﻿THE 
  STEM 
  AND 
  THE 
  LEAF 
  

  

  81 
  

  

  79. 
  Structure 
  of 
  leaves 
  in 
  relation 
  to 
  water 
  supply. 
  Leaves 
  

   of 
  plants 
  which 
  grow 
  in 
  very 
  moist 
  earth 
  usually 
  have 
  a 
  

   moderately 
  thin 
  epidermis 
  l 
  and 
  are 
  of 
  a 
  rather 
  loose 
  interior 
  

   structure, 
  with 
  abundant 
  air 
  spaces 
  - 
  - 
  even 
  more 
  than 
  are 
  

   shown 
  in 
  figure 
  26. 
  

  

  The 
  leaves 
  of 
  

   plants 
  which 
  usu- 
  

   ally 
  grow 
  in 
  places 
  

   where 
  they 
  are 
  some- 
  

   times 
  in 
  great 
  dan- 
  

   ger 
  of 
  dying 
  for 
  lack 
  

   of 
  water 
  commonly 
  

   have 
  a 
  thick, 
  nearly 
  

   moisture-proof 
  epi- 
  

   dermis 
  and 
  closely 
  

   packed 
  cells 
  in 
  the 
  

   interior 
  of 
  the 
  leaf. 
  

   This 
  insures 
  slow 
  

   evaporation 
  of 
  what- 
  

   ever 
  water 
  is 
  car- 
  

   ried 
  into 
  the 
  leaf, 
  

   since 
  the 
  thick 
  

   epidermis 
  prevents 
  

   rapid 
  drying 
  up 
  of 
  

   the 
  cells 
  near 
  the 
  

   exterior, 
  and 
  the 
  

   scanty 
  air 
  spaces 
  

   render 
  the 
  circula- 
  

   tion 
  of 
  dry 
  air 
  com- 
  

   ing 
  into 
  the 
  leaf 
  

   slow 
  and 
  difficult. 
  Such 
  leaves 
  are 
  often 
  densely 
  covered 
  

   with 
  hairs, 
  especially 
  on 
  the 
  lower 
  surface, 
  and 
  this 
  coating 
  

   of 
  hair 
  has 
  been 
  found 
  greatly 
  to 
  hinder 
  the 
  escape 
  of 
  moisture 
  

   through 
  the 
  stomata. 
  

  

  1 
  It 
  is 
  not 
  necessary 
  to 
  discuss 
  here 
  the 
  marsh 
  plants 
  and 
  halophytes 
  

   treated 
  in 
  works 
  on 
  the 
  ecology 
  of 
  plants. 
  

  

  FIG. 
  

  

  The 
  giant 
  cactus 
  

  

  This 
  specimen 
  is 
  about 
  40 
  feet 
  high. 
  The 
  holes 
  seen 
  in 
  

   one 
  of 
  the 
  stems 
  are 
  birds' 
  nests. 
  Photograph 
  by 
  the 
  

   Desert 
  Botanical 
  Laboratory 
  of 
  the 
  Carnegie 
  Institution 
  

  

  