170 THE ELEMENTS OF VEGETABLE HISTOLOGY 



primary to secondary structure may be summarized 

 in the following statements: 



1. The epidermal and hypodermal tissues are replaced by 



elements originating from a phellogen of bark cambium, 

 which develops within the primary cortex. 



2. The endodermis is replaced by tissues developed from the 



cambium. 



3. The original complete nbro-vascular bundles increase in 



size through additions of tissues by the cambium, and 

 new bundles are formed in the broad primary medullary 

 rays. 



4. The cambium, originally in the form of arcs or segments 



between the xylem and phloem bundles, extends laterally 

 and forms a complete ring or circle. 



5. The pith is greatly reduced in size through extension 



of xylem elements and the formation of secondary 

 bundles. 



Owing to the thickness and complexity of the bark 

 structures formed by the phellogen, it is more con- 

 venient to consider these separately from the woody 

 tissues originating from the cambium. However, it 

 must be kept in mind that both bark and wood are 

 parts of the secondary stem. 



BARK STRUCTURE 



From the histological standpoint, the bark of a 

 secondary stem includes all structures external to 

 the cambium. According to this statement it will 

 be apparent that the phloem elements are considered 

 with the bark tissues, rather than with the woody 

 structures. True bark is only present in mature 

 dicotyledonous plants, although monocotyledons pos- 

 sess a thickened epidermis which closely approxi- 

 mates bark in function and appearance. 



