THE OLDER SPOROPHVTE 



167 



For a long time sections of the stem show this single central bundle of crescentic 

 form, at first derived from the coalescence of the third, fourth, and fifth leaf traces, 

 but continued upward in the same form and added to by the addition of the traces 

 from the subsequent leaves. This crescentic stele, which, for convenience may 

 be spoken of as the stele of the stem, is entirely of foliar origin. The crescent nevei 

 becomes completely closed and its opening in the earlier stages of development can 

 not be properly called a foliar gap. The parenchyma which is inclosed within its 

 curve belongs from the first to the ground tissue and is not part of the stele. Some 

 of the surrounding cells show traces of the typical endodermal markings and it is 

 perhaps safe to say that the stele is bounded by an endodermis, as Farmer states is 

 the case in Angwpteris and Brebner in D. simplicifolia. The limits of the endo- 

 dermis, however, especially upon the concave side of the stele, are very vague. The 

 stele, after the complete fusion of the three leaf traces, may perhaps best be described 



Fig. 151. 



Four longitudinal sections of a young sporophyte of Darurn elliptica. X 18. The fourth leaf, / 4 , has the stipules well developed; 

 r 3 , sections of the third root, m, mucilage duct; it, stipules. 



as concentric in structure, with phloem developed all around the xylem, but there 

 are probably traces of phloem also between the three xylems which represent the 

 three confluent leaf traces. 



In the older portions of the stem the bundles become still more completely 

 fused and the compound bundle is oval in outline, but still shows plainly the three 

 xylems of its constituent leaf traces (fig. 150, G). In this region the endodermis 

 is rather better developed than it is higher up, but its limits are still rather vague. 

 At this level the traces of the leaves, I and 2, are still free, but have approached 

 nearer to the central bundle than is the case higher up. 



Still lower down, the trace from the second leaf joins the bundle formed from 

 the later leaf traces, which no longer clearly shows the separate xylems, but ap- 

 proaches the condition which has been described as "protostelic," or "haplostelic," 

 usintr Brebner's terminology. The xylem elements, however, do not form a solid 



