06 Plant Water Relation - part 12 - Transpiration

06 Plant Water Relation - part 12 - Transpiration


Transpiration : 
  • Plants absorb water constantly and continously.
  • Hardly 5% of the total water absorbed by rootsthat is utilised for cell expansion and plant growth. 
  • Remaining 95% water becomes surplus which is then lost into the atmosphere, through its aerial parts. 
  • Hardly 1% of surplus water is lost in the form of liquid and 99% of surplus water, is lost in the form of vapour. 
  • The loss of water in the form of liquid is called guttation. It occurs through special structures called water stomata or hydathodes. 
  • The loss of water in the form of vapour is called transpiration that occurs through leaves, stem, flowers and fruits. 
  • Most of the transpiration occurs through the leaves (called foliar transpiration). 
  • The actual water loss during transpiration occurs through three main sites - cuticle, stomata and lenticels. 
  • Accordingly, three types of transpiration are recognized viz - 
  1. Cuticular
  2. Stomatal and 
  3. Lenticular.
 1. Cuticular transpiration: 
  • Cuticle is a layer of waxy substance- cutin,present on outer surface of epidermal cells of leaves and stem. 
  • Occurs by simple diffusion and contributes 8-10% of the total transpiration. 
  • Occurs throughout the day .
  • Its rate is inversely proportional to thickness of cuticle.
2.  Lenticular transpiration: 

  • Lenticels are small raised structures composed of loosely arranged complementary cells. 
  • Each lenticel is a porous tissue consisting of cells with large intercellular spaces in the periderm of the secondarily thickened organs and the bark of woody stems and roots of dicotyledonous flowering plants. 
  • Lenticels are present in bark of old stem and pericarp of woody fruits.
  • Are absent in leaves. 
  • Contributes only about 0.1-1.0% of total transpiration. 
  • Rate is very slow. 
  • Occurs throughout the day.
3. Stomatal transpiration: 
  • Stomata are minute apertures formed of two guard cells and accessory cells. 
  • Located in the epidermis of young stem and leaves. 
  • Leaves generally show more number of stomata on the lower surface. 
  • Depending upon distribution of stomata on leaves, leaves are categorized into three types namely -
  1. Epistomatic
  2. Hypostomatic
  3. Amphistomaticon
 1. Epistomatic- 
  • On upper epidermis (Hydrophytese.g. Lotus)
2.  Hypostomatic- 
  • On lower epidermis (Xerophytes- e.g. Nerium) 
3. Amphistomaticon :
  • Both surfaces (Mesophytes- e.g. Grass). 
  • Stomatal transpiration occurs only during daytime. (Exception: Desert plants). 
  • 90 to 93% of total transpiration occurs through stomata and that too during day time only.
Do you know ? · 
  • The number of stomata per unit area of leaf, is called stomatal frequency. 
  • The correlation between the number of stomata and number of epidermal cells per unit area, is called stomatal index (I)

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