60 



ANOMALOUS EXOGENOUS STEMS. 



broken up by the intervention of woody fibres, as seen in a vertical 

 section of a woody stem (fig. 121), tangentially to the medullary 

 rays m )•, m r, m r, which are separated by similar interlacing fibres, 

 I I. The medullary rays are usually continuous from the pith to the 





1 1 'nt.r 



Pig. 121. 



bark, additions being made to them as they proceed outwards. But, 

 occasionally, secondary rays arise from the outer cells, which pass 

 only to a certain depth between the vascular bundles, as in the Cork- 

 oak (fig. 118, r ro," r m""). Medullary rays are conspicuous in the 

 Cork-oak, Hazel, Beech, Ivy, Clematis, Vine. They are not so well 

 marked in the Lime, Chestnut, Birch, Yew. 



Anomalies in the Structure of the Exogenous Stem. 



The stems of Dicotyledonous plants occasionally present anomalous 

 appearances in the structure and arrangement of their wood, bark, 

 and medullary rays. In place of concentric circles there are some- 

 times only a few rows of wedge-shaped vascular bundles produced 

 during the life of the plant, additions being made by the interposition 

 of bundles of a similar kind annually, resembling in this respect the 

 formation of woody bundles in the early growth of herbaceous plants 

 (fig. 112). In the Pepper tribe, Aristolochiacese, and Menisper- 

 maceae, these anomalous stems occur. In Gnetum (fig. 122), the 



Fig. 120. Vertical section of a one-year old iDranch of Acer campestre, highly magnified, 

 and extending from the ^ith to the bark, parallel to the medullary rays, m r, A medullaiy 

 ray or plate extending from the pith, p, to the bark, c e, crossing tracheaa, (, fibres of 

 xylera or wood, I, pitted vessels, 6, and cortical fibres, cf. Fig. 121, Vertical section of the 

 same branch at right angles to medullary rays. 1 1, fibres of wood (xylem) which interlace, 

 leaving spaces, m r, m r, m r, where the medullary rays pass. 



