Monocot Root Apex
The root apex is the tip of a root where growth occurs. In monocots, the root apex is characterized by the presence of specific histogens, which are groups of cells responsible for the development of different tissues in the root.

The Four Histogens in Monocot Root Apex:

1. Protoderm:
The protoderm is the outermost histogen and gives rise to the epidermis, which is the protective outer layer of the root. The epidermis helps in the absorption of water and minerals from the soil. It also plays a role in protecting the root from pathogens and other external factors.

2. Ground Meristem:
The ground meristem is responsible for the development of the ground tissues in the root. These tissues include the cortex, endodermis, and pericycle. The cortex is responsible for the storage of food reserves and the transportation of water and minerals. The endodermis acts as a selective barrier for the movement of water and minerals into the vascular tissues. The pericycle gives rise to lateral roots.

3. Procambium:
The procambium is the histogen that gives rise to the primary vascular tissues in the root. These tissues include the xylem and phloem. The xylem is responsible for the transportation of water and minerals from the roots to the rest of the plant. The phloem transports sugars and other organic compounds from the leaves to the roots.

4. Quiescent Center:
The quiescent center is a group of cells located at the center of the root apex. These cells are relatively inactive and act as a reservoir for stem cells. They ensure the continuous production of new cells for root growth and repair.

Conclusion:
In monocot root apex, four histogens are present: protoderm, ground meristem, procambium, and quiescent center. These histogens are responsible for the development of epidermis, ground tissues, vascular tissues, and the maintenance of stem cells, respectively. Therefore, the correct answer is option 'D' - 4.

4 in monocot root and 3 in dicot root....