Sieve plates are the connection sites between sieve elements. During early development of young sieve tubes, sieve plates resemble normal cell walls. As in other cell walls, plasmodesmata connect the cytoplast of adjacent sieve elements. Later in development however, plasmodesmata in sieve plates undergo a significant structural alteration. Callose (a cell wall material similar to cellulose) is deposited around the plasmodesmata. During late developmental stages, when the nucleus and other organelles disintegrate, the callose is removed which leads to a significant increase of the plasmodesmata lumen until a pore is formed. The number and size of pores and plates is very variable and depends on the species and developmental state of the plant. A few examples are shown below.  

Sieve plates of 4 different plant species shown at the same scale. From left to right: pumpkin, green bean, castor bean, tomato. From Mullendore et al. (2010) Sieve tube geometry in relation to phloem flow. Plant Cell 22, 579-593. Copyright American Society of Plant Biologists

The different size of plates as well as size and number of pores have a major impact on the resistance inside the tube. It is yet unclear why the structure of sieve plates is so variable. However, since sieve plates represent a major obstruction in the tube, they are ideal targets for tube occlusion mechanisms. Sieve tube occlusion is as important for the plant, just as blood coagulation is for humans. Mechanical injury of a sieve tube by chewing insects, mammals etc. constantly occurs. Without a sophisticated protection system, plants would bleed to death. There are two known mechanisms. Callose formation on the sieve plate and forisome reaction (see link on the left).

Callose is a cell wall material that can be deposited around sieve pores. The deposition leads to as decrease in pore diameter and eventually to coplete occlusion. Callose formation has been shown to occur within minutes after injury and leads to tube occlusion within 10-20 minutes in some species. In other species callose formation appears to be less efficient.

Callose formation in bamboo. A) Sieve plate in the phloem of a vascular bundle of bamboo. B) Sieve plate in bamboo before injury. C-F) Callose formation around pores after 3 min (C), 10 min (D,E) and 20 min (F).  From Mullendore et al. (2010) Sieve tube geometry in relation to phloem flow. Plant Cell 22, 579-593. Copyright American Society of Plant Biologists