LYCOPSIDA 69 



Lepidophloios Wuenschianus, from the Lower Carbonifer- 

 ous of Arran, is known in considerable detail, for examples 

 have been found in which portions of the stele from various 

 levels had fallen into the rotted base of the trunk before 

 petrifaction occurred. This has made it possible to discover 

 something about the growth processes taking place in the 

 young aerial stem. The primary wood near the base was 

 soUd and only 5-5 mm across, halfway up the trunk it was 

 medullated, while near the top (Fig. 11 A) it was 15 mm 

 across and had a hollow space in the centre of the medulla. 

 It is concluded that, as the stem grew, its apical meristem 

 grew more massive and laid down a much broader pro- 

 cambial cylinder. Meantime, the cambium in the lower 

 regions had laid down more secondary wood than higher up, 

 with the result that the total diameter of the wood (primary 

 and secondary together) was about the same throughout the 

 length of the trunk (about 7 cm). In proportion to the over- 

 all diameter of the trunk (40 cm), however, this quantity of 

 wood is surprisingly small, when compared with that of a 

 dicotyledonous tree, where most of the bulk is made up of 

 wood. The difference probably Ues in the fact that the wood 

 of modern trees is concerned with two functions, conduc- 

 tion and mechanical support, whereas the wood of Lepido- 

 dendrales was concerned only with conduction. Mechanical 

 support was provided mainly by the thick woody periderm 

 which was laid down round the periphery of the trunk. 



The metaxylem was composed of large tracheids with 

 scalariform thickenings, while the protoxylem elements 

 were much smaller and frequently had spiral thickenings. 

 The secondary wood consisted of radial rows of scalariform 

 tracheids and small wood-rays, through which leaf-traces 

 passed on their way out from the protoxylem areas. In most 

 specimens the phloem and even some of the cortex had 

 decayed before petrifaction occurred, but what is known of 

 the phloem suggests that it was small in amount and very 

 similar to that of modern lycopods. 



