35.] CHAPTER 11. THE TISSUES. 207 



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 IV, 



B. The formation of Secondary Extra-stelar Tissue. It is clear 

 that the more or less considerable development of secondary stelar 

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

 considerable effect on the primary extra-stelar ground-tissue (cor- 

 tex), 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 tan- 

 gential growth of the stelar tissue will exert a tangential tension 

 on the external tissues (see p. 202). 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 should, however, be pointed out that the development of 

 secondary extra-stelar tissue does sometimes occur in members in 

 which no development of secondary stelar tissue takes place, as in 

 the roots of some Pteridophyta (Marattiaceae, some Ophioglosseae), 

 and of some Monocotyledons (aerial roots of Philodendron, etc. ; 

 terrestrial roots of Iris, etc.). 



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 stelar 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 

 striatum, 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 

 persist also in some roots (e.g. Faba vulyaris, Alcliemilia rulgaris, 

 Gaillardi'i aristata) in which the development of secondary vas- 

 cular tissue is not very active. The extension of the tissues is 

 effected by tangential growth and radial division of the cells. 



