Bromeliad family (Bromeliaceae)

views updated

Bromeliad family (Bromeliaceae)

The pineapple family (Bromeliaceae) consists of about 1,500 species of flowering plants. Most species are medium-sized herbs with tightly packed, thick, stiff, spiraling leaves that usually have spiny margins. Some species are semi-woody, and a few rare ones, such as Puya raimondii, are trees that can reach 33 feet (10 m) in height. Most species of bromeliads are epiphytes in rainforests, while others are terrestrial, inhabiting dry habitats in mountainous and coastal regions. Bromeliads are native to tropical and subtropical regions of North and South America. The only exception may be Pitcairnia feliciana of coastal west Africa; this species appears to represent a remarkable example of long-distance, natural dispersal, although it may have been introduced into Africa by humans. The range of the bromeliads extends from Tierra del Fuego in southernmost South America, northward into Mexico, then through the southeastern United States to their northernmost limit in southern Maryland.

The most commercially valuable member of this family is the common pineapple, Ananus comosus. A native of South America, the pineapple has been widely planted as a cash crop throughout tropical and subtropical regions, especially in the Hawaiian Islands, the Philippines, South Africa, and southern Asia. The pineapple is a biennial plant, that is, it usually lives for only two years. In the first year this species produces a dense growth of sharp-pointed, overlapping leaves. During the second year a short stalk with many flowers is produced. Each flower gives rise to fruits that are technically berries, but then the stalk bearing the fruit begins to swell. This

results in the development of a thick, sweet, fleshy mass, within are embedded the fruits. This whole thing is the “pineapple,” which is therefore an aggregation of fruits within an accessory structure (the thickened stem). When you buy a pineapple at the market you notice at the top of the “fruit” a tuft of prickly, reduced leaves or bracts, and a cleanly cut-off base. This may lead you to believe that the pineapple was cut off from the plant at ground level, but this was not the case. Pineapples grow at the end of shoots, two to four per plant, so they are cut off from an underlying, large-leafed shoot.

Bromeliads are xerophytes and possess many of the usual, water-conserving adaptations of such plants: a thick epidermis covered with wax, water-storage cells that cause the leaves to appear succulent (that is thick and fleshy), and sheathing leaf bases. One of the notable characteristics of bromeliads is the distinctive, water-absorbing scales on their leaves and stems. These thin scales occur in grooves in the

epidermis, and they resemble opened umbrellas. Their thinness and large surface area make the scales ideal for rapidly absorbing water.

Equally important in terms of water economy are the shape and arrangement of the leaves of bromeliads. In many species the leaves are wide and deeply U-shaped where they join the stem, forming a series of vessel-like compartments. When it rains, water flows down the leaves and pools in the compartments, where it can be absorbed by the umbrella scales. Especially remarkable are the “tank plants,” such as Nidularium and Billbergia. In these species the stem is greatly reduced and the densely packed leaves have broad, overlapping bases, resulting in a pitcher or vase-like center—the tank. Rainwater fills the tank, where some of the moisture is absorbed by the umbrella scales. Because the tank is shaded by the dense crown of leaves around it, the water does not evaporate quickly and can persist, enabling the plant to survive periods of drought. Interestingly, some species of mosquitoes breed nowhere else but in these tanks.

Another important adaptation of some bromeliads to drought-prone environments is seen in their stomata. Stomata allow gaseous carbon dioxide, a necessary ingredient for photosynthesis, into the leaf. In most plants the stomata are open during daylight hours, because that is when light is available to drive photosynthesis. When open during the day, however, stomata also lose water, which is a disadvantage for plants growing in environments where water is scarce. Many plants, including the bromeliads pineapple and Spanish moss, have evolved stomata that are closed during the heat of the day but open at night when temperatures are cooler. These plants trap the carbon dioxide that enters through their stomata and store the gas until daylight, when light energy is available for further processing through photosynthesis.

Why are some plants of wet tropical and subtropical regions, such as bromeliads, specially adapted to dry conditions? The key is that most bromeliads are epiphytic, occurring high in the crowns of forest trees. When it is not raining and the sun is out, these plants can dry out rapidly because their roots are not in soil. In addition most bromeliads that are terrestrial generally occur in rocky habitats, where rainwater rapidly percolates through the soil, leaving it dry.

Spanish moss (Tillandsia usneoides ) is not a true moss but rather is one of the interesting bromeliads known as “air plants.” Most of these species grow as epiphytes high in the crowns of trees, where of course there is no soil. In order to obtain the nutrients necessary for growth, air-plants absorb some directly from the atmosphere, such as gases of sulfur and nitrogen. Other nutrients must be absorbed from rainwater or atmospheric dusts. Air-plants are commonly available in flower shops. These attractive plants need to be misted occasionally with a sprayer, because their foliar scales must be moistened to absorb the nutrients. Because they derive most of their nutrients directly or indirectly from the atmosphere, these bromeliads are called air plants. Spanish moss commonly grows in the southeastern United States, where it often forms large and beautiful drapes in tree canopies. The roots of Spanish moss serve to hold the plant to the tree on which it is growing, and do not function in obtaining water and nutrients.

Bromeliads have become increasingly popular as indoor plants. Air plants are appealing novelty items, but many of the other, leafier bromeliads are also now prized. These have attractive, usually bright red flowers, designed to attract birds as pollinators, and often surrounded by brightly colored modified leaves. Furthermore, as xerophytic plants, bromeliads can withstand the benign neglect of forgetful watering.