380 STUDIES IN FOSSIL BOTANY 



sections to be spirally thickened, are at the prominent 

 angles (J>x), constituting a peripheral protoxylem, such 

 as is characteristic of roots. Towards the interior the 

 tracheides become rapidly wider, and assume pitted 

 structure. They are intermixed with conjunctive 

 parenchyma, which, however, never forms a definite 

 pith. In good preparations the primary groups of 

 phloem, lying in the furrows of the wood, between the 

 protoxylem-angles, can be clearly recognised, separated 

 from the neighbouring wood by one or more layers 

 of conjunctive parenchyma. The delicate, elongated 

 elements of the phloem are quite distinct, as - seen in 

 longitudinal section, from the adjoining parenchymatous 

 cells. We thus see that there was a perfectly regular 

 alternation of the xylem and phloem-strands, just as in 

 recent roots. A pericycle, one or more layers thick, 

 surrounds the vascular tissue ; the endodermis beyond 

 this is often well defined, and in specially favourable 

 cases its radial cell-walls stand out more sharply than 

 the others. The broad inner cortex of the root con- 

 sists of a lax parenchyma (Fig. 141, i.c), in which 

 numerous elongated elements with dense carbonaceous 

 contents, probably secretory sacs, are scattered. The 

 outer border of the root is formed by a very well-defined 

 double layer of large cells (o.c), generally well preserved, 

 and constrasting sharply with the more perishable 

 internal cortex. This double peripheral layer (exo- 

 dermis) is one of the most useful characters in recognising 

 the roots of Lyginodendron, though it was, perhaps, 

 common to those of some closely allied genera. The 

 roots, before secondary thickening set in, had a maximum 

 diameter of about 5 mm. ; they vary from this size 



