vni. ] OF THE STEM. 1OZ 



side, coalesce and thus form one continuous cylinder of 

 multiplying and enlarging cells around a portion of the 

 undirferentiated tissue, which thus becomes isolated in the 

 centre of the stem as the pith, while wholly external to 

 the radiating vascular bundles an outer belt of the same 

 tissue persists as a cellular investment which, together 

 with the collective phlceoid system of the confluent vas- 

 cular bundles, forms what is popularly termed the bark. 

 The consequence of this lateral confluence of the vascular 

 bundles is, that in Dicotyledons all the wood is on the 

 inside of this cambium-cylinder, and all new wood is 

 deposited on the outside of wood previously formed : all 

 the liber and lattice-tubes, on the other hand, are on the 

 outside of the cambium, and immediately within the 

 original cellular investment. Structure such as here 

 described is precisely what we find in the stem of an Oak 

 or any other of our native trees, which are said to be 

 exogenous, from the circumstance that their wood increases 

 by additions to its outside. 



In Monocotyledons, on the other hand, the cambium- 

 cells of the different fibro-vascular bundles never coalesce 



FIG. 71. Diagram showing the arrangement of the fibro-vascular bandies in 

 a cross-section of the stem of a Monocotyledon. v b fibre-vascular 

 bundle?, c t cellular tissue. 



so as to form a cambium-cylinder, but remain permanently 

 scattered through the primitive cellular tissue of the stem ; 

 consequently they do not form continuous rings of wood. 

 The cambium-cells, therefore, soon cease their dividing 

 and enlarging work, and the fibro-vascular bundle is 

 finished. These bundles are especially crowded towards 

 the circumference* and as they are each sheathed, espe- 



