THE YOUNG SPOROPHYTE 65 



these divides and gives off a branch which passes into the terminal lobe of the leaf 

 (see Jeffrey 1, page 23). 



In a section of the older stem, about where the fourth leaf trace becomes joined 

 to the fifth and sixth, the section of the stele appears circular. In the portion be- 

 longing to the fifth leaf trace there are present about half a dozen isolated tracheids, 

 arranged in a row, while in the sixth trace only a single small tracheid is developed. 

 The two xylems of the fourth leaf trace are now no longer distinguishable and the 

 wooily part of the trace has the form of an irregular row on the inner side of the 

 trace section, and is continuous with the xylems of the fourth and fifth traces. 

 liven after the fusion of these three traces is practically complete, it is still evident 

 that the ring-shaped section is composed of three parts, the break between the 

 xylems showing the limits of the three component leaf traces. With the increase in 

 the development of the xylem ring the limits of the individual leaf traces are finally 

 completely lost and the section of the stele shows a thick ring of the tracheary tissue 

 interrupted only at intervals by single rows of parenchyma cells in the medullary rays. 

 Outside the ring of tracheids there can be seen a zone of narrow cells arranged 

 in radially disposed rows. This zone constitutes a genuine cambium, precisely 

 similar in appearance to that found in the stems of Conifers and Dicotyledons. 

 This cambium has long been known to occur in the older sporophytes of the larger 

 species of Botrychium, and Jeffrey first noted it in the young sporophyte. The 

 cambium is well developed in the leaf trace for a considerable distance above its 

 junction with the central stele. 



The primary tracheary tissue is in immediate contact with the pith. Occa- 

 sionally the ring of tracheids is broken by a single parenchyma cell. Outside the 

 tracheary ring the cells are arranged in radial rows and in these the formation of 

 the walls is always periclinal, so that the cambium in these earlier stages presents a 

 very characteristic appearance. The rows of cambium cells continuous with the 

 primary parenchyma cells lying between the tracheids of the primary row do not 

 develop tracheary tissue, but remain parenchymatous and thus give rise to the 

 medullary rays, which later are so conspicuous. 



The extreme outer portion of the vascular cylinder is composed of a ring of 

 thick-walled cells which are probably bast fibers, but the walls of these do not stain 

 strongly with safranine and are hence much less conspicuous than the tracheids. 

 These may possibly be considered as protophloem elements and evidently corre- 

 spond to the zone of thick-walled cells figured by Poirault for B. lunana (Poirault 

 2, fig. 1 1). fust outside of this ring of thick-walled cells lies the endodermis, which 

 has not as yet developed the characteristic radial thickenings which aie so easily 

 seen in the older parts of the stele. These thick-walled phloem elements, as is the 

 case with the protoxylems, do not form an unbroken ring, bur are interrupted at the 

 junctions of the confluent leaf traces (fig. 44, C). 



In the older bundle, a zone of large cells, the young sieve tubes may be recog- 

 nized lying inside the ring of bast fibers. Inside of the zone of sieve tubes lies 

 the cambium, the cells of which gradually pass into the xylem, to which constant 

 but slow additions are made from the cambium; indeed, Jeffrey thinks that we can 

 not speak of primary wood, lie considers that all of the wood owes its origin 

 to the activity of the cambium ring. The radial rows of secondarj wood are inter- 

 rupted at intervals by the medullary rays, ami the resemblance of the stele at this 

 stage to the section of a young coniferous stem is really quite remarkable. I he 

 endodermis has now become exceedingly conspicuous, as the lignificd lateral walls 

 stain very strongly with safranine and stand out strongly in contrast with the cells 

 of the cortex lying outside the endodermis. The inner walls of the endodermal 

 5 



