258 THE ANATOMY OF WOODY PLANTS 



found in the stem of many of the Equisetales as well as of all 

 but the very lowest seed plants, the initial discontinuity of the 

 secondary wood often becomes very marked. The situation 

 presented by these extreme types unco-ordinated with the eluci- 

 dative anatomical features presented by more primitive forms has 

 been the cause of serious misunderstanding as regards the origin 

 of the so-called medullary rays. The case of the siphonostelic 

 lepidodendrids and Sigillariae, as diagrammatically represented in 

 Fig. 1 2 2, page 170, seems to indicate definitely that parenchymatous 

 interruptions in the secondary cylinder resulting from the dis- 

 continuity of the primary wood cannot be interpreted as true rays 

 any more than are the gaps related to the departing traces of 

 appendages to be brought into the category of rays. Much 

 confusion of definition has resulted from the failure to interpret 

 the conditions found hi higher forms in terms of the structures 

 presented in earlier and more primitive types. 



It has been made clear in an earlier chapter that the lepido- 

 dendrids proper, which beyond question represent the more 

 primitive state of organization for the Lepidodendraceae as a whole, 

 supply evidence for the derivation of radial parenchyma as the 

 result of the transformation partial or complete of radial tra- 

 cheary strands into storage parenchyma. The older representa- 

 tives of the Lepidodendraceae show themselves in this respect the 

 most archaic of all the vascular plants with secondary growth. 

 Not only is the origin of radial storage cells in the secondary xylem 

 elucidated by the lepidodendrids, but, as has been pointed out in 

 an earlier chapter, the parenchymatous elements of the primary 

 wood have their origin illustrated in the conditions found in the 

 primary region of the stem in this ancient group of vascular plants. 

 It has been indicated in Figs. 29 and 30 that the living cells occurring 

 in the wood of the primary region of the axis in protostelic lepido- 

 dendrids have been derived by septation from elements belonging 

 to the category of tracheids. Some of the resultant elements 

 persist as short tracheids with thickly reticulated walls, while 

 others maintain a thinner wall and in all probability in life were 

 occupied by living protoplasm. In the siphonostelic Lepidoden- 

 draceae and in the sigillarian forms as a whole there is no evi- 



