Transpiration–The evaporation of water from plants.
Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants (similar to sweating), especially in leaves but also in stems, flowers and roots.
Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants they are more numerous on the undersides of the foliage. The stoma are bordered by guard cells (together known as stomata) that open and close the pore. Leaf transpiration occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants, changes cell's osmotic pressure, and enables mass flow of mineral nutrients and water from roots to shoots.
Transpiration from the aerial parts of a plant occurs from three different regions
- From the leaves through the stomata (stomatal transpiration),
- Directly from the surface of the leaves and stems (cuticular transpiration)
- From the lenticels which are the minute openings on the surface of old stems (lenticular transpiration).
The major part of the transpiration occurs through the stomata, the other two types of transpiration contribute very little.
Guttation–The leaves of certain plants exhibit droplets of water along their margins in the morning.
Guttation: The leaves of certain plants exhibit droplets of water along their margins in the morning. This particularly happens in plants growing in warm humid conditions when the rate of transpiration is low. A humid environment hampers transpiration while the roots continue to absorb water from the soil. This builds up a big hydrostatic pressure within the plant and "forces out" the excess water directly from the tips of veins in the leaf. Special pore–bearing structures called hydathodes are present on the margins of the leaf to allow this exudation. Guttation can be observed in many plants and is quite common in banana. Drops of water found on grass in early morning are often the result of guttation, rather than dew. Sometimes the water contains minerals in solution, such as calcium, which leaves a white crust on the leaf surface as it dries.
Bleeding: This happens only due to injury. The plant sap escapes ("bleeds") from the ruptured or cut surfaces of a plant. The root pressure generated by a plant assists in bleeding. The sap contains dissolved solutes, both organic and inorganic. It is reported in agave, Acer, Betula, vitis etc.