Which of the following cells lack the structure discovered by Robert Brown is orchid plants?

Options

A. Erythrocyte of mammals
B. Sieve tube cells
C. Both (1) and (2)
D. Liquid endosperm of coconut

Answer

To solve the question, "Which of the following cells lack the structure discovered by Robert Brown in orchid plants?", we need to follow these steps:

### Step-by-Step Solution:

1. **Identify the Structure Discovered by Robert Brown**:
- Robert Brown discovered the nucleus, which is a vital organelle found in most cells.

2. **Understand the Role of the Nucleus**:
- The nucleus serves as the control center of the cell, containing DNA that regulates all cellular activities.

3. **Recognize the Context of the Question**:
- The question specifically asks about cells that lack the nucleus, particularly in the context of orchid plants.

4. **Evaluate the Options**:
- We need to analyze the given options to determine which cells do not have a nucleus.

5. **Consider Erythrocytes of Mammals (RBCs)**:
- Mature red blood cells (RBCs) in mammals do not have a nucleus. This adaptation allows more space for hemoglobin, which is crucial for oxygen transport.

6. **Consider Sieve Tube Cells**:
- Sieve tube cells, which are part of the phloem in plants, also lack a nucleus. They rely on companion cells for their metabolic functions.

7. **Consider Liquid Endosperm of Coconut**:
- The liquid endosperm is a multi-nucleated structure, meaning it contains multiple nuclei, thus it does not fit the criteria of lacking a nucleus.

8. **Conclusion**:
- The cells that lack the nucleus are erythrocytes of mammals and sieve tube cells. Therefore, the correct answer to the question is option number 3, which includes both erythrocytes and sieve tube cells.

### Final Answer:
The cells that lack the structure discovered by Robert Brown (the nucleus) in orchid plants are **erythrocytes of mammals and sieve tube cells**.

---