The Word: Deciduous
The winter season is here. In the temperate zones of the world our rhododendrons still hold their leaves but most of the other trees and shrubs, including deciduous azaleas, are bare. What could be a more appropriate word for this season than "deciduous"? Its origin is not particularly mysterious; it derives from the Latin "decidere," to fall off. What's interesting about the word is how and why it happens. Deciduous has been defined as "shedding leaves at a certain season" (Raven, P. et. al. Biology of Plants, p. 728). Typically that happens in the fall as the days get shorter, but not always. Wiliwili (Erythrina sandwichensis), the endemic coral tree of Hawaii's dry forests, and most of its tropical relatives in the same genus and a number of other plants in arid climates lose their leaves in the early summer as the dry season approaches, thus the use of the words "...at a certain season."
The deciduous azalea Rhododendron luteum (Azalea Pontica).
Note bare stems with flowers appearing before leaves.
Photo by Eleanor Philp
Why do so many plants drop their leaves, whether in response to shorter days or some other stimulus? The apparent answer is that broad-leaved plants get caught in a bind. In climates where the winters are cold, the needle-shapes and small cross-sections of conifer leaves can survive nicely, though the same leaves are not highly efficient at photosynthesis. Broad-leaved deciduous plants with their wide, thin leaves are much more efficient at photosynthesis when the days are warm and long but expose all their surface area to freezing when the days get short and frigid. What to do? Make as much food as possible while it's the efficient thing to do and then drop the leaves when they become more of a liability than an asset. It takes less energy to grow new leaves in spring than to use damaged, inefficient ones in preparation for the next warm, sunny season.
What is a broad-leaved deciduous plant doing that causes the leaf color to change followed by a loss of the leaves? Let's start with the leaf color. The predominant green color in leaves is caused by chlorophyll (Greek: chloros, green; phyllos, leaf). Chlorophyll is the molecule that allows the existence of all organisms by converting red and violet light energy to chemical energy trapped in carbohydrate molecules. Green, blue and yellow light are absorbed very little by chlorophyll, thus its green color. Chlorophyll alone does not do the entire job. Accessory pigments called carotenoids (Latin: carota, a carrot) assist. Beta carotene is one of these accessory pigments. In animals, after it is eaten, it is converted to vitamin A, then to retinal for use in vision. The carotenoids are various shades of red, orange and yellow and function to trap visible light for the extraction of energy. The carotenoid molecules are far less numerous than chlorophyll and thus are not visible when the leaf is functioning in summer. They are more stable, though. Chlorophyll is broken down constantly and reconstructed; carotenoids aren't recycled as readily. When the days get shorter, the cells slow down and stop manufacturing chlorophyll, but the carotenoids remain, showing up as the various colors in deciduous leaves in the fall after the chlorophyll is gone.
Once photosynthesis is no longer taking place there's no reason for the plant to continue allowing the free passage of materials to and from the leaves. The plant forms an abscission (Latin: abscissus, to cut off) layer between the stem and the leaf. It is a corky layer that forms a scar which cuts off the transporting vessels and protects the live cells in the stem. The leaf cells die and the leaf drops off. The dead leaves on the ground form good mulch for protection from cold and the materials that didn't get back into the plant can be recycled through the roots the next spring. The cells in the leafless plant subsist on the large stored excess of food that was made by photosynthesis while the leaves were functioning. As the days get longer the plant starts over, using the stored nutrients from the previous warm season to grow new leaves, typically right above the scars where the old ones fell off. It sounds like a lot of extra work, but the deciduous habit has allowed broad-leaved trees to flourish and dominate temperate forests in much of the modern world.
Note: It has come to my attention that in the last publication of "The Word" (JARS, 61, No. 4) I left a redundant sentence in the second paragraph. It is not accurate to say that "each organism can be told from all others..." with a binomial. To do that we must go beyond the narrow species concept and deal with such ideas as subspecies, trinomials and varieties. I did not intend to get into that level of detail. My intended meaning had been conveyed earlier, in the sentence "Let's look first at why every plant has two scientific names, genus and species."
Bruce Palmer is a member of the Eureka Chapter. He was a teacher of biology at Maui Community College in the University of Hawaii System for twenty-five years.