Flower Power: Spadix and Spathe in the Araceae Family
An iconic leaf you often see representing the luxuriant vegetation of the tropical world is an arrowhead or heart-shaped leaf with a drip tip, coming in many sizes, iterations and venations (vein patterns), found in plants of the Araceae (“ar-ray-say-e”) family. Common names are Aroids or Arums.
You may have first learned about this huge plant family in your own home, when your mother installed a large Philodendron in a corner of your living room. You had your first lesson in botany when you saw the leaves turn squarely toward the light and saw the plant quickly climb that roughhewn board wedged into the plant pot. If you are like me, you can’t remember ever seeing the Philodendron flower. In just a few weeks the Philodendron sent out various leafy tendrils, some threatening to take over your dad’s Lazy Boy. Our mothers probably cut them back before they had time to invade or to bloom.
In the Neotropics there are more than 500 species of Philodendron; about 250 species occur in Costa Rica, 3000 species worldwide. Other major genera of the Araceae family found in Costa Rica are Anthurium, Monstera, Syngonium, Spathiphylum and Dieffenbachia.
You may already know the family Araceae from the Jack-in-the-Pulpit (Arisaema triphyllum) and another plant common in the Northeast of the USA, skunk cabbage (Symplocarpus foetidus), one of the first plants to appear in early spring. These, and all the other members of the genera listed above, have the distinctive inflorescence of tiny flowers or florets on a spadix emerging from a bract or spathe. The hooded spathe of the skunk cabbage–blotchy, brownish-purple (the color of rotten meat)–covers the spadix and opens only when the female flowers are receptive.
Like some of its tropical relatives, the skunk cabbage is capable of thermogenesis, a complex chemical process involving impressive generation of heat. The high temperature of the blooming spadix and its covering spathe, the first parts of the plant to appear, melts the snow above. Why do skunk cabbage and many Aroids in the tropics generate heat? Why, to promote pollination: the foul odor of skunk cabbage attracts mostly beetles and flies, and sometimes even butterflies. The heat–as much as 86 F (30 C) higher than the ambient temperature!–volatizes odors broadly and efficiently, thus attracting pollinators from greater distances.
Another possible evolutionary advantage for thermogenic plants is that the beetles that pollinate them prefer their warmth: beetles don’t move much until the sun shines and temperatures rise. More moving beetles in a warm thermogenic environment, protected from the elements by the spathe, with pollen nutrients to eat, means more reproductive opportunities for the plant. It seems like co-evolution at its best.
I first learned about thermogenic mechanisms from a magnificent elephant-ear-sized leaf specimen, a tree Philodendron cultivar (below) at the Wilson Botanical Garden of San Vito, Costa Rica, where I worked from 1989 to 1999. Related to my development responsibilities was giving tours to natural history visitors. What impish fun I had when leading silver-haired Elderhostelers to the robust plant when it was in full flower, a large white spadix thrusting out of its pink lined spathe. “Touch that,” I would dare one of the ladies. Her scream of shock when she put her hand around the hot spadix woke everyone who was starting to doze on the tour. A plant generating such heat always astounded and amazed the ever-curious groups, giving me a chance to explain the advantages of heat generation for pollination by the beetles. The flowers’ fragrance is pleasant in this case and carries far at full maturity.
For those on the West Coast in Northern California and points further north, you are likely accustomed to another member of the Araceae, the common Calla Lily found in moist areas and wetlands—actually, not a lily at all, but an Arum, the Zantedeschia aethiopica. The Calla spadix has a mildly sweet fragrance but does not generate heat. Though lovely as cut flowers, Callas can spread invasively and are sometimes considered pests.
Here at Finca Cantaros, pollination events on non-heat-producing Anthurium and Spathiphyllum are busy affairs, attracting stunningly attractive Euglossine bees, commonly called orchid bees as, indeed, they visit orchids as well. The loud buzzing of the hairy, yellow and black Euglossine bees in the genus Eulaema first attracts my attention if I happen to be walking by a flowering spadix. Then I observe on the same inflorescence the more quiet amber, blue or green metallic orchid bees in the genus Eufriesea. As a photographer, I can get very close to the action: these are stingless bees. They are practically oblivious to the lens just inches away. Nothing matters to them except grabbing the chance to gorge and collect pollen, thus ensuring future flowers and the next generation of bees.
Likewise, in the case of our own thermogenic tree Philodendron bipinnatifidum, scores of large nocturnal scarab beetles, smaller beetles, flies, earwigs and others arrive to dine and copulate, but it’s the beetles that do the heavy lifting of pollination.
As for all the heat-producing Aroids, there is something reassuring in the knowledge that beetles have been pollinating relatives of this plant family since the Mesozoic era, which ended 66 million years ago. We see the simple beauty of natural selection at work today in this ancient, enduring adaptation.
Heat-producing Flowers, by Roger S. Seymour and Paul Schultze-Motel, Endeavor, Vol. 21 (3), 1997.
Willow Zuchowski has an excellent chapter on the Arum and Philodendron Family in her book Tropical Plants of Costa Rica, A Guide to Native and Exotic Flora, pp. 354-365, a Zona Tropical Publication (2007).
Click on any photograph to enlarge it, then hit back button to return.