412 



STUDIES IN FOSSIL BOTANY 



secondary wood attained a considerable thickness, and 

 is divided up into distinct groups, corresponding to the 

 peripheral primary strands, and separated from one 

 another by the principal medullary rays, while numerous 

 secondary rays are also present. The bordered pits 



S| -ffV.fi 



Fig. 158. — Hctcra>igium tiliacoidcs. Radial section through part of secondary wood and 

 phloem. -V-, secondary wood ; //•', fully developed pitted tracheides ; tr, tracheides 

 with pits beginning to form ; cb, cambium ; />/fi, secondary phloem ; sv, sieve-tubes ; 

 sv.J>, sieve-plate ; r, phloem-ray. X 112. Phil. Trans., W. and S. Will. 

 Coll. 162S. 



on the tracheal walls are exquisitely preserved, and 

 in favourable cases their development can even be 

 traced, in the elements bordering on the cambium 

 (Fig. 158, tr). The cambium itself is also perfectly 

 preserved, but the special interest of the specimen 

 centres in the phloem, which is perhaps better shown 



