﻿SEEDS 
  AND 
  SEEDLINGS 
  

  

  165 
  

  

  the 
  cotyledons 
  remain 
  underground, 
  inclosed 
  in 
  the 
  seed 
  

   coats, 
  and 
  gradually 
  empty 
  their 
  stores 
  of 
  reserve 
  food 
  into 
  

   the 
  growing 
  seedling. 
  In 
  the 
  bean 
  the 
  cotyledons 
  come 
  out 
  

   of 
  the 
  ground 
  but 
  never 
  become 
  

   leaf-like, 
  while 
  in 
  the 
  squash, 
  

   castor 
  bean, 
  buckwheat, 
  and 
  

   morning-glory 
  (fig. 
  144) 
  they 
  

   emerge 
  from 
  the 
  ground 
  and 
  

   become 
  short-lived 
  leaves. 
  

  

  157. 
  Reserve 
  food 
  of 
  seeds 
  

   digested 
  by 
  enzymes. 
  One 
  of 
  

   the 
  most 
  surprising 
  things 
  about 
  

   the 
  early 
  growth 
  of 
  seedlings 
  is 
  

   the 
  rapid 
  way 
  in 
  which 
  many 
  

   kinds 
  begin 
  to 
  grow 
  even 
  in 
  

   sawdust 
  or 
  on 
  moist 
  blotting 
  

   paper. 
  Evidently 
  the 
  plant 
  food 
  

   must 
  all 
  come 
  from 
  the 
  seed 
  in 
  

   the 
  beginning, 
  and 
  the 
  removal 
  

   of 
  most 
  of 
  the 
  reserve 
  food 
  of 
  

   the 
  seed 
  greatly 
  retards 
  the 
  

   growth 
  of 
  the 
  seedling 
  (fig. 
  146). 
  

   It 
  is 
  not 
  at 
  once 
  clear 
  how 
  the 
  

   proteins 
  and 
  the 
  starch 
  of 
  some 
  

   seeds 
  and 
  the 
  oil 
  or 
  cellulose 
  of 
  

   others 
  are 
  so 
  quickly 
  withdrawn 
  

   from 
  them 
  and 
  transferred 
  to 
  

   the 
  growing 
  plantlet. 
  Most 
  of 
  

   the 
  reserve 
  substances 
  found 
  in 
  

   seeds 
  are 
  difficultly 
  soluble 
  or 
  

   quite 
  insoluble 
  in 
  water 
  or 
  the 
  

   watery 
  sap 
  of 
  plants, 
  but 
  the 
  

  

  insoluble 
  substances, 
  before 
  being 
  transferred 
  into 
  the 
  seed- 
  

   ling, 
  are 
  transformed 
  into 
  soluble 
  ones. 
  This 
  is 
  due 
  to 
  the 
  

   action 
  of 
  certain 
  substances 
  known 
  as 
  enzymes 
  or 
  soluble 
  fer- 
  

   ments. 
  An 
  enzyme 
  as 
  found 
  in 
  seeds 
  is 
  a 
  substance 
  secreted 
  

  

  FIG. 
  146. 
  

  

  Pea 
  seedlings 
  growing 
  

   in 
  water 
  

  

  A, 
  deprived 
  of 
  both 
  cotyledons; 
  S, 
  

   with 
  cotyledons 
  uninjured 
  

  

  