PRIMARY STRUCTURE OF CERTAIN PALAEOZOIC STEMS. 



335 



correspond roughly to a 2/5 between A and B, and B and C, and about 1/5 between 

 C and A. Where a new trace, D, makes its appearance (diagram 3) it bisects the 2/5 

 gap A — B, and again where a fifth trace, E, appears (diagram 4) it bisects in like 

 manner the other 2/5 gap, B — C. It may be mentioned that all the leaf- traces shown 

 in the other sections, not figured, are likewise referable to the same five orthostichies, 

 A, B, C, D, E, and follow the same order of succession. No other than a 2/5 arrange- 

 ment would account for the facts. The irregularities in divergence which occur are 

 easily explained by the distortion due to cracks, and to the contraction of the pith. 



The course of the smaller strands, i.e., of the lower ends of the leaf- traces where 

 they have become medullary, has not been completely made out, but some light has 



C 



Diagram 2 (K. 628). 



been thrown on it. Thus the entering leaf-trace A, after it has become embedded in 

 the pith and has diminished much in size, obviously united with the adjoining bundle 

 (a) on the kathodic side (diagrams 3 and 4). The arrangement of the strands indicates 

 that this was a general rule. The last leaf-trace to enter, above A, would have lain on 

 the orthostichy E. In this position we see, in diagram 1, two small bundles (e) which may 

 well be the reduced leaf-trace with its reparatory strand. Lower down (diagram 3) these 

 two bundles are fusing, and in the lowest section (diagram 4) they are completely fused. 



The leaf-trace still further above would have been on the orthostichy D. Two 

 small bundles in this position (d) are already uniting in the uppermost section (diagram 

 l), and in the next below (diagram 2) their union is complete. 



Considering next the leaf-traces which enter below A, the small strand b on the 



