Tag: internal structure of root, stem and leaf

In dicot stems, the pith is generally small or reduced compared to monocot stems, where it is often large and well-developed. Dicot roots typically lack a pith or have a very small one.

In a dicot stem, by the end of the first year, the primary structure shows the growth of vascular and cork cambiums. The growth of the secondary xylem becomes continuous with the primary xylem. The secondary growth occurs both in the stele and cortex and though simultaneously, is caused by separate strips of secondary meristem. The cambial ring exhibits mitotic activity on both the sides and ones towards the inner surface results in the formation of xylem cells.

Hypodermis is a region lying immediately below the epidermis. It is represented by a few layers of collenchyma cells with angular thickenings. The cells are compactly arranged without any intercellular spaces. Hypodermis provides mechanical support and additional protection. So, hypodermis of a dicot stem is composed of collenchyma and not parenchyma or sclerenchyma. 
Thus, the correct answer is option B.

In a young dicot stem epidermis is covered by a waxy coating, called as cuticle. Cuticle is the waxy coating of the outer walls of epidermal cells of stem. It's function is to protect against excessive water loss, as well as protecting against mechanical injury.

Collenchyma tissue is composed of elongated cells with irregularly thickened walls. They provide structural support, particularly in growing shoots and leaves. Collenchyma tissue makes up things such as the resilient strands in stalks of celery. Collenchyma cells are usually living and have only a thick primary cell wall made up of cellulose and pectin. Cell wall thickness is strongly affected by mechanical stress upon the plant. The walls of collenchyma in shaken plants may be 40 to 100% thicker than those not shaken. Collenchyma cells are most often found adjacent to outer growing tissues such as the vascular cambium and are known for increasing structural support and integrity. So, stem of few dicot plants are getting mechanical strength mainly because of collenchyma before secondary growth. Parenchyma is a versatile ground tissue that generally constitutes the filler tissue in soft parts of plants. It forms, among other things, the cortex and pith of stems, the cortex of roots, the mesophyll of leaves, the pulp of fruits and the endosperm of seeds. Sclerenchyma is the supporting tissue in plants after secondary growth. 
Thus, the correct answer is option B.

As we know that Intrafascicular or intrastelar cambium is present between xylem and phloem in dicot stem and roots. Pith rays, present between the edges of intrafascicular cambium, divide to form a new cambium strip between the two vascular bundle i.e. interfascicular cambium. Thus, inter and intra fascicular cambium together forms a complete cambium ring called as vascular cambium or just cambium which forms secondary xylem on inner side and secondary phloem on outer side. Differential cambial activity during autumn and spring season result in narrow and wide secondary xylem rings respectively which together forms the annual growth ring. Thus annual growth ring is derived from vascular cambium which is derived from inter and intrastelar cambium.

Pith or medulla, is a tissue in the stems of vascular plants. Pith is composed of soft, spongy parenchyma cells, which store and transport nutrients throughout the plant. In dicots, pith is located in the centre of the stem. In monocots, it extends also into flowering stems and roots. The pith is encircled by a ring of xylem. The xylem, in turn, is encircled by a ring of phloem. The monocot root has a pith, whereas the dicot root does not. Absence of pith is the diagnostic feature of dicot root. Pith is present in dicot stem, monocot stem, monocot root and dicot stem. 
Thus, the correct answer is option D.

Intercalary meristem at base of leaves or nodes of grasses is responsible for  primary growth. Meristems are classified by their location in the plant as apical located at root and shoot tips, lateral in the vascular and cork cambia, and intercalary at internodes or stem regions between the places at which leaves attach, and leaf bases. Intercalary meristem cells possess the ability to divide and produce new cells, as do apical and lateral meristems. They differ, however, in being situated between regions of mature tissue, such as at the base of grass leaves, which are themselves located on mature stem tissue.
Therefore, the correct answer is option A.

Phellogen is the extrastelar lateral meristem that produces extrasteler secondary tissue system. Epidermis is the outermost covering of the primary stem that delimits cotex on outer side and is ruptured by stelar secondary growth, therefore cannot produce phellogen. Cortex is the ground tissue which surrounds the central stele and is derived from ground meristem. Hypodermis refers to 5-7 layers of cortical cells present just beneath the epidermis which resume the meristematic activity and produce phellogen just beneath the epidermis. The innermost layer of cortex forms endodermis while the phellogen is produced just beneath the epidermis. The outermost covering of stele forms pericycle it is an intrasteler tissues present in a stele which means that phellogen (extrastelar lateral meristem) cannot be produced by pericycle. The phellogen or cork cambium produces cork or phellem towards the periphery. 
Therefore, the correct answer is option A.

Hypodermis is a region lying immediately below the epidermis. It is represented by a few layers of collenchyma cells with angular thickenings in dicot stem. The cells are compactly arranged without any intercellular spaces. Hypodermis provides mechanical support and additional protection. Thus, option A is correct.