By definition, succulent plants are
drought-resistant plants in which the leaves, stem, or roots have become more than usually fleshy by the development of water-storing tissue. Other sources exclude roots as in the definition "a plant with thick, fleshy and swollen stems and/or leaves, adapted to dry environments". The difference affects the relationship between succulents and "
geophytes"–plants that survive unfavorable seasons as a resting bud on an underground organ. The underground organs, such as
bulbs,
corms, and
tubers, are often fleshy with water-storing tissues. Thus, if roots are included in the definition, many
geophytes would be classed as succulents. Plants adapted to living in dry environments such as succulents, are termed
xerophytes. Not all xerophytes are succulents, since there are other ways of adapting to a shortage of water, e.g., by developing small leaves which may roll up or having leathery rather than succulent leaves. Nor are all succulents xerophytes, as plants such as
Crassula helmsii are both succulent and aquatic. '' Some who grow succulents as a hobby may use the term in a different way from botanists. In
horticultural use, the term
succulent regularly excludes cacti. For example, Jacobsen's three volume
Handbook of Succulent Plants does not include cacti. Many books covering the cultivation of these plants include "cacti (cactus) and succulents" as the title or part of the title. In botanical terminology, cacti are succulents, but not the reverse, as many succulent plants are not cacti. Cacti form a
monophyletic group and apart from one species are native only to the
New World, the
Americas, but through
parallel evolution similar looking plants in completely different families like the
Apocynaceae evolved in the Old World. A further difficulty for general identification is that plant families are neither
succulent nor
non-succulent and can contain both. In many genera and families, there is a continuous gradation from plants with thin leaves and normal stems to those with very clearly thickened and fleshy leaves or stems. The
succulent characteristic becomes meaningless for dividing plants into genera and families. Different sources may classify the same species differently. Species with intermediate characteristics such as somewhat fleshy leaves or stems may be described as
semi-succulent. Horticulturists often follow commercial conventions and may exclude other groups of plants such as
bromeliads, that scientifically are considered succulents. A practical horticultural definition has become "a succulent plant is any desert plant that a succulent plant collector wishes to grow", without any consideration of scientific classifications. Commercial presentations of "succulent" plants will present those that customers commonly identify as such. Plants offered commercially then as "succulents", such as
hen and chicks, will less often include geophytes, in which the swollen storage organ is wholly underground, but will include plants with a
caudex, that is a swollen above-ground organ at soil level, formed from a stem, a root, or both. ==Appearance==