CHAPTER V 



THE FIB RO VASCULAR TISSUES: SECONDARY WOOD- 

 PARENCHYMA 



Although the present chapter is to be devoted to the presence 

 of parenchyma in the secondary wood, it will be advantageous 

 to consider first the presence of cells belonging to this category in 

 the primary xylem. Parenchymatous elements, as has been made 

 clear in an earlier chapter, are those which are characterized in 

 their functional con- 

 dition by the pres- 

 ence of protoplasm, 

 by simple pitting of 

 their walls, and by 

 their generally more 

 or less isodiametric 

 dimensions. Cells of 

 this nature form a 

 well-marked feature 

 of primary wood 

 that is, ligneous tis- 

 sues which are with- 

 out rays and without 

 regular disposition of 

 their elements. The 

 parenchyma of the 

 ancient arboreal club 

 mosses of the Paleo- 

 zoic, the lepidodendrids, is of special interest from the standpoint 

 of the doctrine of descent, for here the mode of origin of the 

 parenchymatous elements of the primary wood is clearly indicated. 

 Fig. 29 shows a transverse view of the primary woody tissues of a 

 species of Lepidodendron from a "coal-ball" of the Carboniferous 

 of Lancashire in England. A number of elements of large size 

 and thick walls can be distinguished. These are the tracheids. In 



37 



FIG. 29. Origin of wood parenchyma in the pri- 

 mary wood of a lepidodendrid. 



