The T-shaped intersection of South Tiverton and Charles E. Young Drive South is a busy pedestrian crossing on the UCLA campus, where the life sciences of south campus meet the health sciences, and there they share the BioMedical Library. Beginning in 1959, after the completion of the Botany Building, the botanical garden, under the directorship of Dr. Mildred Mathias, took responsibility for landscaping the southwestern corner with interesting botanical specimens. So resulted one of the best opportunities to advertise the botanical garden on campus. The university erected a large sign there after changing the name of the garden in 1979 to the Mildred E. Mathias Botanical Garden. Some gorgeous trees frame the corner, especially the flamboyant African tulip tree (Spathodea campanulata) and floss-silk tree (Chorisia speciosa), but the eye-level character has been created by the shrubs. This corner is where MEMBG presents a small but very interesting collection of plants with white leaves, called by us "Snow in Westwood." These are variegated plants, forms that have two, sometimes more, colors or white patches on the leaves.
To be honest, variegated plants are mutants possessing shoot regions that are not photosynthetic. Most commonly, the genetic mutation results in patches that are white or yellow. White, of course, signifies the absence of pigmentation, and cells there lack the ability to make green chlorophyll, either because the chloroplast contains a genetic mutation of its DNA or, rarely, the mutation has occurred in the nuclear genes required to make the functioning chloroplast. Yellow patches result when chlorophyll is absent but yellow, radiation-screening xanthophyll pigments remain. Some plants have chlorophyll but no xanthophyll screen, so when the leaf is exposed to sunlight it bleaches white.
Ornamental plants are often selected to have red leaves, and there are many variegated forms that have two tones of red. In most cases, the red coloration is produced by water-soluble pigments called anthocyanins, which are produced and stored in cell vacuoles, especially in the outer layer (epidermis) of the leaf. Wherever the anthocyanin occurs on a patch in which chlorophyll is also present, the red plus green combination will yield a dark red to brown; wherever the anthocyanin occurs on a patch in which chlorophyll is totally absent, a pink or rose color results, hence the second tone of red. Pink over yellow will produce yet another color.
In the technical lingo of the plant biologist, the term "chimera" is used to describe a plant organ in which one sector has one genetic makeup and another sector has a different genetic makeup. Your dictionary is worth consulting here, because the original word chimera referred to a mythical Greek creature, a monster that had the head of a lion, the body of a goat, and the tail of a serpent or dragon. Get the parallelism?
Developmental biologists have used visible chimeras as a way to see how cells divide to give rise to each region of an organ. If a dividing cell, call it a mother cell or a meristematic cell, has a genetic mutation and divides repeatedly, all of the offspring of that cell (its daughters or derivatives) will have the same genetic mutation. So it might be that a patch of white on a leaf may have come from the divisions of just one mutant cell traceable to early in the development of that leaf, although more than likely a small cluster of mutant cells were the founders. If the cells forming the earliest leaf are all of the mutant genotype, i.e., they have a defect in the chlorophyll-making process, every cell in that mature leaf, including the petiole, will be mutant. Occasionally an entirely white leaf is produced. When the leaf has green sectors and white sectors, that means that both cell types were present during the earliest stage of leaf development. Thus, to the trained eye, to someone who knows the details of leaf cell divisions, the patterns on leaves or stems can give a clear picture of tissue origins.
In most variegation chimeras, there probably is a mutant chloroplast. A chloroplast has its own chromosome--actually copies of a circular one--that carries the majority of information for that cell organelle. Plastids like these divide by pinching (binary fission), in the same manner as bacteria, and when a cell divides some plastids go to one daughter cell and some to the other. If during that division one cell gets only a mutant plastid, that cell can then begin a lineage of mutant cells.
Most of us are not caught up in describing variegated mutants as products of periclinal, mericlinal, and sectorial chimeras; we are merely fascinated by patterning and design. The most common form of variegation among dicotyledons is white around the edge or margin of a leaf, and for a monocotyledonous leaf, striped is a common form. Other patterns of variegation are blotched (maculate), spotted, and banded or zoned. In both monocotyledons and dicotyledons, one can find handsome variegated forms in which the principal veins are lacking pigments because cells above and below the conducting tissues of the veins are not involved in photosynthesis.
Variegation is also observed occasionally on stems. Particularly unusual are the striped stems of certain monocotyledons, such as a popular form of giant bamboo growing at the north entrance to the Botany Building or certain showy forms of sugar cane (Saccharum officinarum). The stripes are vertical because cell divisions for stem elongation of these grasses occur just above each node, in dividing cells called an intercalary meristem. If a portion of that meristem has cells lacking the ability to make chlorophyll, then all cells derived from and above those cells will have the same defect.
Variegated leaves are freaks, but they are truly freaks of nature. Especially in the tropics one can find species with variegated leaves. Examples are species of aroids (e.g., Dieffenbachia) of the densely shaded rainforest understory and bromeliad epiphytes living on perches in the trees. Horticulturists brought these species into cultivation, and in greenhouses and nurseries people have also searched through their stocks for spontaneous mutants.
Because white patches on leaves are not photosynthetic, we should not expect nor do we find that variegated forms grow as vigorously as do nonvariegated forms under identical conditions. Variegated forms are also sensitive to damage by intense sunlight. This explains much about how these plants are used in landscaping. They grow more slowly and hence may be less prone to grow too large for a space. They not only do well in shade but also benefit by being there, whereas fully green plant forms may not be happy and may require more sunlight. See now why variegated forms are used as accents with green-leaved plants in shady gardens. The presence of light areas on leaves tends to brighten a shaded area of the garden or dark corner indoors.
It is really the luck of the draw, i.e., the source of mutant cells, that determines whether a leaf will be totally albino, have white and green sectors, or be all green. The same can be said about a shoot, which may have all variegated leaves or be entirely green, depending upon what cells gave rise to the shoot tip. On many variegated plants, a branch may form that has only green leaves; it did not revert back to a normal genotype, but it simply arose from nonmutant cells. These fully green leaves are more productive than variegated ones and grow more vigorously and rapidly, and a fully green shoot can soon overtake the rest of the plant. Collectors of variegated forms therefore prune away "normal" shoots so that the health of the variegated portion is not jeopardized. In fact, on the MEMBG corner, the variegated portion of Coprosma X kingii died in fall 2000, and now only a typical green shoot remains.
I would be interested in your reactions to the few variegated plants that we have growing on the corner. Personally I find many variegated forms rather sickly and prefer not to have them at MEMBG. Take a close look at the variegated clown fig (Ficus aspera), the leaves of which appear to have been used as a paint drop cloth. Then, next to the sign and just behind our new plant novelty Plectranthus comosus, witness the fine foliage of Pisonia umbellifera 'Variegata' (syn. Heimerliodendron brunonianum). Sparingly used, variegated plants can be both beautiful and educational.
ARTHUR C. GIBSON, MEMBG Director