To determine which plant has absent roots, we can analyze the options provided in the question step by step.
### Step-by-Step Solution:
1. **Identify the Plants**: The question asks us to find out which plant has no roots. The options include Wolffia, Myriophyllum, Ceratophyllum, Eutrichularia, Podostemon, Pistia, and Lemna.
2. **Analyze Wolffia**: Wolffia is known as a free-floating aquatic plant. It is recognized for having no true roots. Instead, it has modified leaves that serve a similar purpose to roots, providing some anchorage in the water.
3. **Examine Myriophyllum**: Myriophyllum is another aquatic plant, but it does have a poorly developed root system. Therefore, it does not qualify as a plant without roots.
4. **Check Ceratophyllum**: Like Myriophyllum, Ceratophyllum is also an aquatic plant that possesses a rudimentary root system. Thus, it cannot be considered as having absent roots.
5. **Look at Eutrichularia**: Eutrichularia is also a free-floating plant, but it has modified structures that can resemble roots. Therefore, it does not fit the criteria of having no roots.
6. **Investigate Podostemon**: Podostemon is an aquatic plant that has thread-like roots attached to rocks. This means it has roots and is not the answer.
7. **Consider Pistia**: Pistia is an aquatic plant with roots that hang submerged under its floating leaves. Since it has roots, it cannot be the answer.
8. **Evaluate Lemna**: Lemna, while closely related to Wolffia, has a thallus structure with a single root. Therefore, it does not meet the criteria of having absent roots.
9. **Conclusion**: After analyzing all the options, the only plant that is confirmed to have no roots is **Wolffia**.
### Final Answer:
**Roots are absent in Wolffia.**
