358 STUDIES IN FOSSIL BOTANY 



before its connection with the other organs was 

 established. 



i. The Stem. — The stems vary in diameter from 

 about 3 mm. up to about 4 cm. The description will 

 be based on the larger specimens, from 1 cm. in 

 diameter upwards, as the smaller stems are in some 

 ways exceptional, and will be best dealt with later. 



The stem is, in all cases, of monostelic structure ; 

 the middle of the stele is occupied by a large pith, 

 chiefly parenchymatous, but containing irregular groups 

 of dark sclerotic tissue (see Figs. 129 and 130). 



Around the pith are disposed several distinct strands 

 of wood, varying in number from five to about eight or 

 nine, and constituting the primary xylem of so many 

 collateral vascular bundles (Fig. 129,3:) Immediately 

 outside the ring of primary xylem-strands, we find, in 

 all except the youngest stems, a broad zone of secondary 

 wood, consisting of radial series of pitted tracheides, 

 with parenchymatous medullary rays, both principal 

 and secondary, between them (Figs. 129 and 130). 

 This zone was manifestly the product of a cambium, 

 and, in fact, the cambial cells themselves are, in some of 

 the specimens, quite evidently preserved at the exterior 

 of the secondary wood (see Fig. 134, cb). Beyond the 

 cambium is a well-marked zone of phloem, also divided 

 up by the rays, and for the most part of secondary 

 origin. Opposite each of the primary xylem-strands, 

 however, a group of primary phloem can be recognised 

 in well-preserved specimens (see Figs. 129 and 134,/^). 

 We see, then, that the vascular structure, in its broad 

 features, is that typical of the stems of normal Gymno- 



