Mechanism of translocation

Diffusion Hypothesis– according to this hypothesis, the conc. Of food material is higher at their site of synthesis –the leaves and their conc. Is low in roots. So, the food material diffuses to roots.

Protoplasmic streaming theory – According to Devries and Curtis soluble food materials in sieve tubes move from one end to another end due to cytoplasmic streaming.

It was proposed by Hugo de Vries (1885). Organic solutes that enter the sieve tube are possibly carried by the streaming protoplasm from one end to the other of the sieve tube. Organic solutes move by diffusion through the pores of the sieve plate. Thus the streaming protoplasm acts as a conveyor belt. Different substances move in different directions at the same time in the same sieve plate.

Objections:

• The observed rate of protoplasmic streaming is much slower than the rate of translocation.

• Protoplasmic streaming has not been observed in the matured sieve plate of most plants.