A structural polysaccharide found in the sieve plates of the phloem of higher plants, consisting of glucose residues linked through β(l-3) glycosidic bonds. It is deposited steadily throughout the growing season eventually causing blockage of the pores of mature sieve tubes. The functions of blocked tubes are taken over by new phloem formed from the cambium. In some species the blockage is reversible, the callose being hydrolysed in the spring. Callose may be laid down very rapidly in response to injury of the phloem.
Callose was the first structural plant polysaccharide to be synthesized in vitro, using an enzyme isolated from bean (Phaseolus aureus) seedlings.