What do biologists call groups of cells that are specialized to secrete substances outside the cells?

The correct answer is "glands".
Glands are groups of cells which are formed from epithelial tissue and produce substances, such as hormones. They can be divided into two categories, exocrine or endocrine. Endocrine glands secrete these substances into the bloodstream, while exocrine glands release the substances into some of the body's cavities or the outer surface.

That is true tge cactus as a structural adaptation to its envirinment has it leaves reduced to spines. but this doesnt mean it cant produce food it produces food because of the presence of chlorophyll in its stem