THE STEM OF VASCULAR PLANTS. 165 



by degrees the tree increases in size, especially above 

 the cord, so that it appears as if it were buried in the 

 tissue. It is in this manner that climbing shrubs often 

 kill the trees around which they twine, as may be seen 

 even in our climate ; as, for example, in the Periploca 

 Grcsca, or Wisteria frutescens. We shall see, in speak- 

 ing of Endogens, that similar phenomena with them are 

 impossible. 



§ 4.— Of the Medullary Rays. 



If the woody stem of an Exogen be cut transversely, 

 we remark lines springing from the pith and radiating 

 to the bark, like the lines on a dial, or the spokes of a 

 wheel. Grew, who first observed them, called them 

 Medullary Insertions ; they have since been called 

 Medullary Rays (radii meduUares ; productions, pro- 

 longemens, or rayons medullaires ) ; this term is more 

 applicable, because it describes their position without 

 affirming their origin. Between the complete rays are 

 perceived some half ones, which arise from the centre, 

 and appear to stop before they reach the circumference; 

 some authors call them Medullary Appendages 

 (appendices medullaires). Most frequently we see 

 some rays which do not arise from the centre, but from 

 one of the medullary zones of which each annual laver is 

 formed. It results from this production of medullary 

 rays from each of the annual zones, that their number 

 is much larger in the layers of the circumference than in 

 those of the centre. The medullary rays are not simple 

 processes, but radiating and interrupted vertical lamina, 

 directed towards the circumference ; we can be assured 

 of this by making a vertical or oblique section. We 

 are thus enabled to follow them through more or less of 



