Bromeliaceae, members of which are called bromeliads (), is a family of monocot flowering plants of about 80 genera and 3700 known species, native mainly to the tropical Americas, with several species found in the American subtropics and one in tropical west Africa, Pitcairnia feliciana. It is the sister group to the Typhaceae, and together this clade is sister to the remaining lineages in the Poales. Bromeliaceae is the only family within the order that has septal nectaries and inferior ovaries. These inferior ovaries characterize the Bromelioideae, a subfamily of the Bromeliaceae. The family includes both epiphytes, such as Spanish moss (Tillandsia usneoides), and terrestrial species, such as the pineapple (Ananas comosus). Many bromeliads, colloquially called "tank bromeliads", are able to store water in a structure (a "tank") formed by their tightly overlapping leaf bases. However, the family is diverse enough to include the tank bromeliads, grey-leaved epiphyte Tillandsia species that gather water only from leaf structures called trichomes, and many desert-dwelling succulents. The largest bromeliad is Puya raimondii, which reaches 3–4 metres (10–13 ft) tall in vegetative growth with a flower spike 9–10 metres (30–33 ft) tall, and the smallest are some Tillandsia species.
Description
Bromeliads are mostly herbaceous perennials, although a few have a more tree-like habit. Many are more or less succulent or have other adaptations to resist drought. They may be terrestrial or epiphytic, rarely climbing (e.g. Pitcairnia species). Some species of Tillandsia (e.g. Spanish moss, Tillandsia usneoides) are aerophytes, which have very reduced root systems and absorb water directly from the air. Many terrestrial and epiphytic bromeliads have their leaves in the form of vase-shaped rosettes which accumulate water. These rosettes can hold as much as 10 L (2.2 imp gal; 2.6 US gal) of water, and be little biotic communities unto themselves. One individual tank was found to contain four harvestmen, a spider, three species of wood lice, a centipede, a millipede, a pseudoscorpion, various metallic beetles, earwigs, a tree seedling, chironomid fly larva, an ant colony, an earthworm, numerous mites, and a small frog. Individual leaves are not divided and have parallel veins without cross connections. The epidermis of the leaf contains silica. Bromeliad flowers are aggregated into inflorescences of various forms. The flowers have bracts, often brightly coloured, and distinct calyces of three sepals and corollas of three petals. The flowers have nectaries. They are pollinated by insects, birds (often hummingbirds) or bats, or more rarely (in Navia) they are wind-pollinated. Fruits are variable, typically taking the form of a capsule or a berry. Bromeliads are able to live in an array of environmental conditions due to their many adaptations. Trichomes, in the form of scales or hairs, allow bromeliads to capture water in cloud forests and help to reflect sunlight in desert environments. Bromeliads with leaf vases can capture water and nutrients in the absence of a well-developed root system. Many bromeliads also use crassulacean acid metabolism (CAM) photosynthesis to create sugars. This adaptation allows bromeliads in hot or dry climates to open their stomata at night rather than during the day, which reduces water loss. Both CAM and epiphytism have evolved multiple times within the family, with some taxa reverting to C3 photosynthesis as they radiated into less arid climates.
Evolution Bromeliads are among the more recent plant groups to have emerged. They are thought to have originated in the tepuis of the Guiana Shield approximately 100 million years ago. The greatest number of extant basal species are found in the Andean highlands of South America. However, the family did not diverge into its extant subfamilies until 19 million years ago. The long period between the origin and diversification of bromeliads, during which no extant species evolved, suggests that there was much speciation and extinction during that time, which would explain the genetic distance of the Bromeliaceae from other families within the Poales. Based on molecular phylogenetic studies, the family is divided into eight subfamilies. The relationship among them is shown in the following cladogram.
The most basal genus, Brocchinia (subfamily Brocchinioideae), is endemic to the Guiana Shield, and is placed as the sister group to the remaining genera in the family. The subfamilies Lindmanioideae and Navioideae are endemic to the Guiana Shield as well. The West African species Pitcairnia feliciana is the only bromeliad not endemic to the Americas, and is thought to have reached Africa via long-distance dispersal about 12 million years ago.
Radiation of Tillandsioideae and Hechtia The first groups to leave the Guiana Shield were the subfamily Tillandsioideae, which spread gradually into northern South America, and the genus Hechtia (Hechtioideae), which spread to Central America via long-distance dispersal. Both of these movements occurred approximately 15.4 million years ago. When it reached the Andes mountains, the speciation of Tillandsioideae occurred quite rapidly, largely due to the Andean uplift, which was also occurring rapidly from 14.2 to 8.7 million years ago. The uplift greatly altered the region's geological and climatic conditions, creating a new mountainous environment for the epiphytic tillandsioids to colonize. These new conditions directly drove the speciation of the Tillandsioideae, and also drove the speciation of their animal pollinators, such as hummingbirds.
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