Companian Cells Sieve plate parenchyma Figure . : Different types of phloemelements Companion Cells The thin walled, elongated, specialized parenchyma cells, which are associated with the sieve elements, are called companion cells. These cells are living and they have cytoplasm and a prominent nucleus. They are connected to the sieve tubes through pits found in the lateral walls.
Through these pits cytoplasmic connections are maintained between these elements. These cells are helpful in maintaining the pressure gradient in the sieve tubes. Usually the nuclei of the companion cells serve for the nuclei of sieve tubes as they lack them. The companion cells are present only in Angiosperms and absent in Gymnosperms and Pteridophytes.
They assist the sieve tubes in the conduction of food materials. Phloem Parenchyma The parenchyma cells associated with the phloem are called phloem parenchyma. These are living cells. They store starch and fats.
They also contain resins and tannins in some plants. Primary phloem consists of axial parenchyma and secondary phloem consists of both axial Plant tissues Meristematic tissue: Capable of active cell division. Thin walled and living. Compactly arranged.
Found in root and shoot apex. Based on position: . Apical . Intercalary .
Lateral Based on origin: . Primary . Secondary Based on function: . Periderm (Epidermis ) .
Procambium (Primary vascular tissues) . Ground meristem (Cortex and Pith) Based on division: . Mass meristem: Divides in all planes . Rib meristem: Anticlinal division in one plane .
Plate meristem: Anticlinal division in two planes. Permanent tissues: Lose the power of cell division. Have definite shape, size and form. Simple tissues: One type of cells.
Parenchyma: Thin walled,isodiametric, found in all the parts. Types: . Aerenchyma. .
Storage parenchyma. . Stellate parenchyma. .
Chlorenchyma. . Prosenchyma. Collenchyma: Hypodermal position.
Provide mechanical strength. Types: . Angular collenchyma. .
Lacunar collenchyma. . Lamellar collenchyma. Sclerenchyma: Dead cells and lignified walls.
Types: . Sclereids . Fibres Sclereids: . Brachysclereids: Stone cells .
Macro sclereids: Rod shaped . Osteosclereids: Bone shaped . Astrosclereids: Star shaped . Trichosclereids: Hair cells Xylem: Water conducting tissue.