LEPIDODENDRON 153 



was small compared with the much greater development 

 of periderm. 



Although the details of the cambial growth have not 

 yet been satisfactorily cleared up, there is no doubt that 

 it was normal, in the sense that the cambium produced 

 wood internally, and phloem, though probably to no 

 great extent, on its external side. 



The cells of the cambium in Lepidodendreae, where 

 preserved, are not usually found to correspond exactly 

 to the radial series of xylem-elements on their inner 

 side (cf. Fig. 57). It appears probable that the same 

 initial layer was not active throughout the duration of 

 secondary growth, but that new zones of cells may have 

 taken up the cambial divisions periodically. In this 

 respect there would be a certain similarity to the 

 secondary increase in Isoetes, among living plants. 



The phloem of the Lepidodendreae also presents 

 considerable difficulties. Typical phloem, consisting of 

 delicate elongated elements, has not always been 

 recognised, even in the best-preserved specimens, such 

 as that illustrated in Fig. 5 7. In other cases, however, 

 as in the leaf- traces of L. selaginoides and in the 

 vascular bundles of the cones, the phloem appears to 

 have been quite of the normal type, 1 so we are not 

 justified in supposing that there was any fundamental 

 difference in this respect between the Lepidodendreae 

 and their recent allies. 2 



1 See Maslen, " Structure of Lepidostrobus ," Trans. Linn. Sac. vol. v. 

 1899, Plates xxxvi.-xxxviii. Figs. II, 13, 14, and 33. 



2 On the controversy as to phloem in Lepidodendreae, see F. E. Weiss, 

 "On the Phloem of Lepidophloios and Lepidodendron" Mem. and Proc. 

 Manchester Lit. and Phil. Soc. vol. xlv. Part iii. 1901 ; and Seward, "The 

 So-called Phloem of Lepidodendron," New Phytologist, vol. i. 1902. 



