Phloem

One of the few organelles they do contain at maturity is the rough endoplasmic reticulum, which can be found at the plasma membrane, often nearby the plasmodesmata that connect them to their companion or albuminous cells.

[10] The common sidewall shared by a sieve tube element and a companion cell has large numbers of plasmodesmata.

[citation needed] Bast fibres are the long, narrow supportive cells that provide tension strength without limiting flexibility.

They also serve as anti-herbivory structures, as their irregular shape and hardness will increase wear on teeth as the herbivores chew.

[citation needed][15] After the growth period, when the meristems are dormant, the leaves are sources, and storage organs are sinks.

[citation needed] Phloem sap is also thought to play a role in sending informational signals throughout vascular plants.

"Loading and unloading patterns are largely determined by the conductivity and number of plasmodesmata and the position-dependent function of solute-specific, plasma membrane transport proteins.

Recent evidence indicates that mobile proteins and RNA are part of the plant's long-distance communication signaling system.

"[17] Organic molecules such as sugars, amino acids, certain phytohormones, and even messenger RNAs are transported in the phloem through sieve tube elements.

[17] Phloem is also used as a popular site for oviposition and breeding of insects belonging to the order Diptera, including the fruit fly Drosophila montana.

The hormone auxin, transported by the protein PIN1 is responsible for the growth of those protophloem strands, signaling the final identity of those tissues.

Additionally, the expression of NAC45/86 genes during phloem differentiation functions to enucleate specific cells in the plants to produce the sieve elements.

The molecular control of phloem development from stem cell to mature sieve element is best understood for the primary root of the model plant Arabidopsis thaliana.

[22] Phloem of pine trees has been used in Finland and Scandinavia as a substitute food in times of famine and even in good years in the northeast.

Supplies of phloem from previous years helped stave off starvation in the great famine of the 1860s which hit both Finland and Sweden.