NEET Practice Questions (MCQs) with Answers & Solutions

(c) Stomata are tiny pore complexes. Each stomata is surrounded by two specialised green epidermal cells called guard cells.The opening of the stoma is facilitated by the orientation of the microfibril in the cell walls of the guard cells.Cellulose microfibrils are oriented radially rather than longitudinally making it easier for the stoma to open.

(c) Phellem or cork is a tissue formed on the outer side of cork cambium. It is composed of dead cells.
The cell wall become impermeable due to suberisation.

(c) Vascular cambium located between xylem and phloem in the stems and roots of vascular plants- It produces secondary xylem towards the pith and secondary phloem towards the bark. Phellogen is made of narrow thin-walled and nearly rectangular cells. phellogen cuts off cells on both sides. The outer cells differentiate into cork or phellem while inner cells differentiate into secondary cortex or phelloderm. The phellogen, phellem and phelloderm are collectively known as periderm.

The opening and closing of stomata are caused by influx and efflux of potassium ions (K⁺). The increase of K⁺ results in opening of stoma and decrease of K⁺ causes closing of stoma. The turgidity of guard cells induces to open the pores of stomata found on the surface of leaves.

The presence of protoxylem helps distinguish a dicot stem from a dicot root. Protoxylem elements are arranged in an exarch pattern (towards the center) in roots, while in stems, they are arranged in an endarch pattern (towards the periphery).

(a) In dicot stems, the cells of cambium present between primary xylem and primary phloem is the intrafasicular cambium. The cells of medullary rays, adjoining these intrafascicular cambium become
meristematic and form the interfascicular cambium. Xylem parenchyma are living and thin-walled and their cell walls are made up of cellulose. In dicot roots the innermost layer of the cortex is called endodermis. Next to endodermis lies a few layers of thick-walled parenchymatous cells called as pericycle.

(b) Plants have stomata and lenticels involved in gaseous exchange. In stems the living cells are organised in thin layers inside and beneath bark. They have openings called lenticels. Transpiration is the evaporative loss of water by plants. It occurs mainly through stomata in leaves. Food transport occurs by the vascular tissue phloem from a source to a sink. Photosynthesis is a physico-chemical process by which they use light energy to derive re synthesis of organic compounds.

The cork cambium or phellogen cells divide periclinally cutting off cells towards the outside and inside. The cells cut off towards the out side become suberized and dead. These are compactly packed in radial rows without intercellular spaces and form cork or phellem. Cork is impervious to water due to suberin and provides protection to underlying tissues. The cells cut off from cork cambium towards inside add to the cortex and are called secondary cortex cells or phelloderm cells.

Companion cells are characteristic elements of phloem tissue associated with the sieve tubes (sieve elements) in the angiosperms. The siever tubes and companion cells are related ontogenically as they develop from the same mother cell. The companion cells and sieve tubes maintain close cytoplasmic connections through plasmodesmata.

In dicot stems, cambium is present between phloem and xylem. Such vascular bundles, because of the presence of cambium, posses the ability to form secondary xylem and phloem tissues and hence, are called open vascular bundles. On the contrary, vascular bundles in monocots have no cambium. Hence, they do not form secondary tissues, and are referred to as closed.