The difference in environments is related to climate, soil, light and the availability of moisture. These elements play a part in leaf shape as well as plant form, dictating how the tissues in a leaf are arranged. Leaves have adapted to life under a wide variety of conditions. Most plants don't fall neatly into one category or another, but bridge more than one.
From One Extreme to Another
Xerophytes
Dry environments are harsh environments for plants, who, by their very nature, require moisture to function. Dry land plants are adapted for high heat and arid conditions. They will, generally, have small, thick leaves with multiple layers of epidermal cells, a thick cuticle and often hairy surfaces. Their object is to reduce evaporation so that leaf cells don't dry out and die. The spongy cells in the leaves may store water in the air spaces. Stomata are only on the lower leaf surfaces and often sunken into pits to further protect them.
Cacti represent an extreme adaptation to dry conditions. Their entire structure is designed to hold water for long periods. Some can survive with no rainfall for more than a year. Their leaves have been reduced to vestigial cells, seen only with the aid of a microscope, replaced with spiny outgrowths of their epidermis developed to protect their precious water supply from thirsty predators.
Pictured is Mammillaria rhodantha ssp. pringlei (synonym: Mammillaria parensisa), the pincushion cactus.
Members of the Cactaceae family, all cacti are succulents but not all succulents are cacti. Sempervivums, semps for short or hens and chicks, are succulents belonging to the family Crassulaceae who also store water in their fleshy leaves but are not adapted to survive extremely long periods without moisture. They come in myriad sizes, shapes and colors from the high mountains of Europe, Turkey, Iran, Morocco, the Balkans and the Caucasus, whereas most cacti originate in the Americas. Semps are hardy creatures. Those pictured were growing on top of a stone wall at Asiatica Nursery in Pennsylvania.
All semps are rosette forming plants, as are most true alpines. True alpine plants come from mountain regions around the world, generally at an altitude of ten thousand feet (3,048 m) or more, just below the snow line. Alpine summers range from 50 - 59º F (10 - 15º C) and winters are below freezing, often with snow or ice cover. Temperatures can change rapidly, dropping below freezing in a day. The sun, at high altitudes, is harsh and winds are strong. There's little carbon dioxide at high altitudes, essential to photosynthesis.
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