A NEW MEDULLOSEAN STEM. 



91 



Stele 4 (PL 10. fi^. 3 ; PI. 11. fig. 8) in Section A shows a very irregular type of 

 development. Smaller normal and larger inverse parts (a and /3) are present as before, 

 but at one side these are so closely in contact as to appear almost continuous, the lateral 

 break being less evident than in the case of stele 3. At the opposite side, however, the 

 break is marked (PL 10. fig. 3). The position and arrangement of the xylem elements 

 marked y suggest that they belong to /3, the inverse xylem ; the apparent separation 

 may be due to a break in cambial activity, such as mentioned by Dr. Scott for Medullosa 



glica (7, p. 90). At the side of the inversely developed xylem is a small group 



of obliquely running elements, possibly a leaf-trace. The crushed tissue external to 

 stele 4 shows no trace of structure; while between the normal and inverse parts, 

 which, as the diagram shows (PL 10. fig. 3), are very closely ia contact, the tissue 



is obliterated. 



In Section B (PL 11. fig. 7) stele 4 still shows the break in the development of the 

 inverse xylem. The inverse and normal parts show no distinct separation laterally, 

 the elements swinging round and appearing oblique and confused. 



Stele 5 (PL 10. fig. 3 ; PL 11. fig. 8) presents in Section A the same arrangement of 

 normally and inversely developed elements, the product of two independent cambiums. 

 The inverse group apparently consists of two parts, |3 and y; the continuity of the 

 medullary rays at one point, however, suggests that /3 and y are not really independent, 

 although the arrangement of the xylem elements shows evidences of a slight disturbance. 

 At the other side, however, there is a distinct break in the continuity of the medullary 

 rays and of the radial series of elements, owing perhaps to a change of direction of the 

 cambium at that point, or to a temporary cessation of cambial activity. At the left- 

 hand side, the inverse group shows a separating trace ; and at the right, further towards 

 the periphery of the stem, is a separate trace, of interest on account of a few well- 

 preserved cells in the crushed tissue accompanying it on its external border. These 

 cells have deUcate walls, and show evidences of having been arranged originally in a 



series. Their nature and position suggest that they are phloem-cells. Traces 

 of simikr cells occur in the crushed tissue in connection with the outer normal part of 

 stele 5. Kg partial pith is present between the closely-developed normal and mverse 

 parts of the stele. In section B, stele 5 (PL 11. fig. 7 ; PL 10. fig. 10) shows a considerable 

 amount of change in form and structure. The normal part is much reduced and, like 

 the inverse, shows discontinuity of development. The elements of both groups are more 

 or less oblique in section. The inverse part shows a large limb, possibly a trace, bcndmg 

 round towards the partial stele d, the direction of the elements being usually oblique. 

 On the inner edge of the limb, closely approximating partial stele d, the elements 

 run horizontally and show spiral thickenings on their walls. They are evidently the 



w 



protoxylem of the limb. 



Stele 6 which in Section A shows a structure very similar to that of stele 3 (PL 10. 

 fi- 4 • PI 11 fio". 8), is considerably changed in form at the level of Section B (PL 10. 

 fi^ 10 • PI 11 fi- 7). The inverse part shows the break in development as before, and 

 at'he left-hand Tide (towards stele 5) is a large, much broken-up limb or trace with 

 scattered sclerotic nests between the groups of xylem elements. This trace approaches 



radiatin^ 



