The Orchidaceae originated as terrestrial forest under-story herbs approximately 100 million years ago. The transition to an epiphytic canopy habitat required adaptations in plant morphology. Orchids have specialized adaptations in the roots, stems, leaves, and seed. Epiphytic orchids have no vascular connection to the host tree. The host only supplies support in a habitat that has more sunlight than the forest floor. Orchids absorb required nutriments from the surface of the host and rainwater.Orchid roots function as anchorage for the plant, photosynthesis, and water and nutrient uptake and storage. These adventitious roots typically arise from the rhizome. Orchid roots have a spongy layer of cells outside the exodermis known as the velamen that functions for temporary water storage. These cells rapidly absorb rainwater (and nutrients) and hold it until it can be translocated across the exodermis into the vascular system. Roots of epiphytic orchids are exposed to the light and the cells in the roots contain functioning chloroplasts. This is why wet orchid roots appear green in color. Velamen can also be found in Aroids that are adapted to an epiphytic habit.
Epiphytic orchids often have enlarged portions of the stem called pseudobulbs, which are used for water and carbohydrate storage. The pseudobulb may form in one internode or it can consist of several internodes. The pseudobulbs swell or shrink as moisture is stored or withdrawn. This adaptation allows orchids to flourish in areas with seasonal rainfall where the plants experience months without rainfall. The pseudobulbs and leaves have a thick cuticle to reduce moisture loss.The leaves of a plant are the primary photosynthetic organs that are sometimes modified for water storage. Some orchids have thick succulent leaves and no pseudobulbs. Orchids have a modified photosynthetic pathway as an adaptation to the dry canopy habitat. The opening of the stomata to take up carbon dioxide is always connected with large losses of water. To inhibit this loss, Crassulacean acid metabolism (CAM) has a mechanism that allows the uptake of carbon dioxide during the night when relative humidity is higher. The prefixed carbon dioxide is stored in the vacuoles and is used during the daytime for photosynthesis.
Orchid seed are adapted for wind disbursal. The dust-like seed consist of a tiny embryo and a net-like testa. The seed lack endosperm, the 3N tissues that typically feed a developing embryo. In orchids when germination occurs a mycorrhizal fungi penetrates the testa and feeds the embryo. This symbiotic relationship also occurs in the seed germination of terrestrial orchid species.Although Orchidaceae is a member of the monocotyledons, the embryo lacks a cotyledon. However orchids do have the general characteristics of other members of the lower Asparagales: mycorrhizal relationships, simultaneous microsporogensis, sympodial growth, inferior ovary, sepal nectaries, and lateral inflorescences.
Figure 1 Velamen Line Drawing