192 



MORPHOLOGY 



nettitales are lateral, but distributed along the stem; while in Cycadales 

 they are lateral, but restricted to the tip of the stem. 



Vascular anatomy. — The anatomy of the stem resembles that of the 

 Bennettitales, with a thick cortex, a thin vascular cylinder, and a large 



■ pith (fig. 438). The 

 vascular bundles of 

 the stem are collateral 

 and endarch, as usual 

 among gymnosperms; 

 but in leaf traces, or 

 leaf veins, or axes of 

 strobili, the old fern 

 connection is indi- 

 cated by mesarch 

 bundles and some- 

 times even by con- 

 centric bundles. It 

 is in these so-called 

 peripheral regions of 

 the body that the 

 older features of the 

 vascular structure 

 persist the longest. 

 The primary cambi- 

 um of the stem may 

 persist (as in Zamia), 

 although the amount 

 of secondary wood it 

 forms is always small; 

 or it may be of short 

 duration (as in Cycas), 

 in which case a series 

 of successive cambium 

 cylinders is formed in 

 the cortex, resulting 

 in a concentric series of vascular cylinders. The leaf traces differ 

 very much from those of the Bennettitales. Instead of being direct, 

 as in the latter group, some of them (usually two) pass around the 

 stem through the cortex, and often enter a leaf on the opposite side of 



Fig. 435. — Bowenia (oriental), showing tuberous stem 

 with heavy armor of leaf bases. — After Eichler (Englee 

 and Pranil). 



