460 SECOND A Ry CHANGES. 



the bast-zone continues to be divided into main sections by the large medullary rays, 

 \vhich at the bounilary of the cambium are continuously increased by equivalent 

 elements, and here for the time being show the same breadth as in the wood ; each 

 of these main sections is divided into partial sections by secondary, successively 

 smaller medullar}- rays, which are a prolongation of those of the wood. The equiva- 

 lent rays and sections of the wood and bast fit one on another at the cambial 

 boundary ; the younger rays, which in the wood penetrate less deeply towards the 

 pith, penetrate to a lesser distance outwards in the bast ; the wedge-like widening of 

 the portions of the strands is necessarily in the opposite direction in the bast to that 

 in the wood, as seen in cross-section. From this distribution, slight deviations, still 

 to be included under the norm.al type, occur here and there. As such are to be 

 mentioned — 



(a) The discontinuous viedullary rays of Hartig^. In Fagus and in exotic 

 woods certain medullary rays in the wood do not extend to the border of the cam- 

 bium, but end externally within the ligneous strand. It remains, however, to be 

 investigated, whether this phenomenon, which appears on examining cross-sections 

 through medullary rays of no great height, always depends on an actual termination 

 of the ray, and a formation of bundle-elements at its exterior limit, or whether it may 

 not perhaps be due to a vertical upward or downward curvature of the ray, in con- 

 sequence of which its radial prolongation comes to lie in a different surface of cross- 

 section from that through which the knife has passed, and in which the inner 

 portion lies. 



{b) The medullary spot, characteristic of many woods, which will be described 

 below. 



(c) The appearance of secondary intermediate hmdles arising from the cambium, 

 inside the older medullary rays. This phenomenon occurs in the internodes of Atra- 

 gene alpina in a form which quite agrees with the normal processes, especially in the 

 allied species of Clematis. In the specimens investigated the internodes in their 

 first year all showed the six leaf-trace bundles only, distributed as in Clematis (p. 244), 

 separated by six large medullary rays, and, like the latter, furnished with secondary 

 growth by means of a zone of cambium extending all round the stem. In some of 

 the older internodes, at least two years old, this structure is permanently maintained, 

 while the secondary growth continues ; they thus correspond to the type given above 

 under i. In others, however, intermediate bundles appear, in or after the second 

 year, and in fact, in the most regular case, one appears on each side of each median 

 bundle of the trace, so that the total number of the bundles now amounts to ten. In 

 many cases one, two, or three of the four intermediate bundles are absent, and the total 

 number of bundles is accordingly nine, eight, or seven. The longitudinal course of the 

 intermediate bundles is that given in the case of Clematis ; in one case only two of 

 them occurred between two trace-bundles, in a short internode ; they anastomosed with 

 each other irregularly in their undulating course. The structure and subsequent 

 growth in thickness of the intermediate bundles are similar to those of the six trace- 

 bundles. 



Clematis Vitalba frequently shows a similar phenomenon, so that in the twelve 



* Botan. Zeitg. i?^59, p. 94. 



