tube conducts sucrose only. But the photosynthate in chloroplast mostly in the form of starch or triose-phosphate which has to be transported to the cytoplasm where it will be converted into sucrose for further translocation. ii. Sucrose moves from mesophyll to nearby sieve elements by short distance transport.
iii. From sieve tube to sink by long-distance transport. . .
Ringing or girdling experiment Ring of bark removed Xylem Swollen tissue Water f bark oved Swo tiss Ring re Figure . : Ringing experiment The experiment involves the removal of all the tissue outside to vascular cambium (bark, cortex, and phloem) in woody stems except xylem. Xylem is the only remaining tissue in the girdled area which connects upper and lower part of the plant. This setup is placed in a beaker of water.
After some time, it is observed that a swelling on the upper part of the ring appears as a result of the accumulation of food material (Figure . ). If the experiment continues within days, the roots die first. It is because, the supply of food material to the root is cut down by the removal of phloem.
The roots cannot synthesize their food and so they die first. As the roots gradually die the upper part (stem), which depends on root for the ascent of sap, will ultimately die. . .
Direction of Translocation Phloem translocates the products of photosynthesis from leaves to the area of growth and storage, in the following directions, Downward direction : From leaves to stem and roots. Upward direction : From leaves to developing buds, flowers, fruits for consumption and storage. Germination of seeds is also a good example of upward translocation. Radial direction : From cells of pith to cortex and epidermis, the food materials are radially translocated.
- - . . PM However, the theory was rejected because the speed of translocation is much higher than simple diffusion and translocation is a biological process which any poison can halt. .
Activated diffusion theory This theory was first proposed by Mason and Maskell ( ). According to this theory, the diffusion in sieve