The function of a leaf is traditionally for the conversion of Carbon Dioxide and water into sugars and oxygen via the process of photosynthesis. The process is facilitated by the pigment chlorophyll, which absorbs red and blue wavelengths of light and reflects those in the green spectrum. Hence, leaves appear green in color. However, depending upon the plant, leaves can have functions beyond simply serving as the sight of photosynthesis.
In most flowering plants, the showy petals that we see are actually modified anthers. Their function is to serve as an accessory – or in modern day language ‘the bling’ – needed to attract pollinators. In many plants, such as roses, they also serve as the source of the ethereal scents and fragrances that are released to once again attract various pollinators – and Gardeners! However, in certain plants the petals that we see are in fact not traditional petals! In the Christmas Rose (Helleborus niger), the petals have actually became greatly reduced and appear as small, yellow, horn-shaped nectaries adjacent to the anthers. Nectaries contain sugary liquids that serve as a food source, attracting pollinators as well as serving as the sugar source for the production of honey. This is a particularly effective tool for attracting pollinators during warmer days of winter since there is little ‘food’ or other plants in bloom. Yet, however effective nectaries are at luring a pollinator, a flower without ‘petals’ remains less attractive than a flower with that little extra ‘bling’. Most flowering plants have modified leaves called bracts that cover and protect the flower during the bud stage. Upon the opening of a flower, the bracts typically appear as small green appendages that subtend the flower and remain inconspicuous to the eye. They are functionally much different than a typical leaf, since bracts contain less chlorophyll and less stomata, indicating that the plant does not ‘look’ to these structures for photosynthesis, simply protection of the embryonic flower. In Hellebores, along with numerous other plants that lack petals, the plant ‘looked’ to these leaves to perform one more function. As the bracts in Hellebores open to reveal the anthers and pistil, they expand and assume different colors, both looking like and performing the same function as petals! The most common example to gardeners is the North American Flowering Dogwood, Cornus florida, whose 4 white petals are actually bracts. As with Cornus florida, the leafy bracts of Helleborus niger also lack pigmentation and appear white color. They also expand considerably from their embryonic beginnings, yielding flowers that are 3-4” in diameter! In NJ, most Christmas Roses bloom in late February or March, but a selection named ‘Joseph Lemper’ often begins to bloom in mid to late December in central and southern New Jersey!
Another large genus of plants that relies on modified bracts for floral interest is Euphorbia, commonly known as the Spurges. For December, a spurge that is most commonly recognized is the Mexican native Euphorbia pulcherrima or Poinsettia! Named after the botanist Joel Roberts Poinsett, who served as the first US Minister to Mexico in 1825 under James Madison, the bright red bracts were long used by the Aztecs as a reddish-purple dye. Much like the brilliant red fall color of Red Maples (Acer rubrum), the red coloration is created by Anthocyanin pigments. In fact, a total of 11 different pigments within the Anthocyanin group appear in Poinsettia bracts [as seen at right, Source: André Karwath on Wikipedia (CC license)].!
Anthocyanin production begins in late October following the autumnal equinox. It is stimulated during bright sunny days and the passing of at least 5 consecutive nights that have 12 hours or longer of darkness. Obviously, with artificial lighting and light impervious curtains, it is easy to encourage Poinsettias to ‘Bloom’ under greenhouse conditions, allowing this plant to rapidly grew in popularity for the Holiday market! As common as this plant is today, it was not until the 1960’s that techniques were developed to grow a more compact and attractive plant in commercial production. By the year 2000, over $236,000,000 in Poinsettias were produced in the United States alone! All of this success due to a simple yet attractive ‘leaf’!