a) What is secondary growth? b) Describe the process of secondary thickening in a woody stem.

a)  What  is  secondary  growth? Type  of  growth  that  occurs  due  to  cambium  activity  in  woody  plants/stems;  resulting  in  an     increase  in  girth/width  of  plants;  2  mks b)  Describe  the  process  of  secondary  thickening  in  a  woody  stem Facilitated  by  meristematic  cells  (cambium)  located  between  the  phloem  and  the  xylem (intervascular  cambium);  it  divides  radially  to  form  cambium  tissues;  with  xylem  forming  the outer  ring/to  the  inside;  while  the  phloem  forming  the  outer  ring/to  the  outside;  division  of the  cambium  ring;  form  a  secondary  parenchyma;  hence  increases/forms  the  medullary  rays; other  xylems  (secondary  xylem)  are  formed;  hence  pushes  the  phloem  and  cambium  ring outwards;  this  creates  pressure  on  the  outer  cells;  resulting  in  stretching  and  eventual rupturing  of  epidermal  cells;  a  new  band/volume  of  cambium  cells  are  formed  in  the  cortex beneath  the  epidermis  (cork  cambium  cells/phellogen);  to  replace  these  ruptured  cells;  the phellogen  cells  divide  on  either  side;  where  the  inner  cells  become  the  secondary  cortex; while  those  produced  on  the  outside  become  cork  cells;  which  are  tightly  packed;  and become  coated  with  an  oily/waxy  water-proof  material/suberin;  further  multiplication  of  cork cells;  lead  to  formation  of  the  bark;  which  forms  a  protective  layer  (against  water  loss  and damage  by  organisms);  seasons  results  into  annual  rings;  some  cork  cells  form a  loose mass/lenticels  that  allow  gaseous  exchange  through  the  stem;

 Max.  18  mks 

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