Phloem-3

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Phloem-3
Long section through Cucumber stem, stained with Aniline Blue, viewed with Ultra-Violet fluorescence.		Aniline Blue fluoresces when it is illuminated with Ultra-Violet light. Other substances in the xylem walls also fluoresce. The latter is called autofluorescence. The red fluorescence is due to autofluorescence by chloroplasts. How to identify Phloem in this image. 1] Phloem and Xylem are usually found together. 2] Tracheary Elements have characteristic wall thickenings & autofluorescence. 3] Sieve Plates contain a lot of Callose which fluoresces & they tend to be transverse in long sections. 4] Locate the Xylem (easy step) look for Phloem.
		A closer inspection of the Phloem reveals the Sieve Plates which have transverse and somewhat inclined dispositions. A Sieve Tube is clearly seen on the right side of the picture. The lateral walls of STMs may also have Sieve Pores. These also contain Callose and fluoresce as expected. These can be clearly seen in the Sieve Tube on the right.
		Enlargement of the two Sieve Plates from the lower left corner of the image above. Note the blue color as well as the fluorescence. Both are due to the presence of Callose & Aniline Blue.
Sieve Cells		There are two types of Sieve Elements. These are Sieve Cells and Sieve Tube Members. Note the similarity to xylem nomenclature. Sieve Cells lack Sieve Plates. They have Sieve Pores on their lateral walls. These may be concentrated where endwalls overlap but they are less specialized than Sieve Plates. Sieve Cells tend to be narrow and elongate with overlapping end walls. Sieve Tube Members are shorter, wider and are more rectangular in shape. Their endwalls are more or less transverse or slightly inclined. STM are found in Angiosperms. Sieve Cells are characteristic for gymnosperms and seedless plants.