Prize-winning giant fruit at fairs relies on cutting a plant's sugar pipeline
Strip a ring of bark from a branch and you sever its phloem, the living tissue that ferries sugar from leaves to the rest of the plant. Growers exploit this: girdle a branch, leave a single fruit on it, and every bit of sugar those leaves make has nowhere else to go.
The Swiss botanist Carl Nägeli coined the word phloem in 1858, from the Greek for bark, because it forms the innermost layer of a tree's bark. Its job is translocation: moving the products of photosynthesis, chiefly sucrose, along with amino acids, some hormones and even messenger RNA. Unlike xylem, which is mostly dead tubing that carries water upward under tension, phloem is made of living cells and is pushed along by positive pressure.
The pipes themselves are sieve tube elements, which at maturity have shed their nucleus and nearly all their organelles so little blocks the flow. Their ends are perforated by pores reinforced with a polysaccharide called callose. Because they can barely sustain themselves, each depends on a smaller neighbour, the companion cell, packed with ribosomes and mitochondria and linked to it by many tiny channels. Tough sclereids mixed into the tissue give pears their gritty texture and wear down the teeth of animals that chew them.
Flow direction changes with the season. In spring, roots and other storage organs release sugar to growing shoots; later, leaves become the source and storage organs the destination, while developing fruit always draws sugar in. Since sap cannot easily cross between neighbouring tubes, adjacent channels sometimes carry sap in opposite directions at the same moment. Phloem sap is also thought to carry signals, with mobile proteins and RNA forming part of a long-distance messaging system.
Girdling cuts sugar off from the roots, which is why a full ring of stripped bark kills a tree, and why beavers, which gnaw bark at a fairly consistent height, can be deadly to trees near their territory. People have eaten phloem too. In Finland and Scandinavia, dried pine phloem was milled into a flour called pettu and baked with rye into hard, dark bark bread, which helped fend off starvation in the great famine of the 1860s.
Source: Phloem
