66 THE OPHIOGLOSSALES 



cells also stain more or less strongly with the safranine, indicating that they also are 

 partially lignified. 



The junction of the second leaf trace with the central stele is shown in fig. 43, 

 E, F, G, and similar stages in the fusion of the first leaf trace are shown in fig. 

 43, H, I. The junction of one of the early roots, perhaps the third root, with the 

 central stele is shown in fig. 43, /. At the level where the second leaf trace departs 

 from the central stele the xylem ring is composed of about three rows of cells. In 

 both this second leaf trace and the third the endodermis is very conspicuous upon the 

 outer side of the bundle, but is not developed upon its inner face. As the trace 

 approaches the central cylinder it can be seen that the endodermis is interrupted 

 across the leaf gap, but as the leaf trace approaches, endodermal cells are developed 

 between the endodermis of the central cylinder and that of the leaf trace. This 

 union takes place first upon one side, the gap being closed sooner on one side than 

 the other, but later a similar process takes place on the other side of the leaf trace 

 and the closing of the gap is complete, the endodermis now being continued without 

 interruption around the whole of the circular section. The leaf trace where it joins 

 the central cylinder is exactly like it in size and structure; indeed, it is nothing more 

 than a sector of the hollow cylindrical stele, which is evidently built up exclusively 

 of these fused leaf traces. There is no vestige of any tissue in the stele which is not 

 referable to the bases of the leaf traces themselves. This becomes very evident when 

 the structure of the leaf trace is examined before it closes the gap where it joins the 

 central stele. 



An examinatioij- of the outer tissues of the older portions of the stem (fig. 42) 

 show that at certain points there is a conspicuous border of periderm (per), forming 

 a layer some four or five cells deep. This border of periderm occupies about a third - 

 of the circumference of the section and marks the position of an old leaf base which 

 has fallen away. According to Jeffrey, this periderm is an "abciss" layer and 

 results in the cutting off of the old leaf bases. He suggests that the layer of cork 

 cells developed from the periderm closes the scar left by the dead leaf and may be 

 efficacious in preventing infection from fungi in the soil. The presence of a periderm 

 in the outer cortex of the Ophioglossaceae was first pointed out by Russow (Russow 

 1), and was afterward confirmed by Holle (Holle 1). The periderm in B. virgin- 

 ianum is macje up of the usual radially arranged rows of cells (fig. 42, per). 



All of the tracheary tissue in the young sporophyte is composed of rather short, 

 reticulately marked tracheids. The secondary tracheids increase in size and the 

 thickened bars on their walls become broader, so that the larger elements have their 

 walls pitted rather than reticulately marked, and these pits, especially in the older 

 plant, are of the bordered type, not very unlike the round bordered pits found in 

 the wood of Conifers. 



The elongated thick-walled elements in the outer part of the phloem are 

 probably bast fibers. In the older part of the bundle these stain quite strongly with 

 safranine and show irregular pits upon their walls. The sieve tubes do not show at 

 all clearly in sections mounted in balsam. They are evidently large, but the char- 

 acteristic dotted sieve areas can not be made out and no special study was made of 

 these. Between the large secondary tracheids and the phloem are the narrow, thin- 

 walled cambium cells. These probably contribute only to the wood, and there is 

 no secondary phloem. 



