Tag: absorption of water from the soil

The quiescent centre (QC) is a small group of cells with low mitotic activity (they divide sometimes, but only rarely). QC's are to be found at the tips of growing roots, in the root meristem, and are typically surrounded by groups of initial cells. Through rounds of cell division, these initial cells give rise to the various tissues of the root and so there are usually vascular initials, pericycle initials, cortex initials etc. It is thought that the QC sends out signals to these initial cells to ensure that they remain in an initial state and keep dividing to produce whatever tissue they are responsible for. So, root apex has a zone of slow dividing cells in the middle of rapidly dividing cells which is called as quiescent centre. 

Velamen is a spongy, multiple epidermis that covers the roots of some epiphytic or semi-epiphytic plants, such as orchid and Clivia species. The velamen of an orchid is the white or gray covering of aerial roots. It is many cell layers thick and capable of absorbing atmospheric moisture and nutrients. Often, the roots of orchids are associated with symbiotic fungi or bacteria. The latter may fix nutrients from the air. This functionality allows the orchid to exist in locations that provide a reproductive or vegetative advantage such as improved exposure or reduced competition from other plant species. The velamen also serves a mechanical function, protecting the vascular tissues in the root cortex, shielding the root from transpirational water loss and in many cases, adhering the plant to the substrate. Velamen is not found in breathing roots of halophytes, parasitic plants and all aerial roots. Cork, epiblema and hypodermis are not capable of absorbing water from air. 
Thus, the correct answer is option B.

Plants absorb water through the entire surface using roots, stems and leaves. However, mainly the water is absorbed by roots. The area of young roots where most absorption takes place is the root hair zone. The root hairs are delicate structures which get continuously replaced by new ones at an average rate of 100 millions per day. The root hairs lack cuticle and provide a large surface area. They are extensions of the epidermal cells. They have sticky walls by which they adhere tightly to soil particles. As the root hairs are extremely thin and large in number, they provide enormous surface area for absorption. They take in water from the intervening spaces mainly by osmosis. So, most water absorption in plants takes place through root hair zone and not by root apex, root cap or meristematic zone.
Thus, the correct answer is option C.

Root xylem is exarch xylem in which xylem formed first is present towards the periphery and is termed as protoxylem while metaxylem is present near the centre. Exarch xylem refers to the presence of protoxylem and metaxylem in different radii, not in same. 

Dicot root vascular bundles are described as radial and tetrarch. There are four bundles each of xylem and phloem occurring alternately. Xylem is described as exarch. Endodermis is present in dicot roots. Pith is absent in older dicot roots. 
Thus, the correct answer is option B.

According to histogen theory, as proposed by Hanstein, the root and shoot apices have three distinct meristematic regions called as histogens. These are namely; dermatogen, periblem and plerome. The dermatogens make uniseriate epidermis by anticlinal divisions, periblem forms cortex and plerome serve as the progenitor of stele (pericycle, vascular bundles, pith rays or medullary rays and pith). Monocotyledons have a fourth histogen, calyptrogen; being the fourth one, that makes root cap. 
Thus, the correct answer is option C.

According to histogen theory, as proposed by Hanstein, the root and shoot apices have three distinct meristematic regions called as histogens, these are namely dermatogens, periblem and plerome. The dermatogens makes uniseriate epidermis by anticlinal divisions, periblem forms cortex and plerome serve as progenitor of stele (pericycle, vascular bundles, pith rays or medullary rays and pith). Monocotyledons have four histogens; calyptrogen being the fourth one that makes root cap. 

Apex portion of root is made of protective tissue, root cap and the region of cell division is situated below the root cap. So, maximum growth in root occurs behind the apex and not in tips. Thus, option C is correct.