368 STUDIES IN FOSSIL BOTANY 



arrangement becomes completely concentric (cf. Fig. 

 138). Thus Lyginodendron, like Osmunda among 

 recent Ferns, had collateral bundles in the stem, which 

 became concentric in the leaf. 



As regards the other tissues of the stem, little more 

 need be said. Nests of thick-walled cells are abundant 

 in the pith and pericycle, and extend out through the 

 cortex into the petiole (Fig. 137). Sacs with dark 

 contents, probably representing some kind of secretory 

 organs, are frequent in all the soft tissues of the plant 

 (Figs. 131, 132, 134). When considerable secondary 

 growth had taken place, the cells, of the more external 

 tissues have often become much dilated tangentially, 

 and this dilatation is naturally most conspicuous in the 

 outermost tissues, namely in the parenchyma between the 

 fibrous strands of the external cortex (Figs. 1 29 and 1 30). 



Very curious, flask-shaped, multicellular outgrowths 

 (Fig. 137, c) are often found on the surface of the stem, 

 each consisting of an outer sheath of thick-walled cells, 

 surrounding a parenchymatous core ; the two tissues 

 are continuous with the sclerenchyma and parenchyma 

 respectively of the outer cortex. Very similar blunt 

 spines occur on the petioles of some recent Tree-ferns, 

 such as species of Alsophila. These emergences have 

 been of great service in the investigation of Lygino- 

 dendron, for, as they occur on the leaves (Fig. 139, e) 

 as well as on the stem, they rendered it possible to 

 identify these organs as belonging to the same plant, 

 before they had been found in actual continuity. 



Very young twigs, still in the bud condition, which 

 are sometimes met with, either detached or in connec- 

 tion with mature stems, are densely covered with the 



