Phloem-2

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Phloem-2
Origin of Companion Cells - The smaller and densely cytoplasmic brother cell will become a 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. Phloem also contains Parenchyma cells of various dimensions, and these can make it hard to locate Companion Cells due to their similar features. The small Companion Cells are easy to find in the images on the left and above, because of their size, and clear association with STMs.
Sieve Tube Members 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 and cytoplasmic proteins to accumulate at the Sieve Pores. This occludes the Sieve Pores and prevents "bleeding". This makes it easier to locate Sieve Plates.The Sieve Plates are not always perpendicular to the long axis of the Phloem. The image on the left is from cucumber phloem stained with Aniline Blue which stains Callose blue. Locate Sieve Pores & Sieve Plates.
		Cucumber Vascular Bundle Labeled with Aniline Blue & IKI. The densely stained cells are Sieve Tube Members. What compound accounts for the dark blue staining reaction? Locate the Protoxylem & Metaxylem
		High magnification view of a Sieve Plate from the specimen above. What is clogging the Sieve Pores?
		Sieve Plates (SPs) in a prepared specimen - Callose also lines the margins of the Sieve Pores, just outside the Plasmalemma. Increased callose deposition can also block the SPs. 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.