Q.What are sieve tubes ?explain.

Step 1. Place sieve tubes within phloem's cell types. Phloem is a complex tissue built of four cell types - sieve elements, companion cells, phloem parenchyma and phloem fibres. Sieve elements, the actual conducting cells, occur as two kinds: the more primitive sieve cells (found in pteridophytes and gymnosperms) and sieve tubes (found only in angiosperms).

Step 2. Describe a sieve tube's structure. A sieve tube consists of a series of sieve-tube elements placed vertically one above the other, forming long, continuous tubes. Where one element meets the next, their end walls are perforated by sieve pores concentrated into well-differentiated sieve plates - the pores connecting the cytoplasm of successive elements so that dissolved food can move from cell to cell along the tube.

Step 3. Describe the sieve tube's companion cell. Every sieve-tube element in an angiosperm is paired with an adjacent, living, nucleated companion cell, connected to it by plasmodesmata. Because a mature sieve-tube element loses its own nucleus, it relies on its companion cell to help maintain and regulate its metabolic and transport activity.

Step 4. State the function. Sieve tubes are the vascular tissue system's food-conducting pathway - they carry out translocation of organic solutes (the sugars and other products of photosynthesis) from where they are produced (source, e.g. leaves) to where they are used or stored (sink, e.g. roots, developing fruit). …

Place sieve tubes within phloem's four cell types, describe their structure (sieve-tube elements, sieve plates, companion …

• Describing sieve tubes without mentioning the companion cell pairing, which is one of their defining features and what distinguishes them from sieve cells. …