Aroids of the imagination IX - Spirals and Floaters

By LariAnn Garner (LariAnn)
February 25, 2008

On Aroidia, nearly every available niche is populated with some type of plant. Here I investigate those plants that float on the ocean and those that grow as hemiepiphytes on other, larger species.

Growth in suspension?

Many species of Anthurium on Earth grow in a similar manner as familiar orchids, being perched and rooted on tree branches rather than rooted in the soil. These are often known as hemiepiphytes, or plants that spend part of their life cycle growing in soil and another part of it growing up on branches or rocks.

On Aroidia, the genus most like the Anthurium genus on Earth is Spirophyllum, so named because the leaves are always produced in a single spiral arrangement. Shown at right is Spirophyllum cordatum, or the Heartleaf Spiral. The coiled nature of the stem and arrangement of the leaves is typical of plants in this genus. Every leaf is accompanied by an inflorescence, and here the spiral arrangement is right-handed. This indicates that the plant was observed growing in the northern hemisphere of Aroidia. Those found in the southern hemisphere are seen with left-handed spiral structures.

On Earth, observations of Anthurium plants in my own collection have shown me that leaves are produced in a double spiral or double helix pattern. Here is an example to illustrate how this works. Let's say your plant is going to have leaves arranged in two spirals, A and B. The first leaf emerges to start spiral A. The very next emerging leaf will be roughly opposite to the first leaf and begins spiral B. The leaf after that will be #2 for spiral A, and the next, #2 for spiral B, alternating in this manner for as long as the plant grows. Arrangement of leaves on a plant is known as phyllotaxy.

Back on Aroidia, I visualized other species of Spirophyllum. The next one illustrated below, left, is Spirophyllum pinnatum, or the Windmill Spiral. This particular species grows quite large and needs a heavy-trunked support on which to grow. Consequently, S. pinnatum is usually found growing attached to the trunks of Arodendron (not Aroidendron, the Keeper of the Gate, a completely different plant).

Plants in the genus Arodendron are very similar to Philodendron species of the Meconostigma group. On Earth, a familiar example of this type of Philodendron is P. bipinnatifidum or the plant frequently referred to as "P. selloum". These are also known as "tree philodendrons" because they develop thick trunks that can enable them to stand on their own. However, Arodendron plants on Aroidia are, predictably, much larger in every way, forming very heavy trunks that in size often rival those of hardwood trees on Earth.

The third species I envisaged is S. lancifolium, a plant not quite so large as S. pinnatum, but much heavier and with a thicker stem than any of the other Spirophyllum species mentioned so far.

Sailing away . . .

On Earth, a few aroids can be found floating on water in their natural habitats. Two notable genera are Pistia, also known as water lettuce, and Lemna, now considered to be an aroid. Neither of these grow to the large sizes seen in other aroid genera such as Anthurium or Alocasia. Both genera are relatively small growing plants; in fact, Lemna is very small indeed!

On Aroidia, however, this is not the case, as the genera with floating representatives there can hold their own with all the other huge growing Aroidian plants. The most conspicuous of these genera is Pneumatophyllum spathomorpha, or the bladderleaf plant. This genus ranks among the most paradoxical, as the structures that look most like leaves are actually subtending bracts to the inflorescence! The actual leaves are the bladder-like organs beneath the bracts. These modified leaves enable the plant to float on the water's surface, as shown in the picture at right.

To give an idea of scale here, a full grown person could sit easily on one of the lobes of a bladderleaf. These leaves have an internal structure honeycombed with air pockets, giving them their buoyancy. The leaves are produced in groups of three, as are the bracts and inflorescences, although the inflorescences on P. spathomorpha are produced one at a time in succession.

Pneumatophyllum species are not all aquatic or floaters. In fact, the leaf tip of Pneumatophyllum terrestris can be seen at the left edge of the picture above. This large-growing species has a buttressed trunk and leaves that look very similar to a rectangular parachute or parafoil. This morphology helps to support the large leaves in the gentle breezes, which are almost always present on Aroidia. Without the breezes, the leaves would droop towards the sides of the trunk. So while P. terrestris does not float on water, in a sense it "floats" on air! P. terrestris also produces leaves and inflorescences in groups of three, but all three inflorescences emerge simultaneously, unlike those of P. spathomorpha.

The habit of producing plant structures in groups of three is represented in another genus of plants on Aroidia besides Pneumatophyllum. One representative of this genus may have the most beautiful inflorescences on Aroidia. So my next stop was to investigate this wonder. . .

Picture credit: LariAnn Garner, Aroidia Research

(Continued in Part X: Beauty times three)

About LariAnn Garner   LariAnn has been gardening and working with plants since her teenage years growing up in Maryland. Her intense interest in plants led her to college at the University of Florida, where she obtained her Bachelor's degree in Botany and Master of Agriculture in Plant Physiology. In the late 1970s she began hybridizing Alocasias, and that work has expanded to Philodendrons, Anthuriums, and Caladiums as well. She lives in south Florida with her partner and son and is research director at Aroidia Research, her privately funded organization devoted to the study and breeding of new, hardier, and more interesting aroid plants.

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