Ecological Interrelationships

Some plants grow in symbiotic associations with other organisms. The lichen is a classic example. The nodules of bacteria that grow on the roots of bean family plants are another example as is the growth of fungal hyphae on tree roots (mycorrhiza). The reliance of many species of flowering plants on visitations from insects can also be considered symbiotic. The range of the yucca plant is limited by the range of the Pronuba moth, which is involved in the plant’s pollination and seed formation. The development and range of flowering plants is intimately related to the insects associated with them.

Some plants are parasitic, such as Dodder (Cuscuta gronovii) and Indian Pipe (Monotropa uniflora). Indian Pipe obtains its nutrients from the roots of other plants through a mycorrhizal association linking the two. Such plants have entirely lost the capacity to form chlorophyll, and, thus, the capacity to produce their own nourishment. These plants survive through their parasitic dependency on hosts.

Some plants are epiphytes, meaning they grow on other plants. Although green in color, such plants partially depend on the plants on which they grow. Mistletoe (Phoradendron sp.) is such a plant. Dr. Daniel Janzen of the University of Michigan describes an epiphyte that has developed a reliance on ants. Ants deposit parts of dead insects in cavities of the swollen stems, where the dead insects provide some nutritional benefit for the plant. The ants also harvest the seeds and plant them in the stems of young plants.

Insectivorous plants may derive benefits from ingesting insects that they “capture.” An insectivorous plant that never captures an insect, however, may yet fare quite well. It may be that insects merely provide some nitrogen for such plants. Some of the so-called insectivorous plants possess mechanisms that capture insects in the way that flypaper captures flies; the sundew, Drosera rotundifolia, is an example. Others capture insects by drowning them. One example is the pitcher plant (Sarraceniaflava). Nepenthes (another form of pitcher plant), while having the capacity to trap and digest insects, furnishes the home for certain other species of insects said to be able to live in the liquid at the base of the “pitcher” and nowhere else. Some insectivorous plants, such as the Venus flytrap (Dionaea muscipula), capture their “prey” by clamping the victim in a “vise” (the leaves). An aquatic form called the bladderwort (Utricularia vulgaris) sucks its victims through a trap door, which then quickly shuts behind them. This form also secretes proteolytic enzymes, which digest the insects. Thus, two traits are necessary in order for a plant to be considered insectivorous: having a type of trapping mechanism and producing protein-digesting enzymes. Plants that can entrap insects but that do not produce protein-digesting enzymes are not considered to be insectivorous. Likewise, plants that produce such enzymes but do not capture insects are also not considered to be insectivorous.