i9 i4l HILL—ANATOMY OF LYCOPODIUM 67 



from the sides across the center (fig. 4). This gives the parallel- 

 banded type of stele, that is, alternating parallel bands of xylem 

 and phloem. In all of these situations there is a tendency for the 

 protoxylem to grow around the circumference of the cylinder 

 (figs. 8, 9), with very irregular behavior of the groups, consisting 

 of fusions and splitting, resulting in crescent-shaped masses and 

 even in masses completely encircling a central mass of the paren- 

 chyma and phloem, giving a type of stele resembling the amphivasal 

 type found in monocotyledons (fig. 5). 



The phloem as recognizable tissue is differentiated from the 

 parenchyma at a somewhat later period than the protoxylem. In 

 all of the protoxylem situations described above, the phloem arises 

 from the small parenchyma cells located on the same circumference 

 as the protoxylem, giving a true radial stele. 



The study of the strobilus in transverse serial sections shows 

 three types of stele: radial, parallel-banded, and amphivasal, the 

 two latter conditions being derived from the first by modifications 

 and specialization. These changes are not connected with branch- 

 ing. They are the result partly of the behavior of the protoxylem 

 groups, but chiefly of the growth and development of the metaxy- 

 lem. The young stele in the apex is organized at first with a distinct 

 radial arrangement, the protoxylem points, 2 or 3 in number, 

 alternating with phloem groups on the same circumference (fig. 3). 

 Lower in the stem, the sections show the protoxylem extending 

 around the periphery of the cylinder, with the developing metaxy- 

 lem organized as two parallel bands, with the phloem groups at 

 each end, extending inward and forming a central band of phloem 

 (fig. 4). At a slightly lower level in the young strobilus, the 

 amphivasal arrangement is found (fig. 5). This is a further modi- 

 fication of the parallel-banded arrangement. It is formed by the 

 continued peripheral growth of the xylem until a complete ring 

 is formed inclosing the phloem. This ring remains intact only a 

 short distance in the stem. At a lower level it appears broken up 

 into the parallel-banded types, and this condition still lower passes 

 into the distinctly radial arrangement. Since the parallel-banded 

 and radial arrangements are found both above and below the 

 amphivasal, the latter passing into the parallel-banded, and that 



