Transcellular streaming:
Thaine (1961, 62, 64) and Canny (1962) are of the opinion, that sieve elements have a number of cytoplasmic strands called transcellular strands traversing along their path. They have observed movement of solutes in opposite directions in adjacent strands. The existence of transellular strands can conveniently explain bidirectional momement of solutes in one and the same seive element.
According to Thaine, the transcellular strands in sieve elements possess mitochondria like particles, which provide energy rich ATP for the movement of solutes. Radioisotopic experiments conducted by Thaine using C14 have indicated the existence of radioactive strands within the seive elements. Thaine, further proposed that transcellular strands are proteinaceous and have microtubules helping in rhythmic contraction.
Aikman and Anderson (1971), who supported Thaine assumed that peristalic contractions occur as series of waves along the length of transcellular strands when there is movement of the solutes.
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The theory of transcellular streaming is very appealing as it explains easily the bidirectional flow of solutes.
Several objections have been raised against this theory. These are:-
a) How biological energy is transduced into a physical force in a sieve tube?
b) What is the nature of the transcellular strands? Are they solid or tubular?
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c) Whether the solute particles move in or on the strands.
d) Why the transcellular strands are not seen when sieve tubes are observed under electron microscope?