In C4 plants, the Calvin cycle takes place in the stroma of bundle sheath chloroplasts.

Explanation:
C4 plants are a type of plants that have adapted to hot and dry environments. They have a unique photosynthetic pathway called the C4 pathway, which helps them to minimize water loss and increase their efficiency of carbon fixation. The C4 pathway involves the spatial separation of initial carbon fixation and the Calvin cycle.

1. C4 Pathway:
The C4 pathway begins with the initial carbon fixation step, which occurs in mesophyll cells. In these cells, the enzyme phosphoenolpyruvate carboxylase (PEPCase) fixes CO2 to a three-carbon molecule called phosphoenolpyruvate (PEP) to form a four-carbon compound called oxaloacetate. This step occurs in the stroma of mesophyll chloroplasts.

2. Bundle Sheath Cells:
After the initial carbon fixation, the four-carbon compound (oxaloacetate) is converted into another four-carbon compound called malate or aspartate. This compound is then transported into the bundle sheath cells, which are located around the vascular tissue of the plant. Bundle sheath cells are specialized cells that surround the vascular tissue and are responsible for conducting water and nutrients throughout the plant.

3. Calvin Cycle:
Once the malate or aspartate is transported to the bundle sheath cells, it is decarboxylated to release CO2. The released CO2 enters the Calvin cycle, which takes place in the stroma of bundle sheath chloroplasts. The Calvin cycle is a series of biochemical reactions that convert CO2 into glucose or other organic compounds. It involves the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) and other enzymes to catalyze the conversion of CO2 into organic molecules.

Therefore, in C4 plants, the Calvin cycle takes place in the stroma of bundle sheath chloroplasts. This spatial separation of initial carbon fixation and the Calvin cycle allows C4 plants to minimize photorespiration and enhance their efficiency of carbon fixation, making them well-adapted to hot and dry environments.