A brief look at how plants grow is necessary to understand what causes crested, monstrose, and variegated succulents. All plants grow long by increasing the number of cells at the tips of shoots and roots. These regions of active cell division are called apical meristems, and the growth is called primary growth. Some plants, most notably the monocots, have meristems at leaf bases that cause leaf elongation. Additional meristems, called lateral meristems, produce secondary growth in woody plants. The most important lateral meristem is the vascular cambium, which produces wood and bark. Monstrose and crested growths only involve primary growth.
Cell divisions in the apical and intercalary meristems are highly regulated and finely tuned in each plant, resulting in distinctive stem and leaf shapes. For example, rapid apical growth with suppressed branching results in long, slender stems, whereas slower growth results in "fatter" stems. Stems without apical dominance are highly branched. Cacti generally exhibit apical dominance, with slow apical growth, and are "fat" (with cells filled with water, making them succulent). So, in cacti, faster growth results in columnar cacti, and slower growth results in barrel cacti.
Occasionally, a growth mutation occurs that "messes up" the regulated, coordinated cell divisions in the primary meristems. The causes of these mutations range from injury to bacterial or viral diseases. The three most common growth mutations are crests, monstrose growth, and variegation.
In crests, the growth mutation changes the shape of the apical meristem. Instead of a single growth tip, the area of active cell growth becomes a line, resulting in fan-like or crested growth.
In monstrous growth, local apical dominance is lost, and each growth tip tries to grow as if it were the dominant point, resulting in a "knobby" or "lumpy, jumbled" appearance.
Cresting and monstrous growths are not unique to succulent plants. Crests are found in many genera of non-succulent plants, including conifers and many common garden plants.
Crests and monstrose plants are grown like normal plants of the same species, except that crests and monstrose plants tend to be more sensitive. This is one of the many reasons they are often grown as grafts. Monstrose and crested plants flower and produce seeds just as other plants do. However, growth mutations are not generally transmitted by seed, so the best way to propagate these plants is by cuttings.
Variegation in succulents is almost always caused by normal genetic variation, at least initially. Once this mutation occurs, it is selected for and magnified during cultivation. Most variegates in the wild would probably be eliminated due to the tendency for variegated tissues to be naturally "weaker," more prone to insect attack, sun damage, and a reduced ability to photosynthesize in low-light situations (and hence become weak and die), and to infection by fungi and bacteria. But in captivity, under the right environments, or protected from evolutionary pressures, many of these variegated plants perform beautifully and make excellent landscape and potted plants.
Variegation is usually caused by a lack of chlorophyll in a section of the leaf, forming a yellow or white band, streak, or mottling of tissue. Sometimes, variegation is caused by pigments that mask chlorophyll, producing even darker colors (purples, reds, etc.). Variegation streaking down the middle of leaves is referred to as medio-variegation, while variegation down the sides of the leaves (margins) is margin-variegation.
Source: bakersfieldcactus.org
Links
- Succupedia: Browse succulents by Scientific Name, Common Name, Genus, Family, USDA Hardiness Zone, Origin, or cacti by Genus