THE FOSSIL OSMUNDACE^E. 657 



trace is surrounded by a well-defined endodermis, for the most part recognisable as a 

 single layer of cells (figs. 26 and 37, en.). 



Perhaps the most striking and interesting feature of the petiole is the presence in 

 the pericycle of secretory sacs, which no doubt represent the mucilage sacs that occur 

 in the same tissue in the living Osmundacese. In the fossil these elements are large 

 vesicular cells, elongated in longitudinal section, and filled with black contents. Large 

 numbers of them are present in the pericycle on both sides of the trace (figs. 36, 37, and 

 38, S. s.). On the abaxial side, however, they are confined to the median region, and 

 in this Thamnopteris differs from the living Osmundacese, where the abaxial mucilage 

 sacs form two dorso-lateral groups. In the living Osmundacese these sacs do not 

 appear at all until near the top of the stipular region of the leaf-base, but in 

 Thamnopteris they are already present by the time that the trace is in the sclerotic 

 cortex of the stem (fig. 29). 



The Structure of the Roots. 



The xylem strands of the roots are inserted upon those of the leaf-traces, but they 

 vary considerably as regards the point of their insertion. They arise singly or in pairs, 

 sometimes from the back of the leaf-trace xylem before the latter is yet free from the 

 xylem of the stem (figs. 7, 16, 17, and 30), sometimes from the free leaf-trace in the 

 inner cortex. No roots, however, were seen to arise from the leaf-traces once the 

 parenchyma bay has been formed in the xylem. In the free leaf-trace the roots 

 arise from the back or from the sides of the xylem, and in the latter case from 

 the abaxial or even from the adaxial corner (fig. 39). The roots obtain a cortex of 

 their own as they enter the sclerotic cortex of the stem. The root cortex consists of 

 an inner thin-walled and an outer sclerotic zone, which remains still thick-walled even 

 in the outermost regions of our sections. They bore their way in all directions through 

 the stipular tissues of the petiolar coating, but they never perforate the sclerotic rings 

 (fig. 1, v.). They have the structure of a typical diarch fern root with a rather stout 

 xylem strand, and without further description are sufficiently illustrated by fig. 40. 

 The xylem elements bear multiseriate pits similar to those in the stem stele. 



Locality. — Near Bjelebei, government of Orenburg, Russia. 



Horizon. — Upper Permian, " gres cuivreux " (see P 2 of Geological Table, Part II., 

 p. 219). 



