THE YOUNG SPOROPHYTE 77 



In fig. 48, D, there is shown an insertion of a root upon the stele and its very 

 broadly expanded base with the irregular conjunctive tracheids. This very broad 

 base of insertion and the obvious continuity of the tracheary tissue of the root with 

 that of the central stele of the stem suggest that the ventral part of the stele might 

 be made up, in part at least, of root traces. 



Cross-sections of older plants, such as that shown in the series indicated in 

 fig. 51, show essentially the same arrangement of the parts as do the younger ones. 

 At the apex there may be seen in section the three youngest leaves, which we will 

 designate respectively i, 2, and 3, No. i being the oldest leaf shown. In this section 

 the oldest leaf shows two vascular bundles in the petiole, and this is true also of the 

 second leaf, which is contained within the stipular sheath of the oldest leaf. The 

 lamina of leaf No. i already shows the characteristic ternate form. The young 

 leaves make an angle of about 30° with each other. Section B passes directly through 

 the stem apex and shows the sections of the basal part of the sheath of the two older 

 leaves. The stipular sheath of the youngest leaf partially incloses the stem apex, 

 and surrounding the apical region is a loose mass of cells, derived partly from the 

 tissue immediately around the stem apex and partly from the edges of the sheath 

 of the youngest leaves. These cells are really sections of hairs and scales which 

 perhaps secrete mucilaginous matter, but there was no evidence of this in the sec- 

 tions. Turned toward the dorsal side of the section may be seen the leaf trace from 

 the second leaf and separated from it by a considerable space on the ventral side 

 is a rather vaguely defined mass of young procambium, which marks the trace of 

 the youngest leaf. On the ventral side of the latter is the stem apex itself. Further 

 down, as has already been described for the younger stem, the two leaf traces approach . 

 At the level of the stem apex the trace of the second leaf is clearly defined and 

 nearly oval in outline, but more convex upon the dorsal side. The bundle is inclined 

 toward the side of the stem opposite the insertion of the first leaf. Further down 

 the trace becomes much broader, this being mainly due to the development of the 

 tissue upon one side, which begins to bend downward toward the ventral side of the 

 stem. The trace of the youngest leaf is now somewhat better defined and appears 

 somewhat kidney-shaped in section, the convexity, like that of the second leaf trace, 

 being turned toward the dorsal side. This trace also begins to show the extension 

 of tissue toward the ventral side of the stem and this is developed on the side opposite 

 to the extension in the next older leaf trace. Ultimately these ventral extensions of 

 the two bundles meet as the two leaf traces come nearer together, and the dorsal 

 ends also finally come into contact, so that the two bundles, the section of each of 

 which is approximately semicircular, form a complete ring inclosing a greater or 

 less amount of the ground tissue, which thus forms the pith of the hollow stele. The 

 stele resulting from the union of the two leaf traces is not at first perfectly circular 

 in section, but shows plainly for a long time that it is composed of two separate 

 bundles (fig. 51, F). 



The first development of permanent tissue in the older leaf trace is evident 

 before it joins the younger one. The first elements are thick-walled cells (which 

 may be called protophloem) in the outer zone of the phloem, and a little later at the 

 inner limit of the bundle a small group of protoxylem elements appears. Very soon 

 after this a similar differentiation takes place in the younger leaf trace, so that when 

 the two leaf traces are completely fused the protoxylems have the appearance of 

 having arisen toward the central part of the bundle, but in reality the component 

 bundles, at least at their earlier stage of development, may be described as endarch 

 and agree, therefore, with the bundles of the other Ophioglossaceae. In the leaf 

 traces of the older part of the stem, however, there may generally be found inside 



