Phloem-2

	Home Page
Phloem-2
Companion Cells and STM originate from the same progenitor (mother) cell. The smaller and more densely cytoplasmic brother cell will become the Companion Cell. The larger cell in each pair will become a Sieve Tube Member.
Companion Cells also occur in dicots. However, they are not always easy to identify because Phloem also contains Parenchyma cells of various dimensions, and these can make it hard to locate Companion Cells. The small Companion Cells are easy to find in the images on the left and above, because of their small size, and clear association with STMs.
Sieve Tube Members & Sieve Cells produce a carbohydrate called Callose. Callose lines the Sieve Pores and also occurs in the cytoplasm. Aniline Blue stains Callose blue. When STM are severed, internal pressure causes Callose to accumulate at the Sieve Pores. This occludes the Sieve Pores and prevents "bleeding". This staining quality of Callose makes it easier to locate Sieve Plates. The image on the right is from cucumber phloem stained with Aniline Blue which stains Callose blue. Locate Sieve Pores & Sieve Plates.
	High magnification view of a Sieve Plate from the specimen above. Callose is clogging the Sieve Pores.
	Sieve Plates (SPLs) in a prepared specimen - Callose also lines the margins of the Sieve Pores, just outside the Plasmalemma. Increased callose deposition can also block the SPLs. This can happen seasonally and is reversible. Thus, plants can regulate phloem transport based on seasonal environmental stimuli as well as traumatic stimuli.
	Transverse (Surface) view of a Sieve Plate from a prepared slide. Note the size of the Sieve Pores.