1898-99.] | MORPHOLOGY OF THE CENTRAL CYLINDER IN THE ANGIOSPERMS. 625 
tuber passes over into the narrower cylindrical stem characteristic of the 
older plant, a single circular series of collateral bundles can still be 
observed. If the sections through this region be treated with sulphuric 
acid or with phloroglucin and hydrochloric acid, it may be demonstrated 
that the bundles are no longer ‘surrounded by individual phloeotermal 
sheaths, but that a single lignified external phlceoterma surrounds the 
whole complex of bundles. There is present in this species a state of 
affairs comparable to that already described as occurring in the young 
stelar system of Ranunculus rhomboideus, viz., a primitive so-called 
astelic condition followed by medullated monostely resulting from the 
degeneration of the internal phlceoterma. The gaps between the 
bundles in this stage may be compared with those subtending the 
subsidiary smaller leaf-traces of many Ranunculacee ; for in the case of 
the latter, the fundamental tissues of the pith and of the cortex do not 
communicate through the foliar gaps because these are so exiguous 
that they are filled up by the pericycle flanking the adjacent fibro- 
vascular segments. In Symplocarpus fetidus, the gaps are consequently 
patent in the thick tuberous base of the primary axis, but in its narrower 
subsequent cylindrical portion they are occluded by the encroachment 
of the pericycle on the reduced interval between the fibro-vascular 
strands. The foliar gaps are, nevertheless, morphologically indicated by 
the lacunz in the fibro-vascular cylinder, filled with pericycle, which 
subtend each and all of the leaf-traces, no matter how small. Not far 
above the region of transition already described, the central cylinder be- 
comes modified by the appearance of medullary strands and the fibro- 
vascular bundles become forthwith amphivasal. Photograph 12, plate 8, 
reproduces the appearance of the central cylinder in a section passing 
through the cylindrical axis about halfa millimetre above its junction 
with the basal tuber; ¢ is the external phlceoterma; a are the peripheral 
bundles, and @ are leaf-traces which have begun to run in the medulla in 
the characteristic monocotyledonous fashion. 
In Zea mais the epicotyledonary stele is a continuous cylinder of 
fibro-vascular tissue throughout the first internode, which is generally 
two or three centimetres in length. There is a single gap in the.side of 
the cylinder throughout the internode corresponding to the trace of the 
cotyledon or scutellum. Above the second node, the fibro-vascular tube 
is perforated by a large number of foliar gaps, and the leaf-traces cor- 
responding to these immediately pass into the medulla, and pursue 
henceforth the course so characteristic of the foliar strands in 
the monocotyledons. There is a well-marked external lignified 
phlceoterma and a less well-marked and somewhat interrupted internal 
