The process of exudation of liquid droplets from the margin and apex of the leaves of herbaceous plants is termed as guttation. It was first studied by Bergerstein in 1887. All plants do not show guttation. Some common plants which show guttation are wheat, barley, rye, mustard, tomato, nasturtium etc. Guttation takes place through special structures called, hydathodes.
Hydathodes are present on the margins and tips of the leaves. Each hydathode consists of a group of loosely arranged achlorophyllous or colourless parenchymatous cells called epithem. The epithem cells are also known as transfer cells. These cells are specialized cells which help in absorbing inorganic and organic solute present in the exudate.
The epithem cells are present above the vascular strand. The epithem cells connect to the outside through a permanent pore in the epidermis, termed as water pore or water stoma. The development of a positive pressure in the xylem present in the vein ending causes exudation of liquids from the water pore or stoma. The pressure forces the liquid out through the hydathode.
Phenomenon of guttation:
Guttation generally occurs when transpiration is less and the rate of water absorption is high. Under these conditions, root pressure is maximum which forces water upwards to xylem elements and eventually exudes in the form of drops from the hydathode in the leaf margins. Guttation may also occur during daytime if the plants are growing in moist or humid condition which increases root pressure.
An incrustation of salts is formed on the surface of the leaves after the guttation liquid evaporates. Guttation fluid is a dilute solution of both inorganic and organic substances. Poorly aerated soils, heavily salted or mineral deficient soil, dry soils and the atmospheric conditions promoting transpiration inhibit guttation.
Guttation drops can easily be mistaken for dew drops because both occur in the morning. However, dew generally does not fall on cloudy nights. Guttation drops is seen on tips or margins of the leaves while dew drops are present all over the plant and even the soil.
Demonstration of guttation:
A well watered potted plant of nasturtium is taken. The pot is covered by a polythene to check evaporation of water from the soil. It is kept under a bell jar over a flat surface or a glass slab. The edges of bell jar is sealed by vaseline.
The bell jar is connected to an aspirator. Air is sucked from the bell jar through an aspirator. Within an hour small droplets of liquid is seen at the leaf margins or leaf tips.
Difference between Stomata and hydathode
|1. Stomata occur in the aerial parts of all land plants.||1. Hydathodes occur on the leaves of only a few plants.|
|2. They occur on the surfaces of both the leaves and the young parts of the stem, flowers, fruits etc.||2. They occur on the margin and tips of the leaves.|
|3. Stomata open and close by the turgor changes in the guard cells.||3. Hydathodes possess permanent pores because the guard cells surrounding them are immobile.|
|4. Guard cells contain subsidiary cells.||4. Subsidiary cells are absent.|
|5. Stomata pass out water vapours.||5. Hydathodes pass out water droplets.|
|6. Along with water vapours, stomata allow passage of CO2 and O2.||6. Small quantities of solutes also pass out along with the liquid.|
|7. Each stomata leads internally to a substomatal cavity.||7. Hydathode possesses loosely arranged epithem cells below its pore.|
|8. They do not have any connection with a vein ending.||8. Hydathodes contain a vein ending.|
Difference between transpiration and guttation
|1. Loss of water as water vapour through aerial surfaces of plants is termed as transpiration||1. Exudation of liquid droplets from the margins and leaf apex is termed as guttation.|
|2. It occurs through stomata, cuticle and lenticels.||2. It occurs through hydathodes, scars and lenticels.|
|3. Water is lost in pure form.||3. Guttated water is a dilute solution of both inorganic and organic substances.|
|4. It does not leave anything on the surface of the plant.||4. An incrustation of salts is formed on the surface after the guttation liquid evaporates.|
|5. It usually occurs in the presence of light.||5. It usually occurs during the dark.|
|6. It occurs during dry periods.||6.It takes place during humid periods.|
|7. It does not depend on the pressure of xylary sap.||7. It depends on the pressure of xylary sap.|
|8. Transpiration continues even when the plant is under water stress.||8. It does not occur under conditions of water deficiency.|
|9. It keeps the plant cool by dissipating excess of heat.||9. It has no such effect.|
|10. Environmental factors such as heat, light, wind velocity, etc., affect the rate of transpiration.||10. Environmental factors do not affect .|
|11. Photosynthesis has an indirect role on stomatal regulation and hence, on transpiration.||11. Photosynthesis has no role.|
|12. Excessive transpiration causes wilting.||12. Excessive guttation does not cause loss of turgidity.|
It is the exudation of sap or watery solution from the cut or injured parts of the plant,e.g., Agave, Acer, Vitis, Toddy palm. It occurs due to root pressure, phloem pressure, local pressure in xylem (stem pressure) and latex or resin.