Write the elements which are most readily mobilised from senescent parts.

Answer

### Step-by-Step Solution:

1. **Understanding Senescence**:
- Senescence refers to the aging process in plants, where older parts, such as leaves, begin to deteriorate and lose functionality. During this process, certain nutrients are reallocated to younger parts of the plant for growth and development.

2. **Identifying Mobilized Elements**:
- The elements that are most readily mobilized from the senescent parts of the plant include:
- **Phosphorus (P)**: Essential for energy transfer and photosynthesis.
- **Sulfur (S)**: Important for protein synthesis and enzyme function.
- **Nitrogen (N)**: A key component of amino acids and nucleic acids, crucial for plant growth.
- **Potassium (K)**: Vital for various physiological processes, including osmoregulation and enzyme activation.

3. **Contrast with Non-Mobilized Elements**:
- It's important to note that not all elements are mobilized from senescent parts. For example:
- **Calcium (Ca)**: This element is primarily a structural component of plant cells and is not readily mobilized from older tissues.

4. **Final Answer**:
- The elements that are most readily mobilized from senescent parts of the plant are **Phosphorus, Sulfur, Nitrogen, and Potassium**.