Overview of Cactus Evolution: The Evolutionary Journey of Living Fossil Plants

  In the lateral evolutionary history of cacti, even more uniquely shaped species have evolved. Cacti with particularly short lateral teeth evolved into pad-shaped or multi-segmented forms (such as Schlumbergera, the famous Christmas cactus); these species reproduce easily by cutting or by segments that drop off on their own.

  Creeping cacti grow especially fast, because buds lying on the ground quickly take root.

  Such cacti can be cut into several sections without any harm, because each section has its own fibrous roots. Using this method of propagation can also bypass the rootless stage of seedlings or stem segments.

  The succulent bodies of cacti can store large amounts of water, which is why they belong to the succulent plants. As a result, besides arid regions, cacti have also occupied a broader range of habitats. Even in relatively humid regions there is no shortage of fairly dry places (such as on rocks) where plants unable to store water find it hard to survive. That is why we often find cacti and other succulents on flat rocks.

  Moving from growing on rocks to dwelling on the branches and trunks of trees requires no great change. Non-parasitic plants that grow on other plants are called epiphytes. This way of life is obviously very advantageous for small plants; for example, in dense forests, small plants growing epiphytically in the canopy can obtain more light, whereas on the lightless forest floor they could not thrive. The epiphytic way of life requires plants to have the ability to absorb and store water quickly, because the soil in tree forks or bark crevices is, after all, pitifully scarce, and water evaporates completely within a short time. Therefore, epiphytes usually have aerial roots to facilitate the rapid absorption of rainwater. Although epiphytes can store water well, they can only appear in large numbers in tropical regions with extremely abundant rainfall.

  In Germany there are only lower epiphytes such as lichens and mosses; although they dry out after each rain, this does not affect their survival. Higher epiphytes cannot tolerate drought, so they habitually store water. Unlike parasitic plants, epiphytes do not necessarily have to attach themselves to other plants; many epiphytes occasionally also grow on rocks. Cultivated epiphytes likewise grow well in flower pots, even more luxuriantly than when growing epiphytically on tree branches. The reason is that tree branches and trunks usually grow faster than the epiphytes, so even epiphytes growing in the canopy often cannot obtain sufficient sunlight. This has led, in the course of evolution, to the surface area of epiphytic cacti gradually increasing in order to capture more light. Their shoots are either flattened (such as leaf-like cacti or multi-segmented cacti) or only a few millimeters thick (such as Rhipsalis or Hatiora), while the dense spines originally used to protect them from animal attack have also become sparse or transformed into bristles.

  Although in this environment leaves (which had already degenerated and disappeared in the evolutionary history of cacti) would be of some benefit to the plants' growth, cacti did not regrow leaves; instead they produced flattened shoots to replace leaves. Because in the evolutionary history of cacti it is impossible for the same genetic trait to appear twice and reappear in the same way. Complex organs that have degenerated and disappeared (such as leaves) cannot be produced again; at most they will be replaced by completely different new organs (such as flattened shoots). This is an important law in biology (Dollo's law). Epiphytic cacti also originated from ordinary cacti, and this is shown even more clearly in their seedling stage. Adult epiphytic cacti often still retain many ribs and hard spines or stiff bristles. As typical plants of arid zones, cacti are first-rate at absorbing and storing water, and they can use their stored water sparingly to meet needs in dry times. This ability makes them suited to becoming epiphytes, because under such living conditions the water supply is relatively difficult.

  In addition, epiphytic cacti are also relatively difficult to propagate. If the seeds were scattered as with ordinary cacti, the seeds would fall to the dark forest floor rather than easily landing on tree branches. Therefore most epiphytic cacti produce fleshy berries that birds like to eat, and the seeds themselves are sticky, so when birds peck open the berries on tree branches, the seeds adhere to the twigs. Most epiphytic cacti retain the early dwarf shrub-like form, at least with woody characteristics at their base. The vast majority of climbing cacti are the same as epiphytic cacti. Their slender shoots can quickly grow toward high, bright places in the forest. Thin, slim cacti can easily cling to other plants or rocks with their spines without falling off.

  Twining cacti are even more skilled at climbing, because their shoot groups are long and grow in a spiral form, and people often call them snake cacti. Such cacti usually produce aerial roots that aid climbing. In addition, aerial roots also have a more important function, namely obtaining water. Because climbing cacti often have slender stems several meters long, water supplied by the roots is difficult to transport to the tips. When such cacti completely abandon their underground roots, they also evolve into epiphytic cacti, such as Epiphyllum. This kind of cactus, good at "crawling," should be the cactus type with the longest stems. For example, Hylocereus in Mexico can grow to a length of 100 meters. Epiphyllum and its closely related genera clearly demonstrate their kinship with epiphytic and climbing cacti . They are very similar to climbing snake cacti, and hybrids between these two genera have even appeared.

  In the evolutionary history of cacti, the morphology of most cacti resembles the primitive shrub type. New species usually arise from somewhat earlier primitive species, and the seedling-stage morphology is often retained into adulthood. This phenomenon is quite common in the evolutionary history of animals and plants. For example, herbaceous plants are generally explained as descendants of woody plants. As far as cactus evolution is concerned, the general rule is that in the course of evolution, the characteristics of woodiness and branching gradually diminished.