At first glace, this appears to be a picture of a green succulent, Euphorbia caput-medusae, viewed from above the plant--and it is. But next to the gray stone near the bottom of the photo is a brown flower. It belongs to a root parasite, Hydnora africana, which lives on the Euphorbia. Hydnora, from Africa, belongs to a family, Hydnoraceae that also includes a New World genus, Prosopanche. The relationships of this family were quite uncertain, but recent molecular data suggest that Hydnoraceae is a "basal angiosperm," among the more primitive of flowering plants. Parasites are often so highly modified, compared to their non-parasitic relatives, that their relationships are difficult to determine.
The only portion of Hydnora that appears above the ground surfaces is the upper portion of the flower. Much of the flower is below the soil surface. The flower is thick and succulent in texture. The portion of the flower above the ground surface is tubular and has three openings, one of which is shown here. There are three thick structures, which botanically should be called perianth segments, and which might be likened to sepals, that tend to stay united at the top of the flower.
A plant of Hydnora and the two Euphorbia plants to which it is attached have been excavated and washed. This Hydnora plant is the only one ever known to have been cultivated. The seeds were collected in the Karroo Garden, Robertson, South Africa, by Sherwin Carlquist in 1973, and brought back in a fresh condition to his home in Claremont, California. He purchased several rooted cuttings of Euphorbia caput-medusae, a known host plant for Hydnora africana, removed the Euphorbia plants from their pots, lined the pots with Hydnora seeds, and put the Euphorbia plants back into their pots. During the next several years, the Euphorbia plants were repotted, but noting appeared to have occurred. The Euphorbia plants were planted into the ground. In 1979, a flower of Hydnora appeared. Apparently the development of a seedling of Hydnora is slow. Also, the development of the Hydnora may have accelerated when the plants of Euphorbia were planted into the ground and thereby enlarged in size beyond the size they had when potted. In this photograph, portions of two of the Euphorbia caput-medusae plants are shown. The roots of the Euphorbia are white, and thereby they are easily distinguished from those of the Hydnora, which are dark brown. The plant of Hydnora consists of thick succulent roots. Hydnora has no stems. The flowers of Hydnora are borne on the surfaces of the roots.
A longitudinal section of a flower of a flower of Hydnora africana from the cultivated plant. Notice that the flower is attached to the horizontal, knobby brown root (horizontal, at the bottom of the picture). At the top of the flower are two of the "sepals" (perianth segments) that can be seen above the surface of the ground. They are orange inside. Just below where these are joined is a short tube. At the top of the tube, one can see some yellowish-orange structures protruding into the tube. These are two of the three anther-groups. Hydnora flowers don't have stamens in the ordinary sense. There are groups of anthers united into bunches. At the bottom of the tube is a stigmatic area. The basal part of the flower has a cavity in it. At the top of the cavity are white ovules. The ovules will mature into seeds. The flower of Hydnora is a kind of intricate trap. Or, as we will see, a temporary trap.
The flower of Hydnora, when it first opens, has white threadlike structures that cross the gap between the "sepals." The openings between these threads are barely large enough for a beetle to enter. More about that later! Although a beetle may enter a flower, it evidently has difficulty in finding its way out of the flower. This keeps it inside a flower long enough so that the beetle can pick up pollen or deposit pollen on its surface onto the stigmas at the bottom of the floral tube.
The threads that cross the gaps between the "sepals" of the Hydnora flower when the flower opens are pulled apart after a few days. Any beetles that entered the flower through those threads can now easily escape.
The inside of each of the three parianth segments (like sepals or petals) of Hydnora africana. Notice the bright organ color of these structures. On them are downwardly pointing hairs that are effective in directing beetles down into the lower portions of the flower, slowing the beetles from escaping from the flower. In this respect, Hydnora flowers are like the leaf of a pitcher plant (Sarracenia). The beetles are attracted to the flower of Hydnora by the scent emitted by the ivory-colored pad of tissue in the center. It emits an unpleasant dung-like odor. Not surprisingly, the beetles that are attracted are dung beetles. Even though this flower was on a plant cultivated in California, half a world away from where the plant is native in Africa, it attracted dung beetles. Very likely not the same kind of dung beetles as in Africa, but related. The pollination mechanism that occurs in the native habitat in Africa was thus duplicated, amazingly, in the cultivated plants.
Here are two of the three anther-masses or anther groups of the Hydnora flower. They are arranged in a triangle; a gap between their pits permits beetles to fall down onto the stigma. The anther groups shown here have not yet released pollen. The stigmas of this flower are probably receptive at this time, however. Thus, beetles that have pollen on their bodies from an older flower would be able to brush some of that pollen onto the stigmas of this flower, achieving cross-pollination.
Here are the dung beetles that visited the Hydnora flowers of the plant cultivated in Claremont, California in 1979. Notice their bodies have some whitish hairs on them; pollen could adhere to these hairs. Flowers pollinated by beetles usually have some tissue that offers beetles as food. In Hydnora, that's the whitish tissue on the inside of the "sepals." But beetles don't confine their feeding to such special "bribe" tissues. Beetles eat pollen and then eat the cells of the stigma. Beetle-pollinated flowers usually have an abundance of pollen and an abundance of stigma areas, and Hydnora flowers are no exception. By producing excess pollen and stigma cells, beetle-pollinated flowers succeed in achieving pollination and producing seeds despite the feeding habits of the beetles.
The brown underground axis of Hydnora africana has been interpreted as a stem, but it may be, instead, a root, with rows of stubby lateral roots on it (typically five rows of these stubby roots. When one looks at these underground axes in living Hydnora plants, one finds that their growing tips are conical, with no evidence at all of leafy structures, so the look like thick roots. In any case, the stubby lateral roots. The stubby lateral roots do not grow out. Instead, they apparently secrete growth substances that cause the roots of the Euphorbia (white) in the vicinity of the Hydnora to grow into the stubby Hydnora roots.
A root of Euphorbia (white) growing alongside a Hydnora root has been "captured" at several points. The Euphorbia roots form pad like structures. The contacts between the Hydnora and the Euphorbia are thereby widened. The Euphorbia furnishes Hydnora with moisture, food, and all other substances. Hydnora is not green and is completely dependant on a host plant. Succulent underground portions of the Hydnora, however, may fragment and stay alive for indefinite periods of time. Those fragments may at some time be able to contact a Euphorbia root and begin growth again.
A Hydnora africana that has numerous contacts with a Euphornia root. The nature of the underground axis (stem or root?) in Hydnora is not easy to determine with certainty, because there are no close relatives of Hydnora, and Hydnora is highly specialized for parasitism. However, there are reasons to favor the concept that the Hydnora axes are roots rather than stems. The growing tips show no evidence of reduced leaves at all, and there are no reduced leaves adjacent to the bases of the flowers. The five or six longitudinal rows of stubby roots on the axis remind one of patterns of lateral root formation on roots, whereas lateral roots that form on stems of plants don't tend to form neat longitudinal rows. And flowering plants other than Hydnora that are root parasites form roots (as in Orobanche), not underground stems, that interconnect with roots of host plants.