What type of structures do dead cell layers around plant stems act as?

The correct answer is that dead cell layers around plant stems act as physical barriers against pathogens. In many plants, especially those with woody stems, the outer layers consist of dead cells, such as cork, which form a protective barrier. This barrier serves as a frontline defense that helps to prevent the entry of pathogens such as bacteria, fungi, and viruses. By creating this protective layer, plants can limit damage and infection, allowing for healthier growth and development.

The other options do not accurately represent the function of dead cell layers around plant stems. For instance, while plants do need storage structures for nutrients, this function is typically associated with living tissues, such as parenchyma cells in storage organs like tubers or seeds. Conductive tissues for water transport, such as xylem, involve living and functional cells that transport water and nutrients; thus, dead cell layers do not fulfill this role. Additionally, photosynthetic cells are typically found in the green parts of the plant, like leaves, where living cells can carry out the process of photosynthesis, rather than in the dead outer layers of the stem.