﻿THE 
  STEM 
  AND 
  THE 
  LEAF 
  

  

  79 
  

  

  trees 
  grown 
  in 
  woodlands, 
  where 
  they 
  stand 
  moderately 
  close 
  

   together. 
  In 
  some 
  trees 
  such 
  as 
  the 
  so-called 
  snap 
  willows, 
  

   the 
  cottonwood, 
  and 
  the 
  large- 
  toothed 
  aspen 
  - 
  - 
  live 
  twigs 
  fall 
  

   very 
  freely 
  during 
  windstorms 
  and 
  snowstorms, 
  and 
  when 
  the 
  

   tree 
  is 
  loaded 
  with 
  sleet. 
  These 
  twigs 
  may 
  be 
  blow 
  T 
  n 
  over 
  crusted 
  

   snow 
  or 
  floated 
  along 
  by 
  brooks 
  or 
  rivers 
  near 
  by, 
  and 
  often 
  

   lodge 
  in 
  spots 
  where 
  they 
  take 
  root 
  and 
  grow 
  into 
  new 
  trees. 
  

  

  77. 
  Leaves 
  of 
  water 
  plants. 
  Water 
  plants 
  with 
  aerial 
  leaves, 
  

   like 
  the 
  cat-tails 
  and 
  pickerel 
  weeds, 
  are 
  perhaps 
  the 
  commonest 
  

   type. 
  Others, 
  like 
  the 
  pond 
  

  

  lilies 
  (fig. 
  62), 
  have 
  floating 
  

   leaves, 
  with 
  only 
  the 
  upper 
  

   surface 
  exposed 
  to 
  the 
  air. 
  

   Still 
  others, 
  like 
  some 
  pond- 
  

   weeds, 
  have 
  all 
  their 
  leaves 
  

   submerged. 
  There 
  are 
  only 
  

   a 
  few 
  common 
  plants 
  which 
  

   have 
  two 
  types 
  of 
  leaf, 
  like 
  

   the 
  water 
  crowfoot 
  (fig. 
  63), 
  

   one 
  set 
  growing 
  wholly 
  in 
  

   the 
  air 
  and 
  the 
  other 
  set 
  

   wholly 
  under 
  water. 
  

  

  Floating 
  leaves 
  have 
  sto- 
  

   mata 
  only 
  on 
  the 
  upper 
  sur- 
  

   face 
  ; 
  on 
  the 
  lower 
  surface 
  

   they 
  could 
  serve 
  no 
  useful 
  purpose. 
  Submerged 
  leaves 
  often 
  

   have 
  the 
  thread-like 
  form 
  shown 
  in 
  figure 
  63. 
  This 
  form 
  ren- 
  

   ders 
  them 
  much 
  less 
  liable 
  to 
  injury 
  from 
  waves 
  or 
  currents 
  

   of 
  water, 
  and 
  also 
  allows 
  the 
  freest 
  exposure 
  of 
  the 
  whole 
  leaf 
  

   surface 
  to 
  the 
  surrounding 
  water. 
  This 
  offers 
  the 
  best 
  possi- 
  

   ble 
  opportunity 
  for 
  exchange 
  of 
  gases 
  between 
  the 
  water 
  in 
  

   which 
  they 
  are 
  dissolved 
  and 
  the 
  interior 
  of 
  the 
  leaf. 
  

  

  78. 
  Size 
  and 
  shape 
  of 
  leaves 
  in 
  relation 
  to 
  water 
  supply. 
  

   Plants 
  which 
  grow 
  in 
  earth 
  (that 
  is, 
  neither 
  aquatics 
  nor 
  air 
  

   plants) 
  often 
  show 
  a 
  decided 
  relation 
  between 
  the 
  abundance 
  

   of 
  the 
  water 
  supply 
  and 
  the 
  amount 
  of 
  leaf 
  surface. 
  Those 
  

  

  FIG. 
  63. 
  A 
  shoot 
  of 
  water 
  crowfoot 
  

  

  A, 
  air 
  leaves 
  ; 
  B, 
  thread-like 
  water 
  leaves. 
  

   After 
  Askeiiasy 
  

  

  