Q.In plants, soluble food is always translocated in the form of ______.

Because photosynthesis happens only in the chloroplast-bearing green cells of leaves — the source — every non-green part of the plant, such as roots, developing fruits and storage organs, is a sink that must receive its food supply by transport rather than by making its own. This transport is carried out by the phloem, chiefly through its sieve tubes and their associated companion cells, and evidence from ringing experiments, phloem structure, phloem-sap chemistry and carbon-14 tracer studies all confirm the phloem, and specifically its longitudinal downward-flowing sieve tubes, as the main channel.

Food is always transported as dissolved sucrose, always moving along a concentration gradient, and it can move in two directions: vertical (longitudinal) translocation, most often downward from leaves to stem and root, but reversing to upward flow from storage organs during seed/bulbil/corm germination or from mature leaves toward growing points, flowers and fruits; and lateral (horizontal) translocation across the stem or root via the medullary rays, called radial translocation when it moves from phloem to pith, and tangential translocation when it moves from phloem to cortex. Transport through the phloem is genuinely bidirectional, and the sap itself carries not just sucrose but other sugars, amino acids and hormones as well. …