SECONDARY TISSUES. CORK FORMATION 299 



the cylinder. This layer forms a wavy line as seen 

 in cross section, sweeping out round the primary 

 xylem and in round each primary phloem (Fig. 49, E, c). 

 Opposite the primary xylems it sometimes forms nothing 

 but parenchymatous tissue (secondary rays), but in 

 other species it forms xylem to the inside and phloem 

 to the outside, just as it does opposite the primary 

 phloems. Owing to its greater activity at first opposite 

 the primary phloems and to the formation of hard 

 masses of secondary xylem in these regions the cambium 

 soon straightens out and becomes circular in cross- 

 section. In the root of a tree it continues its activity 

 year by year, and eventually forms a' bulky cylinder 

 of secondary wood, and a much thinner cylinder of 

 secondary phloem. 



(3) Cork Formation. — Another secondary meristem 

 besides the cambium also arises in woody roots, usually 

 in the pericycle. This is calledUhe cork cambium or 

 phellogen, because it cuts off cells to the outside, certain 

 layers of whose walls are formed of suberin. These are 

 called cork cells, and owing to the impermeability of the 

 corky walls to water and solutes, the cortex and endo- 

 dermis are cut off from any functional connexion with 

 the vascular cylinder and soon scale off. This formation 

 of " bark " on woody roots is quite parallel to that which 

 takes place on tree-trunks. On the inside the phellogen 

 often forms parenchymatous tissue, which is sometimes 

 called the " secondary cortex." 



Modified Roots. — In some species the roots, or some 

 of them, depart more or less from the typical root 

 structure and behaviour. In some plants (climbers 

 and epiphytes) they may turn green and carry on 

 photosynthesis for the plant. Later they may grow 

 down to and enter the soil, then assuming typical 



