Biology Department

The term "tropical forest" brings to mind images of lush green jungles of the Amazon. Tropical forests, however, embrace more than rain forest, they also include seasonally wet forests and dry forests. Likewise the term "temperate forest" conjures up thoughts of a Pennsylvania oak-hickory forest or a Michigan beech-maple mature forest. However, in spite of their name, temperate forests do not always exist in a temperate environment. They occupy topographic positions that range from low-lying lands to mountaintops, and environmental condi tions that range from warm and semiarid to cold and wet. Temperate forests include coniferous, deciduous, or mixed stands.


Tropical East

The cycads are palmlike plants (to your right and left on the tables just past the carnivorous plants) found mainly in the tropics and subtropics. These bizarre plants, though relatively uncommon today, were so numerous in Mesozoic times (220 million years ago) that this era is often called the "Age of Cycads and Dinosaurs". We have two representatives of this group in the Bucknell collection, Zamia and Cycas. In cycads, pollen and seed cones are borne on different plants.

The ferns are unique among seedless vascular plants in their possession of megaphylls, a large leaf with several to many veins. Ferns are abundant in the fossil record from the Carboniferous period (some 300 million years ago) to the present. In both form and habitat, ferns exhibit great diversity, ranging from the small aquatic Azolla to huge tree ferns to epiphytes (air plants). Many ferns have a fleshy underground rhizome with adventitious roots arising near the bases of the leaves. The leaf or frond is the conspicuous part of the sporophyte. Commonly the fronds are compound with the lamina divided into leaflets, or pinnae, that are attached to the rachis, an extension of the leaf stalk, or petiole. Typically the young leaves are coiled in the bud and appear like "fiddleheads". Most ferns are homosporous with the sporangium (spore producing structure) on the lower leaf surface, on specially modified leaves, or on separate stalks. The sporangia commonly occur in clusters called sori.


Croton Lvs

Forests that occur at relatively low elevation and have year-round rainfall are usually known as "tropical rain forests". The following features are characteristic of such forests.

Evergreen plants. "Evergreen" does not mean that such plants never lose their leaves. It means that individual leaves remain on the plant for one or more years. Thus, the plants always have younger leaves present when older ones are shed.

Trees predominate. The high rainfall encourages growth of trees. The canopy formed by the dense tree growth lets very little light through to the forest floor, so an understory of herbaceous plants and woody shrubs is discouraged. The floor of a rain forest is generally open as rain forests are not the "impenetrable jungle" that you may have heard about. Jungle occurs along streams and rivers or where disturbance has resulted in enhanced light levels at the forest floor.

Rich in vines. Over 90% of known species of climbing plants occur in tropical rain forests. Climbing plants have an advantage over trees in that they can grow rapidly into the high light environment of the forest canopy without expending a lot of energy on trunk growth.

Epiphytes are abundant. One solution to the problem of obtaining light in a dense rain forest is to grow attached to the trunks or branches of trees. About 25,000 epiphytic species are known (10% of all vascular plants) with the vast majority being epiphytic orchids.

Shade-tolerant epiphytes grow on the trunk or on the lower branches of trees and usually do not have special adaptations for preserving water since they are not exposed to direct sunlight. Shade-intolerant epiphytes, growing in the upper branches of the canopy where they are exposed to the desiccating effect of bright sun and wind, are invariably xeromorphic.

Simple leaves with entire margins are common. Simple refers to leaves that are not divided into leaflets (compound) and entire refers to a lack of lobes on leaf margins. Leaves on trees such as oaks or maples have well-developed lobes while Magnolia leaves are entire.

Drip tips, pointed extensions of the leaf, are common in tropical rain forests. Of 41 species studied in tropical rain forests of Sri Lanka, 37 had drip tips. Their function is uncertain, but is has been well argued that drip tips enhance water runoff and thus leaf drying - a feature that would decrease the epiphytic load of algae, fungi, lichens, and mosses on leaf surfaces.

Dieffenbachia and Monstera spp. (Araceae, same family as philodendrons and skunk cabbages). These plants are climbers in the rain forest, although here they are potted.

Monstera deliciosa owes its wonderful name to its delicious, edible fruits. These plants look much like the philodendrons, but notice the holes in the leaves. The holes allow light to reach the lower leaves that would otherwise be completely shaded.

Ficus elastica (fig), Psidium cattleianum (guava), and Aglaonema sp. are excellent examples of plants with drip tips.

Strelitzia reginae (bird-of-paradise) is bird pollinated; it provides a wobbly landing platform from which birds can feed. However, as the birds rock back and forth, they get pollen spread on their bodies. Try wiggling the platform. Can you envision how it works?

Look for: EPIPHYTES (air plants)

Epiphytic orchids. These plants are mostly in pots in our greenhouse growing in Osmunda fiber, but in the tropical forests where they are native they would be attached to tree trunks and branches. Examine our demonstration tree trunk (on the west windowsill) with epiphytic orchids and bromeliads growing in a more natural state. Look for those xeric features that are associated with orchid species which are shade-intolerant epiphytes.

-thick, leathery leaves
-pseudobulbs (swollen, water-storing stem regions)
-air roots covered with velamen (a special water-storing
tissue...appears white on dry roots)

Look at any orchid flowers that may be in bloom and note their complexity. The various colors, shapes, knobs, and bumps of each flower are the end product of coevolution with some particular species of insect, usually a bee. The specialization is often so narrow and the relationship so symbiotic that neither the plant nor the insect species can survive if the other goes extinct.

Epiphytic ferns. Look for the staghorn ferns hanging from the ceiling, Platycerium sp. These ferns have 2 kinds of leaves: the antler-shaped leaves do most of the photosynthesis while the rounded shield leaves are pressed flat against tree trunks to create a space in which debris collects. Staghorn fern roots penetrate this debris to pick up water and minerals. Often there is a symbiotic relationship between this fern and ants which colonize the debris. The ants feed on an amino acid-rich nectar produced by glands on the leaves and the plants gain minerals from debris more rapidly because of ant-enhanced breakdown of debris.

Epiphytic bromeliads. There are two types of epiphytic bromeliads (pineapple family) in this room.

Tank plants. We have numerous examples of this type growing along the glass wall separating the research house from the museum greenhouse. You can easily see how the tightly imbricated leaves (overlapping and pressed against each other) produce water-storage tanks. The tanks, evolved to catch and hold rain water, are miniature ecosystems with many other organisms living in them. Animals found in bromeliad tanks range from mosquito larvae to frogs.

"Air plants". You will see another type of epiphytic bromeliad on the misting water supply over the carnivorous plants. This one is the familiar "Spanish moss", Tillandsia usneoides. It is a common epiphyte in the southeastern U.S., growing not only in tree canopies but also on telephone wires. They have no mechanism at all for storing water like their relatives, the tank plants. The white fuzz you see covering the plants is an adaptation which these plants evolved to obtain water. This dense coating of complex hairs or scales is highly specialized to absorb dew or occasional rain water and transfer water to the internal photosynthetic tissues. Minerals are obtained in small quantities from dust or from rain water.

If you look carefully, you may see a tiny greenish-yellow flower here and there. Vegetative form is so variable that it is nearly useless in classifying plants. It is the flower which enables botanists to place this plant in the pineapple family.

Flowers are very conservative, changing little within a family, while there is rapid and extreme evolution in vegetative form. Can you think of an explanation for this fact?

Look for: CONIFERS

The conifers include the redwoods, pines, firs, spruces, and the like -- many examples of these can be seen on our campus. The history of the conifers extends back at least to late Carboniferous times (290 million years ago). Their leaves have many drought-resistant features, and perhaps the origin of conifers occurred during the dry Permian period, when increasing world-wide aridity may have provided a powerful evolutionary stimulus.

Two conifer families, both primarily from the southern hemisphere, are represented in our greenhouse collection, both growing in the rain-forest center section, the Podocarpaceae by Podocarpus macrophylla and the Araucariaceae by Araucaria bidwillii. Podocarpus ranges north of the equator in Central America, Africa, and from Malaya to Japan. Curiously, the Podocarp family was formerly widespread in Asia, Europe, and North America, as well as the southern hemisphere. Withdrawal to its present range occurred in later Tertiary times (7 million years ago). Araucaria has about 16 species ranging from New Guinea to Australia and also occurs in Chile, Argentina, and Brazil.

Asparagus Fern


The Bucknell Greenhouse contains a good number of economically important plants of the tropics. Coffee, the savory culmination of the aristocratic banquet, substance of the beggar's plea, and morning eye-opener, is produced in the New World tropics and is largely consumed in the Americas and Europe. There are many species but the prominent of these is Coffea arabica. The coffee plant is a shrub with glossy, deep-green, opposite leaves which like the fruit, contain caffeine. Fragrant white axillary flowers are borne two or three times a year in flowering seasons that correspond to times of dryness. The best and most productive coffee is grown in highland habitats, where constant, moderate temperature and frequent cloud mists stimulate growth.

Tea consists of the dried tip leaves of Camellia simensis native to southwestern China and northeastern India. The tea plant, left unpruned and unplucked, grows as high as 10 m. It bears thick, alternate, elliptic, serrate leaves, which possess numerous oil glands (the essential oils they contain produce the tea flavor) and the alkaloid caffeine. Some teas (Brazilian) contain high tannin content and give "body" to the beverage.

Among the wide diversity of tropical fruits, the banana (Musa species) stands out as one of the finest. The plantain (a starchy banana) bears a starchy, green-skinned fruit that is an important staple in the tropics. It is eaten baked, boiled, or fried. The banana plant is a tall, coarse herb rising as much as 10 m from a fleshy rhizome. The ensheathing bases of the petioles form the "stem" of the banana "tree". Each "stem" bears a terminal pendulous inflorescence in eight to 14 months, and after fruiting dies.

Citrus fruits, represented in our collection by Citrus limon (lemon) and C. mitis (calamondin), include oranges, lemons, limes, tangerines, grapefruits, kumquats, and more. The majority of citrus species are native to southeastern Asia but are now cultivated world-wide in subtropical climates. The commonly cultivated species are small trees with mycorrhizal roots and oil-gland-bearing leaves.

The pineapple is a native of the western hemisphere tropics. The pineapple genus, Ananas, is a member of the Bromeliaceae, a family predominantly composed of epiphytes. It is a coarse, rosette-form terrestrial biennial with long, stiff leaves that are usually spiny-margined. An erect central stalk bears an oblong head of flowers that upon maturation produces the characteristic multiple fruit.

The loquat (Eriobotrya japonica) bears plum-size fruits reminiscent in flavor of apples or pears (it is in the same family, the Rosaceae). It is widely grown in Japan, China, India, and the Mediterranean area, and to some extent in the New World subtropics.

Guava (Psidium species) has been distributed to practically all tropical lands from the Amazon basin. It often becomes naturalized and behaves as a weed. The guava is a small tree with coarse, opposite leaves. The white axillary flowers arise on new wood and develop the globose, gritty acidic, apple-like fruits used to make a dessert marmalade. Guavas can also be eaten fresh or in pie, or the juice extracted to make a drink.

Cardamom, an ingredient of curry powder and seasoning in many types of sausage, is the highly aromatic, dried fruit and seed of Ammonum cardamom. Cardamom plants are tall, coarse, large-leaved perennial herbs typical of the ginger family.

Vanilla is obtained from the fermented pod of climbing orchids (genus Vanilla) indigenous to tropical America.


Dischidia sp. (milkweed family). These are fleshy epiphytic plants of SE Asia. Many species bear inflated, hollow leaves which are specialized as "ant leaves". During droughts which cause abscission of other leaves, the ant-leaves persist.

Myrmecodia sp. (coffee family). Notice the fattened, tuberous stem. Although it may not be apparent, this plant is also an epiphyte. The tubers contain a network of large cavities in which ants live. The usefulness of the ants to the plant is their provision of mineral elements to the plants. In return, the plants bear ant-attracting nectaries. What are some of the possible ways in which the presence of ants might be advantageous to plants?


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