﻿THE 
  STEM 
  AND 
  THE 
  LEAF 
  

  

  57 
  

  

  all 
  the 
  year 
  round, 
  and 
  in 
  these 
  photosynthesis 
  is 
  carried 
  on 
  

   by 
  the 
  green 
  layer 
  of 
  the 
  cortex 
  beneath 
  the 
  epidermis 
  of 
  

   the 
  stem. 
  

  

  In 
  many 
  dry 
  

   countries 
  there 
  

   are 
  switch 
  plants, 
  

   which 
  either 
  bear 
  

   a 
  few 
  little 
  leaves 
  

   during 
  a 
  small 
  

   part 
  of 
  the 
  year 
  

   or 
  are 
  entirely 
  

   leafless, 
  as 
  is 
  

   shown 
  in 
  figure 
  

   37. 
  Like 
  the 
  cacti, 
  

   such 
  plants, 
  and 
  

   others 
  with 
  flat- 
  

   tened 
  stems 
  and 
  

   little 
  leaf 
  sur- 
  

   face 
  (fig. 
  38), 
  do 
  

   the 
  greater 
  part 
  

   or 
  all 
  of 
  their 
  

   photosynthetic 
  

   food-making 
  in 
  

   the 
  green 
  cells 
  

   of 
  the 
  cortex 
  of 
  

   the 
  stem. 
  

  

  Many 
  of 
  our 
  

   common 
  forest 
  

   trees 
  and 
  shrubs 
  

   do 
  food-making 
  

   in 
  the 
  green 
  layer 
  

   of 
  the 
  bark 
  of 
  

   the 
  young 
  twigs, 
  

   until 
  this 
  is 
  shut 
  

   away 
  from 
  the 
  

   light 
  by 
  layers 
  /, 
  flowers 
  ; 
  I, 
  leaves 
  ; 
  s, 
  steins. 
  One 
  half 
  natural 
  size 
  

  

  FIG. 
  38. 
  Branches 
  of 
  Muehlenbeckia, 
  a 
  plant 
  with 
  

  

  scanty 
  leaf 
  surface 
  and 
  flattened 
  stems 
  which 
  do 
  

  

  most 
  of 
  the 
  photosynthetic 
  work 
  of 
  the 
  plant 
  

  

  