Problem of sieve-tube slime.
It is proposed that sieve tubes contain a stationary, living, filamentous reticulum that persists throughout their functioning life. The filaments are about 100 to 150 angstroms in diameter and often striated. By swelling they block the pores of old sieve plates after slime has been lost from the elements. In Cucurbita, slime may be loosely bound to the filaments; in many species it is hydrolyzed, swept out by the assimilate stream, and eventually metabolized.
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