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A T.S. of stem is stained first with safranin and then with acid fast green. What would be the colour of phloem?


A
Red
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B
Green
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C
Orange
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D
Purple
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Solution

The correct option is B Green
Safranin stains chromosomes, nuclei, lignin, suberized or cutinized walls red while fast green stains cytoplasm and cellulosic cell walls green. As we know that phloem is a composed of four elements namely, sieve elements, companion cell, phloem fibres and phloem parenchyma. Phloem fibres are dead sclerenchymatous elongated cells very thick lignified walls and lack cytoplasm and nucleus. A mature sieve tube is composed of vertical sieve elements with non-lignified cellulosic walls and protoplasm; they lack nucleus at maturity. Companion cells are living cells with the prominent elongated nucleus and dense cytoplasm. Thus, lignin (present in phloem fibres) and nucleus (present in companion cells) are stained with safranin. Fast green stains the cytoplasm of sieve tube elements and companion cells; the nucleus of companion cells and their cellulosic cell walls. Therefore, when stained with safranin and fast green, phloem appears green rather than red. 
Thus, the correct answer is option B.

Biology

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