This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Echeverias, Crassulas, and Aloes, from South Africa and Mexico, have fleshy stems and leaves. Cacti, including Epiphyllums from Brazil, Mamillarias and Phyllocacti, from Mexico and other warm and dry parts of America, are also fleshy but have dispensed with leaves to economize their liquids. In their native habitats they get very little rain, and make a point of storing up what they do get, while their structure is such as to prevent the liquids from escaping too freely. Under cultivation many of them enjoy liberal treatment in summer, when the temperature is high and they are making their growth, but they must be kept relatively dry in winter when at rest and the light is bad. With few exceptions they like a relatively high temperature even in winter. They cannot give off moisture like thin-leaved, green plants, consequently water must be withheld almost entirely during winter, otherwise they would decay wholesale. A gardener can readily diagnose a plant of this character, without knowing its name or from what country it comes, and give it the proper treatment accordingly.
Fig. 10. - Illustration of Co-partnership or Symbiosis.
1, Roots of the White Poplar with Mycelial covering. 2, Tip of a Root of the Beech with closely adherent Mycelial covering; x 100 (after Frank). In these cases the Mycelial threads on the roots are of fungous origin, deriving nourishment from the roots on which they grow, but at the same time supplying food material to the roots.
Other plants of dry countries produce hard and wiry leaves, like the Grass tree of Australia (Xanthorrhoea), and must likewise be kept on the dry side during winter. The Rushes (Juncus) of our marshes and river banks belong to the same family, but their stems are very largely made up of loose, spongy tissue, surrounded by a thin layer of more solid structure, almost like a skin. They are therefore capable of giving off large quantities of water at any time when circumstances require it. Some plants of dry climates and arid soils and situations clothe themselves with a more or less dense coating of hairs; and in proportion to the density of this covering must they be kept dry in winter, otherwise they would sooner or later get into an unhealthy condition and ultimately perish. The roots are usually the first to suffer from an excess of moisture, but the functions of the leaves and other parts also get deranged. The common Stock, in a wild state, inhabits dry chalk cliffs, and all parts of the stems, leaves and calyx are densely covered with star-shaped, branching hairs. Seedlings under cultivation are extremely liable to damp off while quite young, if kept too close and moist in the seed pans or boxes. The excessive moisture renders them liable to attack by the "damping-otf" fungus (Pythium debaryanum). It is not usually regarded as a desert plant, but it serves to explain a similar difficulty when brought under cultivation from its dry, wild habitats.