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Unformatted text preview: are needed. This type of transport is known bulk flow. Sugar and water entering the STM has a positive pressure gradient. When sugar and water exit the STM there is a less positive pressure gradient. Aphids stab their style into the plant to attempt to strike one of the sieve tubes. The positive pressure forces the sugar into the aphid, they are small enough to not evoke wounding responses. Biology Notes March 3, 2008 Vocabulary: Sieve tube member Companion cells Pressure gradient...
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