The Stem

The question asks about the location of Casparian strips, which are an essential component of plant root anatomy. Let's analyze each option to determine where Casparian strips occur:

1. Epidermis: The epidermis is the outermost layer of cells in plant roots and leaves. It primarily serves as a protective layer and is involved in water and nutrient absorption in roots. However, it does not contain Casparian strips.
2. Cortex: The cortex is a layer found between the epidermis and the vascular tissues in plant roots and stems. It functions mainly in storage. Casparian strips are not found in the cortex.
3. Pericycle: The pericycle is a layer of cells found just inside the endodermis and is involved in the initiation of lateral roots. Casparian strips are not present in the pericycle.
4. Endodermis: The endodermis is the innermost layer of the cortex in plant roots and is characterized by the presence of Casparian strips. These strips are bands of suberin located in the cell walls of endodermal cells. They play a crucial role in regulating the flow of water and nutrients into the vascular system by blocking the passive flow of materials between the cortex and the vascular cylinder.

Therefore, the presence of Casparian strips in the Endodermis helps control the movement of water and dissolved substances into the plant's vascular system, thereby functioning as a selective barrier.

Thus, the correct answer is Endodermis.