What are adventitious roots?

Adventitious roots are indeed roots that develop in unexpected places, which is exactly what option C describes. These roots can emerge from various parts of the plant, such as stems or leaves, rather than solely from the root system. This characteristic allows the plant to adapt to its environment and can provide additional support, stability, and the ability to access water and nutrients from diverse sources. Adventitious roots can be seen in many plant species, including some types of grass, and they are particularly useful in situations where the plant may need to anchor itself or take advantage of available resources.

The other options do not capture the unique nature of adventitious roots. Roots that grow underground refer to the general nature of most plant roots, which does not highlight the unexpected nature of adventitious roots. Similarly, roots that support the main stem describe stabilizing functions but do not address their occurrence in unusual locations. Finally, while roots can absorb nutrients, option D specifically mentions air absorption, which is not a typical function of roots like adventitious ones. The defining characteristic of adventitious roots is their ability to emerge from non-root parts of the plant in response to various environmental stimuli.