Torrey Pines Docent Society 2019 Cactus Adaptation and Conservation Cacti are in the Lodge garden, the Whitaker Garden, and almost all the trails in TPSNR, but there is a lot more to simply look for the silver Argiope spider and the cochineal. Cacti exhibit many specialized adaptive features to an arid climate. For children, the discussion can revolve around adaptation. For adults, one can highlight conservation issues that confront slow-growing cacti. Key points: • Green stem with spines? This is the first visual clue that cacti are specialized for something. What we observe are adaptations to survive in a dry climate. To reduce evaporative water loss (transpiration loss), cacti have several adaptations: ◦ Cacti make spines, which are modified leaves, to reduce surface area and to protect themselves from thirsty animals. (Nonetheless, there may be tiny foliage leaves on a very young stem.) ◦ The stem takes over the task of photosynthesis. Not only that, the stomata (microscopic openings for gas exchange) on the stem are closed during the day, and are only opened at night. ◦ Without sunlight, the plant cannot make use of carbon dioxide immediately, and so it stores the gas as malic acid, which is the taste of sour apples. During the day, the plant converts malic acid back to carbon dioxide for photosynthesis. ◦ To reduce surface area, the entire plant is made of stems or stem segments. At the extreme, Barrel cactus takes on a globe-like shape. ◦ Cacti may have ribs or tubercles which allow the stem to expand or contract depending on their water content. ◦ The stem has a heavy waxy surface (cuticle). • Some specialized spines on the barrel cactus make a sugar solution to attract ants. The ants in turn drive away insect herbivores in a food-for-protection mutualism. • For protection, prickly-pear-related cacti also make glochids, essentially tiny, hair-like spines. They appear at the base of spines and on the fruits, which become the prickly-pears. • The adaptations to survive in a dry climate come at a cost and that is cacti grow very slowly. As a result, many cacti and some succulent species are threatened because of loss of habitat and illegal poaching for gardening and home décor. What is interesting about cacti? Most people know what a cactus is. They are the ones with succulent green stems and fierce spines. We may consider cacti as iconic of the American Southwest deserts, but the cactus family, Cactaceae, is quite diverse throughout the Americas. There are about 127 genera and close to 2,000 species in the family. The plants also have a large variation in shape and size. Members of the family can be found in hot deserts, cold deserts, dry grasslands, coastal sage scrub, chaparral, and even alpine zones above the treeline. Regardless, these are all semi-arid or arid habitats. Where are the leaves? What are the roles of the spines? Why doesn’t the Indian fig have spines? Why are the stems green? Is this how I can survive in the desert? In short, cacti are extremely well adapted to dry environments—they are xerophytes. As aforementioned, cacti live in arid or semi-arid habitats, and as such they are highly specialized to survive in such environments. They carry out photosynthesis in their stems, and the spines can be considered as modified leaves to protect them from thirsty animals. The long answer follows. We shall look into their unique adaptations or specializations. The descriptions will focus on the key species in TPSNR. They are the Coast prickly- pear (Opuntia littoralis), Coastal cholla (Cylindropuntia prolifera), the rare Coast barrel cactus (Ferocactus viridescens), and the introduced Indian fig (Opuntia ficus-indica). A few illustrations also make use of species from Anza-Borrego. Figure 1. Left: Coast barrel cactus (Ferocactus viridescens) with prominent ribs; Right: A close up on the two different types of spines. (The smaller, finer white ones might have secretory glands at the base, but no confirmation of this yet.) 2 Figure 2. Coastal cholla (Cylindropuntia prolifera). Left: A young stem made of tubercles; Right: A grown, expanded stem with tubercle-like undulations. Cactus morphological features The cactus stem has many specialized features. For one, the stem has taken over the task of photosynthesis and this will be revisited below. Here, we look into the structural characteristics, and see how the stem is specialized for water conservation. Stem structure and shape: Without leaves, cacti are easily recognizable by their stems. The stems generally can be cylindrical or flattened. Formally, a modified stem that replaces the functional role of leaves is a cladode.1 Columnar cacti are the ones with a main cylindrical cladode that we commonly call chollas. One example is the Coastal cholla (Cylindropuntia prolifera). We refer flattened cladodes as pads. Two examples are the Coast prickly-pear (Opuntia littoralis), and Indian fig (Opuntia ficus-indica). All cacti take on some compact shape that may reduce the surface area. Here, the Coast barrel cactus (Ferocactus viridescens) is probably the most specialized. Its globular (globe-like) shape has a very small surface area to volume ratio, and is ideal for water storage.2 Many cacti have ribs and tubercles, which are small Figure 3. The woody nodule-like projections. Ribs are most distinct with a barrel skeleton inside Buckhorn cholla (Cylindropuntia cactus (Fig. 1). These structures allow the cactus stem to acanthocarpa) in Anza- expand or contract depending on water content. The ribs in Borrego. 1 Another term that one may come across is phylloclade, a modified branch. For cactus, cladode is the proper term. There is another similar spelling term, phyllode, which is a modified petiole that resembles and functions as a leaf. This is what we find in acacia (genus Acacia in Fabaceae) plants. 3 particular make a folded surface that impart strength to the structure. The groves on a ribbed surface also help to channel rainfall to the roots. When the sun is at an angle, the ribs provide a partial shade. Some cacti, notably the barrel cacti (genus Ferocactus) are known to tilt slightly toward the sun, especially on a slope, such that only the smaller top faces direct sun. Cacti with flattened stems have neither distinctive ribs nor tubercles. A columnar cactus such as the Coastal cholla can have tubercle-like undulations (Fig. 2). The Coastal cholla is unusual; its young cladode is made of tubercles. As the stem grows, the tubercles swell and the bases stretch out to become tepee-like. This is also the growth pattern with pincushion cacti (genus Mammillaria). Unlike coconuts, we will not find liquid water. Cacti use water storage tissues, specialized thin-walled cells with a large water vacuole. The stem of a well hydrated Indian fig is close to 90% water. Some cacti can convert water into a mucilaginous substance that does not evaporate readily and lowers the freezing point of the tissue such that the cacti can survive cold winters. Figure 4. Two views of glochids and spines arising from areoles in Coast prickly-pear (Opuntia littoralis). To seal in moisture, cacti have thick, waxy cuticle on the surface. To reduce transpiration loss, cacti practically have no leaves (more on this with the spines below), and the stem takes over the photosynthesis function. Even though a cactus is mostly water, it is considered a woody plant. All cacti make a vascular cambium that produces both secondary (woody) xylem and secondary phloem.3 However, the xylem does not become heartwood—the plant uses the space for water storage tissue. The secondary xylem forms instead a hard fibrous matrix. This is most 2 Given a solid object, the sphere has the smallest surface area to volume ratio. 3 New, primary growth makes xylem that does not contain lignin, the substance of woody tissue. Subsequent growth of a stem laterally makes xylem that is lignified, and is referred to as secondary xylem, or as sapwood. 4 evident with a dead and partly decomposed cholla that exposes a cylindrical chain link fence-like woody skeleton (Fig. 3). Figure 5. Two cacti with dense spine clusters. Left: Velvet cactus (Bergerocactus emoryi) planted near the Lodge garden; Right: Teddy-bear cholla (Cylindropuntia bigelovii) at Anza-Borrego. Areoles, spines and glochids: With cacti, we do not find recognizable branches. Instead, the plants bear areoles, structures unique to the Cactaceae family. An areole is a much reduced shoot (axillary bud4) that appears as either a little pad or even a slight depression or pit under dry conditions, but from which grows the spines, glochids, flowers, stems (cladodes), and stem arms on the saguaro in the Sonoran Desert.5 In short, the small oval thing at the base of the spines of a cactus is an areole (Fig. 4). Among things that grow from an areole, spines are the most evident, and they generally appear in clusters. We can say that spines are modified leaves.6 To be more precise, cactus spines are modified scales grown from an axillary bud (areole). As a young spine grows from a bud, it quickly turns into a dead lignified fiber.7 Sometimes, the spines are covered by a loose sheath formed from the dried epidermis. This is presumably common with the Cylindropuntia species, but this is not evident in the Coastal cholla. (But in Anza- Borrego, we can observe that with the Buckhorn cholla, Cylindropuntia acanthocarpa.) 4 Areole is from the French aréole, which is taken from the Latin, areola, the diminutive of area, referring to a small open space or garden. In green plants, an axillary bud is an embryonic shoot located on the upper edge at the base of a petiole.