So 



GYMNOSPERMS 



The smallest of all cycads is Zamia pygmaca, with adult stems i 

 or 2 cm. in diameter, and rarely reaching 3 cm. 



In transverse section the stem shows a large pith and large cortex 

 with a scanty zone of wood between (fig. 70). The vascular cylinder 

 is an endarch siphonostele in the adult plant, but the seedling shows 



Fig. 68. — Bowenia spcclahilis: plant with a female 

 cone, a, apogeotropic root. The elongated, fusiform 

 stem contrasts sharply with the short, broad stem of 

 B. serrtdata. — After Chamberlain."^ 



Fig. 69. — Bowenia serrtdata: short, broad stem with 

 numerous branches at the top, some bearing male 

 cones. — .\fter Chamberlain."^ 



a distinct mesarch condition, and there is some centripetal wood in 

 cone axes and in the stalks of sporophylls. 



'J'he amount of xylem in most stems is surprisingly small. A ma- 

 ture plant of Zamia floridana, with a stem 1 5 cm. in height and 6 cm. 

 in diameter, had a zone of xylem 2 mm. wide. The zone of phloem 

 had the same diameter. A plant of Ccratozamia mcxicana^ 30 cm. 

 high and 15 cm. in diameter, had a zone of xylem 3 mm. wide and 

 phloem 2 mm. wide. Dioon cdule, 60 cm. tall and 21 cm. in diameter, 

 had zones of xylem and phloem each 5 mm. in width. 



