About Chamaerops humilis L.
Chamaerops humilis L. is a clumping, shrub-like palm that grows multiple stems from a single base. It produces shoots from an underground rhizome, and these shoots bear stiff, leathery palmate leaves. Its stems grow slowly, often growing tightly together, and mature stems reach 2–5 m (10–20 ft) tall with a trunk diameter of 20–25 cm (8–10 in). This species is a fan palm belonging to the Arecaceae tribe Corypheae, so its leaves have petioles that end in rounded fans made of 10–20 leaflets. Each full leaf can be up to 1.5 m (5 ft) long, with individual leaflets measuring 50–80 cm (20–30 in) long. The petioles are covered in many sharp, needle-like spines, which likely protect the growing stem tip from grazing animals. Flowers grow in dense, short inflorescences at the tops of stems. Most, but not all, individuals are dioecious, meaning male and female flowers grow on separate plants. A prophyll covers the inflorescence’s flowers until anthesis (the sexual flowering phase), when it splits open at the top into two triangular lobes. The number of flowers per inflorescence varies widely for both male and female plants, depending on the size of the inflorescence. Female flowers have three ovules. Unripe fruits are bright green, and ripen to a dull yellow or brown during autumn, between September and November. Most individual seeds weigh 0.6–0.8 g (1⁄50–3⁄100 oz), and contain a small cylindrical embryo surrounded by distinct layers from inner to outer: a nutritious endosperm, a wide woody endocarp, a fleshy fibrous mesocarp (pulp), and a thin outer exocarp. Chamaerops humilis is one of only two palm species native to continental Europe, the other being Phoenix theophrasti. It occurs naturally mainly in southwestern Europe, across all southern coasts of Spain and Portugal, central and southern Italy, parts of the southern Mediterranean coast of France and Monaco, and the islands of Malta, Sicily, Sardinia, Corsica, and the Balearic Islands. It is also native to North Africa in Morocco, Algeria, and Tunisia. It is one of the northernmost naturally occurring palm species in the world, with its northernmost native population growing in Genoa, Italy at 44°N. While it does not grow naturally far north of the Mediterranean, it grows well in cultivation as far north as southern England. This species is widely distributed across uncultivated land, and is adapted to survive frequent wildfires by re-sprouting from its underground rhizomes and fire-damaged stems. This ecological trait makes it important for preventing erosion and desertification, and for providing shelter and food to many animal species. Chamaerops humilis flowers in spring, typically from April to May. It may be partly pollinated by wind (anemophilous), but it is at least partly pollinated by insects (entomophilous). Only one insect species is known to pollinate it: the specific weevil Derelomus chamaeropsis, which belongs to the family Curculionidae. The relationship between the palm and this weevil is a form of nursery pollination mutualism. When a pollinating weevil finds a suitable plant (either male or female), it usually stays on that plant until the end of anthesis, using the inflorescence for shelter, as an egg-laying site, and as a food source. During anthesis, both male and female plants attract pollinators with chemical compounds, which is common among flowering plants. An unusual feature of this interaction is that the scent attractive to weevils is released by the palm’s leaves, not its flowers. Near the end of anthesis, weevils leave the plant to seek a new host plant, which may be either male or female. Larval development of Derelomus chamaeropsis occurs inside the inflorescence rachises of male plants during autumn and winter. At the start of the next flowering period, adult weevils emerge from the dry, brittle stems of the previous year’s male inflorescences only. Weevil larvae that hatch in female plants die before completing development. This is an adaptation of the palm to keep pollinating weevils from destroying female inflorescences that carry developing seeds. Weevils will lay eggs inside female inflorescences, but as soon as seeds begin to develop, the eggs or larvae cannot complete their life cycle. Male inflorescences finish their reproductive role after pollination, so it benefits the palm for weevils to complete their life cycles and shelter in male inflorescences, so adult weevils are available to pollinate new flowers when they emerge the following season. The ripe fruit pulp of Chamaerops humilis serves several key functions. Ripe pulp has a strong odor similar to rancid butter, which acts as a foraging cue for nocturnal frugivores, which are often primarily carnivorous mammals like badgers and foxes. The pulp inhibits seed germination, which ensures seeds do not germinate before they have been dispersed away from the parent plant. The pulp also acts as a chemical or physical barrier against invertebrate seed predators, most commonly beetles, particularly weevils. This combination of functions in the pulp means the palm experiences both benefits and drawbacks when carnivores feed on its fruit and remove the pulp. Seeds swallowed by carnivores germinate more often than seeds that remain inside whole fruits, but ingested seeds are more likely to be destroyed by invertebrate pests than non-ingested seeds. Even so, the dispersal service provided by mobile carnivores is important for this palm, because most of its native populations are severely fragmented and isolated across the increasingly densely populated Mediterranean basin. The fungus Coniothyrium palmarum, which has been found associated with leaf spots on the date palm Phoenix dactylifera in India and Cyprus, has also been found on a dead petiole (leaf stalk) of Chamaerops humilis in Italy. Chamaerops humilis is valued for gardening and landscaping in many regions around the world. Once established, it is very tolerant of drought. It can survive temperatures as low as −12 °C (10 °F), and prefers hot summers. It grows very slowly. A blue-colored form of the species, native to high elevations in the Atlas Mountains, has recently been made available in the horticultural trade, and early reports indicate it can tolerate temperatures 12 °C (22 °F) or more colder than the common green form. It is widely cultivated across the Mediterranean region of Europe, as well as in southern portions of the United Kingdom. In the United States, it is cultivated along the West Coast from southern California to the Pacific Northwest coast, along the East Coast from Georgia to Maryland, and in several areas of the drier southwestern United States. It is also cultivated in New Zealand and several parts of Australia. This species has received the Royal Horticultural Society's Award of Garden Merit. In addition to its practical uses, this palmetto holds cultural value as a recognizable iconic component of Mediterranean coastline garrigues and macchias. Leaves from adult plants have been used to make brooms, and to weave mats, carrier baskets, and similar items. Young, unopened leaves are treated with sulphur to soften them for finer work, producing supple fiber. The young husk, called "higa" in southern Spain, is edible before it becomes too tough as it matures. The mature fruit is not used for human food due to its bitterness and high tannin content, but it has been used as an astringent in traditional medicine.
