35] CHAPTER II. THE TISSUES. 149 



It will be remarked that the development of secondary vascular 

 tissue takes place in those plants the stems of which branch more 

 or less (e.g. an Oak), while it usually does not take place in those 

 plants the stems of which do not branch (e.g. the Palm), or do so 

 only slightly. It is obvious that, wlien the ste'm is of branching 

 habit, the number of leaves must increase year by year, whereas 

 when the stem does not branch the number of leaves does not vary 

 materially. Hence the whole matter may be summed up thus, 

 that the development of secondary vascular tissue in a stem is 

 directly correlated with an increase in the area of leaf -surf ace : as 

 in each year the leaf-surface of a tree increases in consequence of 

 repeated branching, so does the annual ring of secondary vascular 

 tissue become larger in circumference and possibly also of greater 

 thickness ; when, however, the tree begins to grow old, and its 

 branches, instead of increasing in number, begin to die off, then 

 the annual growth in thickness becomes arrested. Some further 

 explanation of this is given in Part III. 



B. The formation of Secondary Tegumentary and Cortical 

 Tissue. It is clear that the more or less considerable development 

 of secondary tissue in the interior of a young stem or root, must 

 have a very considerable effect on the primary cortex, and on the 

 primary tegumentary tissue. This effect will be one of pressure 

 and tension ; the radial growth of the stelar tissue will exert a 

 radial pressure upon the external tissues, while the tangential 

 growth of the stelar tissue will exert a tangential tension on the 

 external tissues. The radial pressure of so firm a structure as is 

 usually that of the secondary vascular tissue tends to cause more' 

 or less rapid obliteration of the softer cortical tissue ; whilst the 

 tangential tension stretches the cortical cells and tends to cause 

 them to grow tangentially, and to multiply by radial division. 

 According to the predominance of the radial pressure or of the 

 tangential tension, the primary cortex is either rapidly destroyed, 

 or it persists for a very considerable period. 



It may be stated generally that the epidermis and the primary 

 cortical tissue of herbaceous dicotyledonous stems keep pace by 

 growth with the formation of new tissue in the interior. This is- 

 true also of most woody shoots during the first year of their- 

 growth and in certain cases (e.g. Mistletoe, Holly, Acer striatuirij- 

 etc.) of woody shoots during their entire existence ; in some cases 

 (e.g. Euonymus) the epidermis persists and grows for several 

 years, but is at length disorganised. These primary tissues per- 



