ANOMALOUS EXOGENOUS STEMS. 47 



the base of the tree. They are broken up by the intervention of woody 

 fibres, as seen in a vertical section of a woody stem (fig. 103), perpen- 

 dicular to the medullary rays m r, m r, m r, which are separated by 

 interlacing woody fibres, 1 1. The medullary rays are usually continu- 

 ous from the pith to the 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. 100, r m'", r m", r m""). Medul- 

 lary 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. 



90. 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 sometimes 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 simi- 

 lar kind annually, resembling in this respect the formation of woody 

 bundles in the early growth of herbaceous plants (fig. 95). In the 

 Pepper tribe, Aristolochiacea?, and Menispermaceas, these anomalous 

 stems occur. In Gnetum (fig. 104), the vascular bundles, b b b b b b b, 

 form zones, which are each the produce of several years' growth, and 

 are separated by layers, 1 1 1 1 1 1, which may be considered as repre- 

 senting different zones of liber. 



In some of the Menisper- 

 mum tribe, the separating lay- 

 ers are of a cellular and not 

 of a fibrous nature. In Banis- 

 teria nigrescens (fig. 105), the 

 young stem (1) presents a 

 four-lobed surface; the lobes 

 gradually deepen (2), and ulti- 

 mately (3) the stem is divided 

 into a number of separate por- 

 tions, the central one of which 

 alone exhibits pith and medul- 

 lary rays. The portions are 

 separated by interposed cortical layers. 



Many of the Malpighiacea?, Sapindaceas, and Bignoniacese of Brazil, 

 exhibit stems in which the woody layers are arranged in a very irregu- 

 lar manner. In the stem of Calycanthus floridus, and of some Bra- 

 Fig. 104. Horizontal section of stem of Gnetum. m, Pith, e m, Medullary sheath, b b b 

 6666, Woody bundles forming seven concentric zones, each of which is the produce of several 

 years. I II II ?, Fibres of liber forming interposed circles, equal in number to the woody zones. 



