Biology Test 267

These special roots, called pneumatophores or knees, develop in mangrove plants, i.e., plants growing in saline marshes. These roots grow vertically upward and are negatively geotropic. Air enters these roots through minute breathing pores called pneumathodes, present on the tips of vertical roots. These plants include Rhizophora, Heritiera, Avicennia, etc.

Clinging roots arise from the nodes of stem and penetrate the stem of the host plant. It helps in fixing the plant to the host. It is found in orchids. Orchids bear three types of roots - clinging roots for fixation, absorbing roots for absorbing mineral salts and water and epiphytic roots for absorbing moisture from air.

Lateral meristems are the meristems which are present along the lateral sides of stem and roots. They divide only in radial direction. Intrastelar or vascular cambium ring formed by intra-fascicular (also called fascicular) and inter-fascicular cambium; and cork cambium (phellogen) are examples of this type of meristem. These meristems are responsible for increase in girth of stem and roots.

Companion cell is a type of cell found within the phloem of flowering plants. Each companion cell is usually closely associated with a sieve element. They remain connected with sieve cells by plasmodesmata. They help in loading of phloem sieve cells with sugars through active transport.

Smooth muscle fibres are elongated and spindle shaped (fusiform). Each fibre contains a single oval nucleus surrounded by cytoplasm (sarcoplasm). In cytoplasm, myofibrils are arranged longitudinally. These fibres lack striations and sarcolemma. However, are enclosed by plasma membrane.

Rudolf Virchow (1855) observed that new cells develop by division of the pre-existing cells— “Omnis cellula-e-cellula” (theory of cell lineage or common ancestry). The finding gave cell theory its final shape.

Phosphoglycerides are the triesters of fatty acids (either saturated or unsaturated) and glycerol to which a phosphate group is also attached.