Orchid roots have many unique features. One of these is the green root tips found on aerial roots in some epiphytic species. Green orchid root tips indicate a healthy, well-watered plant that indicates areas of active root growth. Conversely, excessive salts in the soil and over-fertilization will shrivel the roots and turn them brown.
Green orchid root tips contain photosynthetic structures with green pigmentation: chlorophyll and chloroplasts. These are typically only found on a plant’s leaves and stem. Like other parts of the plant that contain photosynthetic structures, if a portion of the green root tip is blocked from receiving sunlight, they will become pale. This is what happens when green aerial roots find their way into the ground: they lose their chlorophyll and become more branched. Once exposed to light, however, the roots supplement foliar photosynthesis (photosynthesis that occurs in the leaves).
Photosynthetic structures are also present in the rest of the orchid root away from the tips. However, most of the roots are covered by white velamen, a layer of dead cells. The white velamen of orchid roots conserves water during dry periods when the bark would otherwise draw water out of the plant. When the velamen is wetted, its cells become translucent to reveal the green layer beneath.
In most orchids, net photosynthesis (the difference between the total photosynthesized energy and loss of photosynthesized energy through respiration and death of plant parts) occurs through leaves. In these cases, the green-tipped aerial roots will perform just the amount of photosynthesis necessary for maintaining basic physiological function. Other orchids such as the epiphytic Microcoelia (below) lack major shoots and leaves. These leaf-less orchids conduct all their photosynthesis via their roots.
Orchid species can be divided into epiphytic and lithophytic kinds. Epiphytes grow off trees rather than in the soil at ground level, while lithophytes grow in the soil. Both kinds of orchids can have aerial roots that draw moisture and ions from the air, much like conventional plant roots draw them from the soil. In humid tropical environments, the air contains a lot of moisture, and the ability to utilize this environmental feature offers an evolutionary advantage to the plant.
However, most orchids are epiphytes. Their roots grow along the bark and sometimes hang off it. It is usually only epiphytic orchids whose roots are capable of photosynthesis and have green tips.
Why is it that epiphytic orchid roots are green and capable of photosynthesis? The most likely explanation is that a multi-functional root that anchors the plant, absorbs moisture, and functions as a photosynthetic organ is an efficient way to survive in a relatively nutrient-poor and precarious environment.
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