(CoCl . 2H O or CoCl . 4H O) due to water vapours coming out through stomata. The rate of transpiration is more on the lower surface than in the upper surface of the dorsiventral leaf.
. . Significance of transpiration Transpiration leads to loss of water, as stated earlier in this lesson % of absorbed water is lost in transpiration. It seems to be an evil process to plants.
However, number of process like absorption of water, ascent of sap and mineral absorption directly rely on the transpiration. Moreover plants withstand against scorching sunlight due to transpiration. Hence the transpiration is a “ necessary evil ” as stated by Curtis . .
Translocation of Organic Solutes Leaves synthesize food material through photosynthesis and store in the form of starch grains. When required the starch is converted into simple sugars. They must be transported to various parts of the plant system for further utilization. However, the site of food production (leaves) and site of utilization are separated far apart.
Hence, the organic food has to be transported to these areas. The phenomenon of food transportation from the site of synthesis to the site of utilization is known as translocation of organic solutes . The term solute denotes food material that moves in a solution. .
. Path of Translocation It has now been well established that phloem is the path of translocation of solutes. Ringing or girdling experiment will clearly demonstrate the translocation of solute by phloem. - - .
. PM . . Source and Sink Source is defined as any organ in plants which are capable of exporting food materials to the areas of metabolism or to the areas of storage.
Examples: Mature leaves, germinating seeds. Sink is defined as any organ in plants which receives food from source.Example: Roots, tubers, developing fruits and immature leaves (Figure . ). Figure .
: Source and Sink . . Phloem Loading The movement of photosynthates (products of photosynthesis) from mesophyll cells to phloem sieve elements of mature leaves is known as phloem loading . It consists of three steps.