Describe the structure of the phloem and cardiac tissues.

Structure of phloem

• This is a vascular conducting tissue whose structure is composed of sieve tubes, phloem parenchyma and companion cells.
• Sieve tubes are living, slender, elongated tubular cells connected end-to end. Their cell walls are thin but made up of cellulose. They have large cavities with transverse walls obliquely placed and perforated with numerous sieve pits. These are called sieve plates. The cells have no nuclei but the living layers of protoplasmic strands have direct continuous connection through sieve pits from one cell to another.
• Companion cells are living, elongated, thin walled cells with prominent nuclei and dense cytoplasm lying close and parallel to the sieve tubes.
• Phloem parenchyma and sclerenchymatous fibers lay in association with other cells giving them support.

Structure of cardiac muscle.

• This consists of a network of branched muscle fibers in the walls of the heart.
• The fibers are short, cylindrical, branching and separated by loose connective tissue rich in blood capillaries.
• The fibers are joined end-to end and are interconnected by oblique bridges to form zigzag junctions called intercalated discs.