226 



Professor Williamson on some 



[Feb. 16, 



outline of a transverse section of a young branch less than an inch in 

 diameter. We have a cellular pith, a, enclosed within a vascular zone. 

 The latter consists of detached clusters of vessels at b, surrounded by 

 a regular exogenous cylinder, c, composed of thin, radiating, vascular 

 laminae, separated by large medullary rays. This zone was surrounded 

 by a cambium layer, through which it obtained additions to its 

 periphery until it became a large, exogenously-developed tree. Exter- 

 nally to the vascular zone was a thick bark, chiefly composed of two 

 layers g, h. Two other facts of morphological importance may be 



Fig. 7. 



Diagramatlc section of a branch of Lyginodendron Oldhamium. 



noticed. Four or five pairs of isolated vascular bundles, e, pass verti- 

 cally through the inner bark, close to the periphery of the vascular 

 zone, each pair of which eventually moves obliquely outwards to some 

 unknown, but most probably foliar, appendage. Upon the nature of 

 these appendages ten years of diligent search has failed to throw any 

 light. 



The second fact is connected with the vascular zone, c, when in its 

 very young state. The vascular bundles, h, the vessels of which were not 

 arranged in radiating order, then constituted an almost, if not absolutely 

 unbroken ring, and were surrounded by the equally uninterrupted 

 exogenous zone, c. But the general growth of the latter and of the 

 pith was not accompanied by any corresponding growth of this inter- 

 mediate vascular zone, h ; hence this became broken up into the 

 detached portions (Fig. 7 h) already mentioned. So long as the 

 exogenous cylinder, c, retained its integrity, it was surrounded by an 

 equally uninterrupted zone of cambium, the instrument of its exoge- 



