The following three points will highlight the three major types of transpiration.
The three major types of transpiration are: (1) Stomatal Transpiration (2) Lenticular Transpiration and (3) Cuticular Transpiration.
Transpiration mainly takes place through surface of leaves. It is known as Foliar transpiration (more than 90%). Transpiration occurs through young or mature stem is called as Cauline transpiration.
Depending upon the plant surface, transpiration is classified into three types:
Type # 1. Stomatal Transpiration:
Water vapour diffuses out through minute pore (stomata) present in soft aerial part of plant is known as Stomatal Transpiration. Of the total water loosed, near about 85 – 90% of water loosed by the stomatal transpiration.
Type # 2. Lenticular Transpiration:
Sometimes water may evaporate through certain other openings present on the older stems. These openings are called Lenticels and the transpiration that takes place through term is known as Lenticular Transpiration.
Huber observed in some plants that water lost by lenticular transpiration was about 0.1% of the total transpiration loss. He further noted that coating the bark of the trees reduced the total loss by 20% from total bark surface, showing that some water loss was taking place through general surface of the bark also.
Type # 3. Cuticular Transpiration:
Loss of water may also take place through cuticle, but the amount so lost is relatively small and make up only about 5 to 10 percent of the total transpiration. This type of transpiration depends upon the thickness of the cuticle and presence or absence of wax coating on the surface of the leaves. Xerophytic plants generally have very thick cuticle and wax coating on the leaves and stem in order to check cuticular transpiration.
The phenomenon of transpiration can be demonstrated by a simple experiment (see Fig. 4.1) with a small plant, such as Geranium. In order to eliminate evaporation, the surface of the pot and soil is covered with polythene sheath or an oil cloth. The potted plant is then put under a glass bell-jar. After sometime drops of water appear on the inner side of the bell-jar wall.