| 1. A division of the bryophytes, liverworts are relatively small plants which |
| 2. have adapted to different habitats. Two species of liverworts, Riella and |
| 3. Ricciocarpus, thrive in aquatic habitats. Some are found in the company of |
| 4. other vegetation such as mosses, lichens, and sedges in the tundra in |
| 5. Antarctica, while most others prefer moist, shady floors and tree trunks of |
| 6. tropical forests. Leafy liverworts, with two or three rows of lobe-shaped |
| 7. leaves which overlap incompletely, are discovered plentifully in the |
| 8. tropical forests. These plants develop water storage pockets which become |
| 9. home to a host of very small animals. They have a prostrate growth, and |
| 10. single-cell rhizoids -- hairlike projections -- anchor the plant but are |
| 11. incapable of transporting nutrients to the plant. The absence of a midrib is |
| 12. quite common in bryophytes. Sphaerocarpo, a Thallus liverwort, |
| 13. sometimes produces round rosettes or extended, flattened lobes. |
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| 14. The bryophytes not only aid soil formation on rocky and unproductive land |
| 15. but balance the moisture content of the soil. Their epidermal cells -- outer |
| 16. cells of the plant -- fused with significant air pores enclose the |
| 17. photosynthetic cells. These pores play a major role in the photosynthetic |
| 18. process in which carbon dioxide is taken in and oxygen is given off. |
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| What does the statement " The absence of a midrib is quite common in the bryophytes" in lines 11-12 mean? |
