Stem surface of a prickly pear cactus (Opuntia spp.) showing a sunken stomata, coloured scanning electron micrograph (SEM). Cacti typically have sunken stomata, which are an adaptation to drought conditions. An additional adaptation to a harsh environment is the fine covering of wax across the leaf or stem surface. Stomata (plural of stoma) are tiny openings in the epidermis of a plant that are surrounded by pairs of crescent-shaped cells, called guard cells (specialized parenchyma cells), that facilitate gas exchange. Stomata are present in the sporophyte generation of all land plant groups except liverworts. Prickly pear cacti typically grow with flat, rounded platyclades that are armed with two kinds of spines; large, smooth, fixed spines and small, hair like spines called glochids that easily penetrate skin and detach from the plant. Magnification: x220 when shortest axis printed at 25 millimetres.
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