54 SECONDARY INCREASE IN THICKNESS 



where the landmarks of the primary xylem and phloem can 

 still be made out. 



The purpose of the radial arrangement of the primary xylem 

 and phloem of roots appears to be to allow the water absorbed 

 by the root hairs to pass into the xylem highways without first 

 traversing the food highways in the phloem. The changes 

 brought about by secondary thickening would not interfere 

 with this purpose because they occur in older parts of the 

 root where the root hairs have already died away. 



Increase in the Cortex. — The growth of the vascular bun- 

 dles subjects the primary cortex to a good deal of tangential 

 tension and its thin-walled parenchyma cells commonly undergo 

 enough increase by radial division to keep from breaking apart, 

 but increase in thickness of the primary cortex takes place in 

 woody perennials and in the underground parts of herbaceous 

 perennials by the formation and continued activity of a zone of 

 cork cambium or phellogen. This takes its origin in the tangential 

 division of the epidermis or in a similar division of the cells 

 immediately beneath the epidermis (compare Figs. 23 and 24). 

 When the origin is in the epidermis the inner cell cut off by the 

 tangential wall takes part in the formation of the cork cambium, 

 while the outer cell enlarges and remains epidermal. If the origin 

 is in the cell layer just beneath the epidermis, it is the outer 

 row of daughter cells that becomes cork cambium. The for- 

 mation of the cork cambium commonly takes place before the 

 end of the first year's growth, and by frequent tangential as well 

 as radial divisions it soon gives rise to layers of cork cells toward 

 the outside, and frequently to thin-walled parenchyma cells 

 toward the inside, called the phelloderm. These new tissues, 

 including the cork cambium which forms them, are termed 

 the periderm. Since the walls of the cork cells are suberized 

 and in uninterrupted union, the passage of water and gases is 

 prevented. The epidermis being thus shut off from the water 

 supply from within soon dies and gradually sloughs away; but 

 provision is made for the aeration of the tissues lying within the 

 cork zone by the production of a loose mass of cells which inter- 



