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Even single cells have astonishing regenerative abilities. Acetabularia, the mermaid’s wineglass, is a single-celled green alga about five centimeters long, with three main parts: root-like structures called rhizoids that attach it to a rock, a stem and a cap about a centimeter wide (Figure 5.2). This very large cell has a single nucleus in one of the rhizoids. As the plant grows, its stem lengthens, it forms a series of whorls of hairs that later drop off, and finally forms the cap. If the cap is cut off by snipping the stem in two, after the cut has healed, a new tip grows and the stem forms a series of whorls of hairs and then a new cap, in a similar way to the normal pattern of growth. This can happen over and over again if the cap is cut off repeatedly.2 As discussed in the following chapter, the usual assumption is that genes somehow control or “program” the development of form, as if the nucleus, containing the genes, is a kind of brain controlling the cell. But Acetabularia shows that morphogenesis can take place without genes. If the rhizoid containing the nucleus is cut off, the alga can stay alive for months, and if the cap is cut off, it can regenerate a new one. Even more remarkable, if a piece is cut out of the stem, after the cuts have healed, a new tip grows from the end where the cap used to be and makes a new cap (Figure 5.2).3 Morphogenesis is goal-directed, and moves toward a morphic attractor even in the absence of genes. FIGURE 5.2. Regeneration of the alga Acetabularia mediterranea, an unusually large single-celled organism, up to 5cm tall, containing a green cap at the top of a long stalk, anchored at the base by root-like rhizoids. There is a large nucleus (shown as a black oval) in the basal part of the cell. When the stalk is cut off near the bottom, the basal part of the cell regenerates a new stalk and cap (shown on the right). When a part of the upper stalk is cut out, it grows a new cap and more stalk, even though it contains no nucleus.
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