X] ARTHROPITYS. SURFACE FEATURES. 315 



an outgoing leaf-trace which is enclosed by the strongly curved 

 tracheids of the stem. The section is taken from the node of 

 a stem where several leaf-trace bundles are passing out to a 

 whorl of leaves ; the few cells intercalated between the tracheids 

 belong to the parenchyma of the secondary medullary rays. 



In the small portion of a stem represented in fig. 74 B, the 

 cortical tissues have been partially preserved ; at the inner edge, 

 next the hollow pith, there are two xylem groups, each with a 

 carinal canal, and between them is part of a broad " principal " 

 medullary ray\ The cambium has not been preserved, but 

 beyond this region we have some of the large cells, c, of the 

 inner cortex : these are followed by a few remnants of a smaller- 

 celled tissue, and external to this part of the cortex there is a 

 series of triangular groups, h, consisting of small thick-walled 

 cells alternating with spaces which were originally occupied by 

 more delicate parenchyma. The darker groups constitute 

 hypodermal strands of mechanical tissue or stereome which 

 lent support to the stem. The surface of a stem possessing 

 such supporting strands would probably assume a longitudinally 

 wrinkled or grooved appearance on drying; the intervening 

 parenchyma, contracting and yielding more readily, would tend 

 to produce shallow grooves alternating with the ridges above 

 the stereome strands. 



The complete section of the stem of which a small portion 

 is shown in fig. 74 5, is figured by Williamson^ in his 12th 

 memoir on Coal-Measure plants. The section was obtained 

 from Ashton-under-Lyne in Lancashire ; it illustrates very 

 clearly a method of preservation which is occasionally met 

 with among petrified plants. The walls of the various tissue 

 elements are black in colour and somewhat ragged, and the 

 general appearance of the section is similar to that of a section 

 of a charred piece of stem. It is possible that the Calamitc 

 twig was reduced to charcoal before petrifaction by a lightning 

 flash or some other cause. 



It is often said that the surface of a Calamite stem was 

 probably marked by regular ridges and grooves similar to those 



1 Vide footnote, p. 811. 



2 Williamson (SH^), PI. xxxiii. fig. 19. 



