Spineshrub, Adolphia californica S. Watson (family Rhamnaceae), is living quietly, and mostly unnoticed, in the California native section at MEMBG. This green-stemmed, virtually leafless shrub has grown well beneath our encina (Quercus agrifolia, the coast live oak and, yes, the namesake of nearby Encino). Nobody disturbs this plant, because its rigid stems are spine-tipped and can easily puncture the skin. Nevertheless, the armature of Adolphia has not protected it from disappearing from our state; small populations survive in the chaparral of southernmost California. But you may spot this species if you are botanizing in northwestern Baja California, for example, along the highway to El Rosario de Arriba. That's where I first encountered the species, with wife Linda, in 1977.
Adolphia is a North American genus of only two species, one here and the other, strikingly similar, A. infesta from Mexico and Trans-Pecos Texas. The genus was named to honor Adolphe Brongniart (1801-1876), who was an expert taxonomist on the Rhamnaceae. Rhamnaceae, you may already know, includes the species of Ceanothus, the gorgeously flowered California-lilacs, of which the state proudly has 43 currently recognized species. In fact, the evolution of Ceanothus occurred essentially in California, and Adolphia not only occurs with Ceanothus but also is a very closely related cousin.
In contrast to its cousin Ceanothus, A. californica produces flowers that are simple and also very instructive. In this family the flower typically has a disk- or cup-like extension around the ovary, called an hypanthium, and thereon sepals and petals are attached. Sepals are triangular. Petals are clawed (each petal has a stalk), as in Stigmaphyllon of family Malpighiaceae (see Fall 1999 MEMBG newsletter). These important and diagnosed features are very easily observed and understood in the flowers of Adolphia, which has white sepals and white petals. The ovary has three styles and stigmas and later may form a spherical capsular fruit. At MEMBG, I have not observed mature fruits, although various insects visit the flowers to drink the nectar, which collects on the hypanthium.
Adolphia californica is a shrub about a meter tall that generally has opposite leaves, but our plant is somewhat taller and often forms robust shoots with alternate leaves, thus expanding the official description of this species. Leaves are short petiolate, oblong, and simple, and each leaf is subtended by a pair of minute triangular stipules. The blade can be as long as one centimeter, but the leaves are early deciduous, meaning that they drop off the stems several weeks after they are formed. Then the green stem becomes the chief photosynthetic organ of the essentially leafless shrub.
Some shoots grow for several years to become the longer branches of the shrub, but most shoots stop growth during the first year, and so are termed determinate shoots. The stem begins to harden as soon as its internodes elongate completely, and terminal growth ends when the growing tip (the apical meristem) matures as a lignified sharp tip. This structure is called a stem spine, meaning that the stem is the organ that became the spine. Cacti, on the other hand, have leaf spines, meaning that the spine is a modified form of a leaf. In other cases, a spine may be a modified stipule. Many armed shrubs form stem spines in a similar manner as occurs in Adolphia.
This species is a fine example of how certain nonsucculent shrubs and some trees native to dry habitats have evolved stem photosynthesis. For a plant organ to be efficient photosynthetically, it must intercept reasonably high levels of sunlight, possess well-developed tissues with abundant chloroplasts wherein photosynthesis occurs, and have a pathway for supplying carbon dioxide from the atmosphere to those chloroplast-containing cells. To achieve the first requirement, plants typically expose flattened surfaces to the sun; slightly curved surfaces are another, less efficient version. To achieve the second requirement, cell layers located directly beneath the outer surface become elongate cylinders, called palisade cells, arranged like canisters barely touching and with air species between the cells. The third requirement is fulfilled by controllable air pores, the stomates, placed on the surface of the photosynthetic organ to permit gas exchange between the atmosphere and the photosynthetic cells located just inside the plant organ.
Generally, older stems of shrubs and trees are incapable of measurable photosynthesis because they are covered with bark, which prevents efficient movement (diffusion) of carbon dioxide to the cells and also blocks sunlight from reaching living cells of the stem. In cases where stems serve as the chief photosynthetic organs, bark development is suppressed, delayed sometimes for many years, and the stems are covered with stomates while the green tissues maintain palisade cells that take up and use the carbon dioxide.
Some people claim that green-stemmed shrubs are xerophytes (plants of dry habitats) that lose leaves to reduce transpiration. However, it is more important, and probably more correct, to say that leaves block sunlight from reaching the photosynthetic stem surface and thereby are lost through evolutionary processes to permit full exposure of the green stems to sunlight. The process of natural selection has created a set of leafless or nearly leafless woody plants with green stems that can exploit sunlight using an alternative lifestyle. Green-stemmed shrubs and trees tend to have very deep roots and can lose reasonably high quantities of water from deeper water sources. These green-stemmed life forms have been quite successful, especially in desert and chaparral-type habitats around the world.
ARTHUR C. GIBSON, MEMBG Director