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RELATIONSHIPS. 



from within outwards. One rather significant structural variation remains to 

 be noted. In Macrozamia {cf. fig. in) and Bowenia a tertiary cambium occa- 

 sionally arises in the parenchyma between the normal and first anomalous or 

 between any two of the anomalous vascular zones, and forms wood and bast with 

 partly or wholly inverted orientation, that is, with the xylem more or less exactly 

 adjacent to that of the next outer bundle zone. It has been suggested (205) that 

 these inverted bundles may indicate a possible reversion in the direction of some 

 ancestral Medullosan type with a cylinder of concentric bundles only, whence as 

 the result of the functional weakening and final elimination of the inner meristem 

 of such bundles there might arise the stronger and more compact existing type of 

 cycadean stem with common collateral bundles. 



Fig. 1 1 2.— Cycas revoluta. From De Bary. 



I. Transverse section through summit of young trunk. Natural size, a. Leaf insertion; g, girdle leaf 



trace; r, xylem zone. 



II. Slightly enlarged thick transverse section cut just below the punctum vegetationis of a lateral shoot and 



cleared with potash for acroscopic view. Leaf-trace pairs of superposed planes drawn together in 

 plane of paper and numbered at point of exit, beginning with the youngest I I } of nine successive 

 leaves. The course of the bundles is shown up to the point where each curves sharply into the leaf base. 

 Anastomosis of bundles in cortex but slightly developed. 



That some of these irregularities iu the alignment of the xylem and phloem may 

 be mainly the counterpart of a more perfect connection, aside from that afforded by 

 the leaf traces, between the several vascular zones of the stem is, however, apparent 

 in radial sections of Macrozamia figured by Worsdell (205). In these, broad bands 

 of wood and bast connect at intervals especially the normal and first anomalous 

 vascular zones in both the upward and the downward direction, both kinds of 

 connection occurring at times in close proximity, so as to form a complex kind of 

 loop. The frequent passage of leaf traces from the inner zone all the way across 

 the anomalous zones to the cortex is also noteworthy. 



In general, the cycad stem, being such a loosely compacted one, undergoes much 

 growth accommodation, its lower and older portions showing the most marked 

 crowding and distortion from the normal order of the several segmental series of 

 the vascular zones. So far as known the primary network of bundles is developed 

 in its full complexity close below but not at the apex of the stem, and the succes- 

 sive transverse portions of the growing stem undergo further increase in thickness 

 by expansion in the first place of pith and cortex, and later by the cambiogenetic 

 duplication of vascular zones just described. 



