Plants supplement their income

…with a dabble in heterotrophism

Green plants are called ‘autotrophs’, meaning that they feed themselves . (This is in contrast to all animals, which are ‘heterotrophs’ that gain nutrition by consuming other organisms.) These green plants feed themselves by photosynthesis, converting carbon dioxide and water into sugars (and oxygen). They also draw water and minerals from soils, and sometimes from water. So we are inclined to think of them as functionally independent entities, in terms of nutrition.

There are some salient exceptions to this simple plant-autotroph versus animal-heterotroph dichotomy. The carnivorous plants consume insects as a dietary supplement, so they are, in effect, both autotrophic and heterotrophic (see also A few plants are not green at all and live a totally parasitic existence, drawing nutrition from host plants; they could be called heterotrophic too. For example, dwarf mistletoe that infects hemlocks and other conifers in our forests is not capable of much photosynthesis, and depends on its host tree for nutrition. Heavy infestations can kill the host tree. (However, the witches’ brooms that they create are useful to squirrels and birds). Northern ground cone, which is common near the Visitor Center at the glacier, is parasitic on the roots of alders (and a favorite food of local bears).

However, most of the other, supposedly autotrophic, plants actually live in association with other organisms that supply nutrients. Many species, including orchids and blueberries, associate with fungi that supply important minerals to the plant; these associations are called mycorrhizal (fungus-root) (see also Some species, such as alders and lupines, form root nodules that are inhabited by nitrogen-fixing bacteria that turn atmospheric nitrogen into a form usable by plants. Many trees form natural root grafts with their neighbors, drawing water and nutrition from each other (and sometimes diseases too).

Then there are the so-called hemi-parasitic plants, which I mentioned a couple of weeks ago in this space. They are green and can photosynthesize carbohydrates and live independently, but which also commonly parasitize other plants. They often grow better and set more seeds when they tap a host’s resources, but a host is not absolutely necessary. Their effect on host plants is generally negative, reducing growth and seed production. As far as I can determine (so far), we have three kinds of hemi-parasitic flowering plants in our flora.

Indian paintbrush (genus Castilleja; about twelve species in Alaska): They grow from sea level to the alpine zone. The colorful bracts of the inflorescences range in color from red to pink to yellow. Some are pollinated by hummingbirds, some by butterflies (especially Down South) and some are pollinated by bumblebees. Paintbrushes can accumulate selenium from soils and become toxic to humans and other vertebrates. They parasitize the roots of grasses, herbs, and some trees.


Yellow rattle (genus Rhinanthus; one species here): It is also known as rattlebox or rattlepod. The yellow flowers are bee-pollinated. When the petals drops off, after pollination, the remaining green calyx contains the loose (rattling) seeds. A root parasite mostly of grasses and legumes, it is known to decrease the productivity and survival of grasses. Therefore it is used in some regions to restore meadows and prairies where cultivated grasses have been grown; by decreasing the cover of grasses, there is more room for wild flowers and thus a diverse community of plants. And the helpful yellow rattle plants eventually get shaded out.

Louseworts (genus Pedicularis; about twenty species in Alaska): Louseworts have their unfortunate name because of an old, very silly, belief that they caused grazing cows to have lice. There are hundreds of species of lousewort in the world, with flower colors of all hues. Most are pollinated by bumblebees or other relatively large bees, but at least one is also pollinated by hummingbirds. Only some have nectar in the flowers. Louseworts are root parasites, often of members of the heath family, such as blueberries.

The bottom line of all this is that green, flowering plants are not such independent entities as one might think. Many, if not most, of them interact with other plants, fungi, or bacteria to supplement their nutrition. Our forests and meadows would be impoverished without these interactions.


Carnivorous plants

turning the tables on the usual story

Carnivorous plants have fascinated people for over a hundred years or more. We are so familiar with the idea that animals eat plants, that the notion of plants eating animals seems quite anomalous. Yet there are hundreds of species of animal-eating plants around the world.

Carnivory by plants came to popular culture in the form of a show called Little Shop of Horrors, featuring an entirely fanciful plant that grew big enough to eat humans. In reality, carnivorous plants usually eat nothing larger than a tadpole. They concentrate on insects or, in some cases, on tiny aquatic organisms such as mites, nematode worms, and micro-crustaceans.

All carnivorous plants usually grow in nutrient-poor, open habitats, although they can sometimes occur in richer conditions when competition from other plants is low. Carnivory is considered to be an adaptation that provides supplemental nutrients to these plants—nitrogen and perhaps especially phosphorus. When Darwin fed insects to carnivorous plants, he observed that they grew bigger and reproduced better, and most experiments since then have found similar results. Thus, in most cases, captured prey is directly beneficial to the plant, although to varying degrees, depending on many other factors.

In Southeast Alaska, and Alaska at large, we have three basic kinds of carnivorous plants. Sundews (two species) grow in muskegs, along with Sphagnum moss. Butterworts (two species) seem to grow on sandy soils and in muskegs. And bladderworts (three species) are aquatic, living in freshwater ponds, sending up aerial shoots with flowers. All of these are potentially insect-pollinated; the flowers are white or blue-violet or yellow, respectively, but their pollination has been studied much less than carnivory.

Long-leaf sundew with prey. Photo by Bob Armstrong

Sundew leaves bear numerous hairs with sweet, sticky secretions at the tips. Insects are attracted by the sweetness, and perhaps also by colors on the leaf. When an insect touches a sticky hair, other hairs on the same leaf lean toward the unhappy bug, so that it is then caught securely by several hairs. Digestion takes place in part by the sticky hairs and in part by glands at the bases of the hairs. The sticky stuff is carbon-based, so its production is dependent on photosynthesis, and thus the sundews are limited to open habitats with solar radiation.

Butterwort. Photo by Bob Armstrong

Butterworts have sticky hairs on the leaves too, but they don’t move. The leaf margins are often slightly curled up, to help corral the hapless insect. Again, digestion is a two-step process, partly by the hairs and partly by glands in the leaf surface. Insect juices enter the leaf through holes in the surface, but these holes also let water escape, making the plant vulnerably to drying out.

Bladderwort. Photo by Bob Armstrong

Bladderworts work in an entirely different way. They capture their prey in small traps. In the resting position, the trap is closed and flat. When trigger-hairs near the opening are touched by a passing invertebrate, the trap suddenly expands, and the inrushing water takes the prey in with it. If the prey happens to be too large for the trap, it can be ingested bit by bit. Re-setting the trap after digestion can happen in less than thirty minutes but requires energy.

The traps of bladderworts often contain rich and abundant communities of bacteria, algae, and other tiny organisms. Some researchers suggest that these organisms actually live there and contribute to prey digestion. Others say that they eventually become prey themselves. The jury is still out on that.

These three kinds of carnivorous plants are well-known and fairly well studied. I suspect, however, that others remain to be discovered. In my former life as a professor, one of my students found that nitrogen markers from insects placed on sticky patches outside the flowering heads of a certain thistle or on the hairs of the inflorescences of penstemon were taken up and circulated through the plant. This suggests that previously unsuspected carnivores may be out there, waiting to be discovered.