HETERANGIUM 413 



in H. tiliaeoides than in any other fossil plant. On 

 entering the phloem-zone, the principal rays become 

 widely dilated, broadening out into conspicuous wedges, 

 and exactly resembling the phloem-rays of the lime- 

 tree, a fact to which the fossil owes its specific 

 name. 



The phloem attained a great thickness, scarcely 

 inferior to that of the secondary wood. To the outside 

 of each group, the primary phloem, corresponding to 

 the primary xylem-strand within, can be distinctly 

 recognised. The greater part of the phloem is 

 secondary, and has a definite radial seriation of its 

 elements. 



The phloem proper, as distinguished from the 

 secondary rays which pass through it, consists of 

 alternating bands of long parenchymatous cells, and 

 of sieve-tubes. The latter are so perfectly preserved as 

 to show, in some cases, the compound sieve-plates on 

 their radial walls, closely resembling similar structures 

 in recent Cycads. Details of this kind are necessarily 

 among the rarities of fossil preservation (see Fig. 158 

 sv.p)} 



Beyond the phloem-zone is a very wide pericycle, 

 the structure of which agrees closely with that in 

 Lyginodendron, as it contains groups of stone-cells, 

 elements which, in the case of Heterangium Grievii, 

 are limited to the cortex. The pericycle in H. tiliae- 

 oides also resembles Lyginodendron in showing the 

 beginning of periderm-formation in its outer layers. 



The cortex, which is less perfectly preserved than 



1 For details of the sieve-tubes, see Williamson and Scott, ' ' Further 

 Observations," etc., Part iii., Plate xxix., Figs. 38 and 38 A. 



