Botany and Plant Science

Sclereids or stone cells are a reduced form of sclerenchyma cells. It has highly thickened, lignified cellular walls that form small bundles of durable layers of tissue in most plants. They are type of sclerenchyma cell that differs from the fibre cell by not being greatly elongated. Often occurs singly (an idioblast) or in small groups, giving rise to a gritty texture in, for instance, the pear fruit, where it is known as a stone cell. May also occur in layers, for example in hard seed coats. A type of cell with thick cell walls, consisting of lignin, that is commonly found on the outer 'skin' of fruits. So, sclereids or stone cells are modified sclerenchyma.

Sclerenchyma is composed of dead tissue which mainly acts as protective coating on fruits and nuts. The reduced form of sclerenchyma cells with highly thickened, lignified cellular walls are known as sclereids. It forms small bundles of durable layers of tissue in most of the plants. 

Senescence is also known as aging. It leads to metabolic degradation. It is the characteristic feature of the period between reproductive maturity and death of a plant or plant part or cell.

Parenchyma cells are thin-walled, flexible and loosely packed cells. They are alive at maturity. Their shape changes as per function. 
Collenchyma cells are living, elongated and irregularly thickened at corners with less intercellular space. 
Sclerenchyma is a type of permanent tissue. They are dead, elongated, thin, narrow cells with thickened walls without any internal space. Therefore, option C is correct.  

Collenchyma contains living, elongated cells with protoplasm at maturity. 
Parenchyma cells are thin-walled, flexible and loosely packed cells. They are alive at maturity. Their cells show protoplasm at maturity.
Sclerenchyma cells are dead, long, thin, narrow with thickened walls without any internal space. They do not possess any protoplasm at maturity. Hence, option A is correct. 

• Sclereids are a reduced form of sclerenchyma cells with highly thickened, lignified walls. These have the shape of a star. They are small bundles of sclerenchyma tissue in plants that form durable layers, such as the cores of apples and the gritty texture of pears (Pyrus communis). 
• Sclereids are variable in shape. The cells can be isodiametric, prosenchyma, forked or elaborately branched. 
• They can be grouped into bundles, can form complete tubes located at the periphery or can occur as single cells or small groups of cells within parenchyma tissues.
•  But compared with most fibers, sclereids are relatively short. Sclerenchyma, tracheids, and collenchyma do not cause the Grittiness of the fruit wall. 

The shape of the sclerenchyma cells are elongated and cell walls are thickened by lignin. The plant gets very strong support and strength since these are heavily deposited with lignin. It is responsible for the stiffness of coconut fibres. 

The chief mechanical tissue of the plant body composed of highly thick walled cell with little or no protoplasm is called as sclerenchyma. The thickening of cell wall is due to deposition of cellulose or lignin or both. Lignin deposited cells are said lignified. Sclerenchyma is the tissue which makes the plant hard and stiff.

• Osteosclereids are also known as bone cells. It is a type of sclereid that forms the hypodermal layer in seeds, fruits, and leaves of some plants (dicots).
• It was first seen in the seed coat of Pisum during the maturation. These are the columnar cells that are dilated at the ends.
  So, the correct option is B.

In stems and leaves of monocots, collenchyma is absent. Instead, sclerenchyma is present in monocots. Xylem parenchyma consists of living cells of parenchyma, which are thick and their secondary cell wall is lignified. Sieve tube member is associated with one or more specialized parenchyma cells called as companion cells. Phloem fibres are generally present in the secondary phloem.
Thus, the correct answer is option B.

Parenchyma, collenchyma and sclerenchyma are called as simple tissues as they are made up of similar kind of cells. Vascular tissue is a complex tissue because it is heterogeneous in nature with different types of cells. The chief elements are xylem, phloem and pericycle.
Thus, the correct answer is option B.

Fibres are usually long, spindle shaped structures, with tapering or blunt ends. Secondary thickening by lignin occur in 90% of the area of the cell. Fibres are grouped into xylary fibres and extraxylary fibres. Xylary fibres are also called wood fibres and are the longest among xylem elements. 
Thus, the correct answer is option A.

Sclerenchyma cells occur in many different shapes and sizes, but two main types occur: fibres and sclereids. Fibres are greatly elongated cells whose long, tapering ends interlock, thus providing maximum support to a plant. They often occur in bundles or strands and can be found almost anywhere in the plant body, including the stem, the roots, and the vascular bundles in leaves

• Sclereids or stone cells are a type of reduced sclerenchyma cells.
  
• These stone cells form hard structures such as seed coats and nut shells and provide support to the soft tissues.
• Their cell wall is highly thickened and lignified which form small bundles of durable layers of tissues.
• The presence of numerous stones cells that makes pear and guava have a gritty texture is brachysclereids (unbranched, short and isodiametric with pits).
• The stone cells in tea leaves that make it tougher are astrosclereids (branched as star-shape).

Sclereids are a reduced form of sclerenchyma cells with highly thickened, lignified cellular walls that form small bundles of durable layers of tissue in most plants.