowenieae. Leaves bipinnate (fig. 391), strobili as in Zamieae. Bowenia. 



m 



Distribution. The most widely spread genus, Cycas, occurs 

 Siam, India, the Nicobar Islands, Ceylon, Madagascar, and 



" Worsdell (00) ; (06). See postea. Chap. xxx. 2 WorsdeU (982), 



For agures, see Mettenius (60) B. ; de Bary (84) A. ; le Goo (14) : Marsh f 14) 

 Wieland (06) pp. 62, 63. 

 Matte (04) PI. vn. fig. 111. 6 goott (97); Matte (04) p. 164. 



