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Plant  Science  
 
Tissues  in  plants  
 

A  land  plant  is  made  up  of  three  types  of  major  organs:  
 
roots,  stems  and  leaves
...
 Some  roots  also  absorb  reserves  of  food
...
 It  also  contains  tissues  for  the  
transport  of  materials  from  the  root  to  the  leaf  and  for  the  distribution  of  the  
products  of  photosynthesis  from  leaves  to  the  growing  parts  of  the  plant
...
 
 
The  phloem  is  a  two-­‐way  transport  system,  from  the  leaves  to  any  part  of  the  
plant  and  back  to  the  leaves
...
 Embedded  in  the  cortex  are  several  tissues,  particularly  the  
xylem  and  phloem  vessels,  which  make  up  vascular  bundles
...
 
 
Some  plants  have  just  a  single  large  leaf  but  many  plants  have  many  leaves  
distributed  on  shoots  or  branches  that  arise  from  the  main  stem
...
 The  upper  epidermis  
of  leaves  is  covered  by  an  outer  cuticle  of  wax,  which  allows  the  light  to  pass  
through  but  prevents  water  loss  through  the  upper  surface
...
 
These  help  protect  the  leaf  from  invading  pathogens  such  as  fungal  spores
...
 The  cells  are  closely  packed  together  to  maximise  the  
amount  of  photosynthesis  that  can  take  place
...
 
Below  the  palisade  layer,  spongy  mesophyll  cells  exist  also  containing  some  
chloroplasts,  are  usually  packed  together  with  large  air  spaces  surrounding  
them
...
 Diffusion  of  gases  to  and  from  the  stomata  can  easily  occur  
through  these  airspaces
...
 The  guard  cells  contain  chloroplast  and  have  an  uneven  
thickening  of  cellulose  in  their  cell  walls
...
 This  allows  
carbon  dioxide,  oxygen  and  water  vapour  to  move  in  and  out  of  the  leaf
...
   
 
Leaves  also  contain  vascular  bundles,  which  are  also  continuous  with  those  in  
the  stem
...
 Water  is  necessary  for  photosynthesis  and  
minerals  are  necessary  for  the  growth  of  the  plant
...
 
Apart  from  the  number  of  seed  leaves  there  are  a  few  more  differences  between  
the  two  types  of  plant
...
 
The  Roots  
Unbranched,  fibrous  
Main  taproots  with  
roots  usually  growing  
branches  from  it
...
 
 
There  are  about  50,00  species  of  monocotyledons  but  there  are  over  200,000  
species  of  dicotyledons
...
 
 
Some  plants  have  large  taproots  for  food  reserves,  e
...
 carrots,  turnips
...
 Many  cacti  have  extensive  surface  roots,  

which  can  quickly  absorb  any  rain  that  falls  before  it  evaporates  in  the  hot  
conditions
...
 These  enable  plants  to  survive  over  winter  when  photosynthesis  
may  not  be  possible
...
 
 
Leaves  
 
Many  plants  survive  in  very  arid  climates  where  water  loss  from  leaves  would  
cause  problems
...
 Plants  in  both  
temperature  and  tropical  region  shave  involved  modified  leaves  called  tendrils,  
which  has  a  mechanism  for  getting  their  leaves  into  sunlight  even  when  they  are  
shaded  by  surrounding  vegetation
...
   
 
Stem  Tubers  
 
Tubers  are  stems  that  grow  below  the  ground  and  are  used  to  store  food
...
 
 
Meristems  
 
Meristems  are  the  growing  part  of  a  flowering  plant,  where  cells  may  divide  by  
mitosis  throughout  the  life  of  the  plant
...
 Number  one,  apical  meristems,  found  at  the  tip  of  the  root  and  
the  shoot  and  number  two,  lateral  meristems,  found  in  the  vascular  bundles  of  
the  stem
...
 Many  plants  also  grow  ate  lateral  meristems,  which  makes  
the  stems  and  the  roots  thicker  and  is  known  as  secondary  growth
...
   
 
 

Auxin  and  Phototropism  

 
Plants  produce  growth  regulating  substances  that  act  to  control  growth  and  
development
...
 It  was  
first  observed  by  Darwin  that  the  plant  shoots  grow  towards  the  light  because  of  
some  influence,  which  he  proposed  was  transmitted  from  the  shoot  tip  to  the  
area  immediately  below
...
   
 
Auxin  is  found  in  the  embryos  of  seeds  and  in  apical  meristems,  where  it  controls  
several  growth  responses
...
 
 
The  exact  mechanism  of  auxin  action  is  not  fully  understood  but  it  has  been  
suggested  that  auxin  has  been  redistributed  to  the  side  of  a  shoot  tip  that  is  away  
from  any  light  source
...
   
 
Some  proteins  in  the  plasma  membrane  of  certain  cells  in  plant  shoots  are  
sensitive  to  light
...
   
 
 

Transport  in  Angiospermophytes  
 
Roots  
 

Roots  are  responsible  absorbing  water  and  mineral  ions  from  the  soil
...
 In  addition  as  new  roots  grow,  
numerous  root  hairs  develop  to  increase  the  surface  area  even  more
...
 
 
Plants  require  a  number  of  minerals  to  make  a  variety  of  substances  necessary  
for  growth  as  given  below:  
 
Mineral  Ion  
Importance  
Calcium  
Constituent  of  cell  walls  
Magnesium  
Needed  to  make  chlorophyll  
Iron  
Required  as  a  co-­‐factor  for  many  
enzymes  
 
Minerals  are  present  in  the  soil  as  salts,  e
...
 calcium  occurs  in  the  form  of  
carbonates
...
 Both  of  these  processes  are  passive  
and  do  not  require  energy  in  the  form  of  ATP
...
 Potassium,  nitrate  and  phosphate  are  
usually  absorbed  using  active  transport
...
 Experiments  have  shown  that  
potassium  ions  stop  moving  into  root  cells  from  the  soil  when  potassium  cyanide  
is  added
...
 
 
(3)  In  some  cases  there  may  be  a  close  association  known  as  a  mutualistic  
relationship  between  the  roots  and  a  fungus,  and  the  name  of  the  fungus  is  
hyphae
...
 Minerals  
can  pass  directly  from  fungal  hyphae  to  root  cells
...
   
 
Support  is  provided  by  cellulose  cell  walls,  cell  turgor  and  by  thickening  certain  
structures  with  lignin  and  other  materials
...
 Over  time  some  cells  may  thicken  their  cell  
walls  with  other  carbohydrates  such  as  lignin,  which  provide  additional  
structure  around  the  cell
...
 This  fluid  exerts  pressure  on  
the  cell  wall,  so  the  cell  becomes  rigid  and  presses  on  the  adjacent  cells
...
 This  is  sufficient  to  support  in  leaves  and  in  new  soft  tissue
...
   
 
Here  xylem  tissue  in  the  vascular  bundles  not  only  carries  water  but  provides  
support
...
 Lignin  is  a  
complex  substance  that  is  very  hard  and  resistant  to  decay
...
 
 
 

 
 

Transpiration  
 
Transpiration  is  the  loss  of  Water  vapour  from  the  aerial  parts  of  the  plants  (the  
leaves  and  stems)
...
 
 
Most  plants  will  grow  in  areas  where  the  amount  of  water  in  the  air,  the  
humidity  is  less  than  in  the  leaves
...
 The  evaporating  water  is  drawn  from  the  
vascular  bundles  in  the  leaf  and  stem
...
 This  is  known  as  the  transpiration  stream
...
 
 
 

Cohesion  -­‐  Tension  Theory  

 
The  movement  of  water  in  xylem  can  be  explained  by  the  cohesion  -­‐  tension  
theory
...
   
• Water  vapour  re-­‐enters  air  spaces  in  the  leaves  from  the  xylem  vessels
...
   
• Cohesion  is  due  to  hydrogen  bonding  in  water  molecules  and  adhesion  is  
caused  by  the  hydrogen  bonds  between  water  molecules  and  molecules  in  
the  walls  of  the  vessels
...
 The  
thickening  provided  by  lignin  prevents  this  happening
...
 Tension  caused  by  transpiration  also  causes  water  to  be  
drawn  into  the  roots  from  the  soil
...
 Guard  cells  have  unevenly  shaped  cell  walls  with  more  
cellulose  on  the  side  adjacent  to  the  stomata
...
 When  the  guard  cells  lose  water,  the  cell  walls  
relax  and  the  stoma  close
...
 In  darkness,  these  ions  leave  the  guard  cells  and  move  
into  surrounding  cells
...
 This  creates  an  increased  solute  
concentration  so  that  water  enters  by  osmosis,  making  the  cells  turgid  
and  opening  the  stomata
...
 When  abscisic  acid  is  present,  potassium  ions  leak  out  and  
water  follows  by  osmosis,  this  means  that  guard  cells  lose  turgor  pressure  
and  stomata  close  thus  conserving  water
...
   
 
(1)  Light  affects  transpiration  directly  by  controlling  the  opening  and  closing  of  
stomata
...
   
 
(2)  Temperature  affects  transpiration  because  heat  energy  is  needed  for  the  
evaporation  of  water
...
 
 
(3)  An  increase  in  atmospheric  humidity  reduces  the  rate  of  transpiration
...
 
 
(4)  An  increase  in  wind  speed  increases  the  rate  of  transpiration  because  it  
blows  away  the  air  just  outside  of  the  stomata,  which  is  saturated  with  water  
vapour
...
 
 

Transpiration  in  Xerophytes  

 
Xerophytes  are  plants  that  live  in  arid  climates
...
 
 
In  both  cases  plants  have  evolved  specialisations  that  enable  them  to  survive  
shortages  of  water  while  reducing  water  loss
...
 Stomata  are  protected  deep  inside  pits,  which  themselves  are  rolled  up  

inside  leaves
...
 Hairs  prevent  water  loss  by  diffusion
...
 At  night  the  stomata  open  and  carbon  
dioxide  diffuses  into  the  leaf
...
   
 
 

Translocation  in  the  Phloem  

 
Translocation  is  the  movement  of  organic  molecules  through  the  phloem  tissue  
of  the  plant
...
 
 
(2)  Companion  cells  -­‐  which  are  connected  to  the  siebe  tube  cells
...
 
Translocation  moves  materials  from  the  source  where  it  is  made  or  stored  to  a  
sink  where  they  are  used
...
 Once  
in  the  phloem,  they  are  translocated  into  sink  regions  such  as  growing  tissues  in  
the  meristems  of  roots,  buds  and  stems  or  storage  organs  like  fruits  and  seeds
...
 Sugar  is  usually  
carried  as  sucrose,  which  enters  and  leaves  the  phloem  by  active  transport  using  
energy  provided  by  the  companion  cells
...
 
 
Sink  and  source  can  change  depending  on  the  needs  of  the  plant  or  seasonal  
changes
...
g
...
 Flowers  
also  differ  in  the  way  they  are  pollinated
...
 
 

When  the  flower  is  at  the  bud  stage,  it  is  surrounded  by  sepals,  which  fold  
around  the  bud  to  protect  the  developing  flower
...
   
 
Petals  of  animal  pollinated  flowers  are  often  brightly  coloured  to  attract  insects  
or  other  animals  that  may  visit
...
 
 
Pollen,  containing  the  male  gametes  is  produced  in  the  anthers,  which  are  held  
up  on  long  filaments  in  many  flowers,  so  that  as  pollinators  enter,  they  brush  
past  the  anthers  and  are  dusted  with  pollen
...
 The  stigma  receives  pollen  grains,  which  arrive  with  pollinators  
as  they  delve  into  the  flower  to  obtain  nectar
...
 
 

Pollination  and  Fertilisation  

 
Pollination  is  the  transfer  of  pollen  (containing  the  male  gametes)  from  the  
anther  to  the  stigma
...
   
 
If  the  pollen  travels  from  the  anthers  of  one  plant  to  the  stigma  of  the  other  plant,  
the  process  is  known  as  cross-­‐pollination
...
 Self-­‐pollination  produces  less  
variation  than  cross-­‐pollination
...
 
 
When  pollen  grains  from  a  plant  of  the  same  species  arrive  at  the  stigma,  they  
germinate  and  produce  a  pollen  tube,  which  grows  down  the  style  to  the  ovary
...
 
 

Seeds  

 
Fertilised  ovules  develop  over  time  into  seeds,  which  protect  the  developed  the  
embryo  inside
...
 
 
Plants  have  evolved  many  ingenious  means  by  which  thy  disperse  their  seed,  e
...
 
(1)  some  seedpods  such  as  those  in  the  pea  family,  mature  and  dry  out  so  they  
eventually  snap,  causing  the  seedpod  to  open  quite  suddenly,  ejecting  the  seeds  

some  distance  from  the  plants
...
 These  seeds  may  appear  long  distances  from  the  parent  plant  and  are  
contained  within  a  rich  fertiliser
...
 They  may  burry  several  
groups  of  nuts  and  fail  to  dig  them  up  during  the  winter
...
 
 
Seeds  have  all  the  necessary  components  to  insure  successful  germination  and  
the  growth  of  a  new  plant
...
 Once  a  seed  has  been  formed  in  the  ovary,  it  
loses  water  so  that  it  can  enter  a  dormant  phase  and  not  develop  further  until  
conditions  for  growth  are  favourable
...
 The  cotyledons  are  surrounded  by  a  
hard  protective  seed  coat  called  the  testa
...
 In  the  wall  of  the  testa  there  is  a  wall  called  
the  micropyle  through  which  water  is  absorbed  to  begin  the  process  of  
germination
...
 A  dormant  seed  
needs  three  vital  factors  to  be  in  place  for  germination  to  occur:  
 
(1)  Temperature  -­‐  a  suitable  temperature  is  essential  for  the  enzymes  in  a  seed  
to  become  active,  this  cannot  work  in  cold  conditions  and  very  high  
temperatures  also  inhibit  their  activity
...
 
 
(2)  Water  -­‐  most  things  only  contain  about  ten  percent  water,  so  water  must  be  
taken  in  to  start  the  germination  process
...
 The  enzymes  breakdown  food  stores  to  provide  
energy  for  the  emerging  root  and  stem
...
 
 
 

Metabolism  and  Germination  

 
Germination  begins  as  the  seed  in  a  process  called  imbibition  absorbs  water
...
 Water  rehydrates  stores  of  food  
reserves  in  the  seed  and  in  a  starch  seed,  such  as  Barley  grain,  triggers  the  

embryo  plant  to  release  a  plant  growth  hormone  called  gibberellin
...
 The  amylase  hydrolises  
starch  molecules  in  the  endosperm  converting  them  to  soluble  maltose  
molecules
...
 Absorption  of  
water  by  the  seed  splits  the  testa  so  that  the  radical  and  plumule  can  emerge  and  
grow
...
 
 
 

Control  of  flowering  
 

At  certain  times  of  the  year,  shoot  meristems  produce  flowers
...
 It  is  also  shown  that  
the  period  of  darkness  is  the  critical  factor  in  controlling  flowering
...
 
 
There  are  two  types:  
 
(1)  Long  day  plants  flower  when  days  are  longest  and  the  nights  are  short
...
 
 
(2)  Short  day  plants  flower  as  nights  are  become  longer
...
 Short  day  plants  cannot  flower  if  the  
period  of  darkness  is  interrupted  by  a  pulse  of  artificial  light  shone  on  them  even  
for  a  few  minutes
...
 
This  pigment  can  exist  in  two  interconvertible  forms:  
 
(1)  Inactive  Pfr  
 
(2)  Active  PR  
 

Active  Pfr  is  produced  from  inactive  Pr  during  daylight  hours
...
 In  darkness  Pfr  reverts  slowly  to  the  more  stable  Pr  (this  happens  
whenever  the  wavelength  is  beyond  is  660nm  -­‐  independent  of  the  time  of  day)
...
 
Flowering  in  short  day  plants  is  inhibited  by  Pfr  but  during  long  nights  sufficient  
Pfr  is  removed  to  allow  them  to  flower
...
 
 



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