FIBRO VASCULAR TISSUES: PARENCHYMA 39 



which instead of being simple are septate. In some instances the 

 short segments of the longer cells are obviously tracheary, since 

 they have scalariform sculpture on their lateral and terminal walls. 

 In other instances the septations of the long elements are quite 

 thin-walled and without any scalariform or reticulate thickenings. 

 Cells of the latter type are parenchyma tous, and the origin of such 

 elements in the case of the primary wood of Lepidodendron is 

 clearly indicated. It is obvious that, as a consequence of the 

 subdivision of originally long cells, tracheary in their character, 

 two sorts of products result namely, short thin-walled cells (the 

 wood parenchyma), and equally abbreviated elements, sisters to 

 these, which constitute merely segments of the subdivided tra- 

 cheids. It thus becomes clear that the parenchymatous elements 

 of the primary wood in the very ancient genus Lepidodendron are 

 derived from the subdivision of elements which were primitively 

 tracheids. It will be shown in subsequent paragraphs that the 

 origin of wood parenchyma in the secondary xylem is likewise due 

 to the septation of elements originally of the nature of tracheids. 

 In the present connection the question naturally arises as to the 

 derivation in turn of the tracheids. The following up of this 

 problem would lead us too far into the field of purely speculative 

 evolution. This much, however, may be stated, that, accepting 

 the thallose liverworts as the probable starting-point of the evolu- 

 tion of vascular forms, it becomes clear that the mass of elaters 

 (with spirally thickened walls) which constitute the columella in 

 Pellia, Aneura, and some species of Anthoceros, etc., obviously 

 constitute the prototype of the fibrovascular bundle of the ferns 

 and higher forms. If this hypothesis is to be admitted, and it is 

 the one assumed by the greater number of those who have worked 

 on the somewhat meager facts which throw light on the probable 

 origin of the Pteridophyta, it appears clear that in the spirally 

 thickened elaters we have the originals of the spiral elements of the 

 protoxylem. It follows that elaters are the forerunners of the 

 tracheids, which in turn, by subdivision, as evidenced by the lepido- 

 dendrids, have given rise to the parenchymatous elements of the 

 primary wood. There does not seem, however, to be any clear 

 indication in the case of other and existing lycopodineous forms 



