Are palms giant herbs?

The largest inflorescence in the world 
is that of a palm, Corypha
umbraculifera, which like the well-
known Century Plants in the New
World, dies after its massive
flowering.  The inflorescence
is said to contain approximately 
24 million flowers. 
Photo courtesy Scott Zona.

The  Palm Family (Arecaceae) includes some of the largest monocots in the world, and one could argue, the largest perennial herbs.

To call a massive palm tree an herb may seem like a strange statement, since it has a sturdy upright stem, and may live for 100 years or more. Palm trunks may be a meter or more in thickness (Roystonea or Jubaea), and they hold the records for the largest inflorescences (Corypha umbraculifera), the largest seeds (Lodoicea maldivica) and the largest leaves (Raphia regalis) in the plant kingdom.

The largest seed in the world, weighing
up to 55 pounds, is that of Lodoicea maldivica,
from the Seychelles Islands.  The large
seeds are thought to be an adaptation
for survival of seedlings in a thick forest
 with nutrient- poor soil. Posted on
Wikipedia, Creative Commons license.

Members of the genus Raphia in Africa have the largest leaves of any plant. Pictured is R. australis, which is truly huge,
but a camera-shy relative, R. regalis, has the largest leaves, measured at over 25 meters in length.
Photo posted on Wikipedia, Creative Commons  License.

So is a palm a herb? The traditional definition of a herbaceous plant (or simply herb, in a botanical rather than culinary sense) is that it lacks permanent, above-ground woody stems, though they may have woody underground parts. Tulips and dahlias are examples of perennial herbs, while pansies and marigolds are examples of annual herbs. The alternate category is woody perennials, which include trees, shrubs and lianas.  There are, in fact, some dwarf palms that do not produce upright stems.  They would clearly be perennial herbs.  But what about larger palms?

The  vegetation of herbaceous plants is produced entirely through primary growth, in which all tissues arise from the apical meristems, or buds, at the tips of the stems. In contrast, woody plants exhibit secondary growth both above and below ground.  It is important to note that wood is the production of concentric layers of secondary xylem.

Are bamboos, perennial herbs or trees? Photo by Alain Van den Hende,
posted on Wikipedia, Creative Commons License.

Tropical plants, and tropical monocots in particular, severely strain the distinction between those two categories. First of all, no monocot, even a palm "tree," has true woody tissues. Their stems, no matter how thick or dense, are produced entirely through primary growth, and are strengthened by dense masses of fibers, rather than by layers of secondary xylem.  For that reason alone, all monocots could be considered herbaceous.

Many botanists would consider that too picky, and would use the term "woody" in a broader sense to refer to the dense wood-like tissues of palms.  And there are a few monocots, such as the dragon trees, giant aloes and some dracaenas, that have a specialized form of secondary growth, but such growth adds only layers of fibers and vascular bundles, not layers of secondary xylem.

Even if we accept that palms and other giant monocots are trees, there are still many gray areas where one is not quite sure where herbaceous perennials end and trees begin, and so there is value in pointing out the distinction between the very different ways that monocots and dicots form tree-like growth forms (see The invention and reinvention of trees).


Monocots abandoned the ability to form true wood as their ancestors adapted to a growth form based on rhizomes, with leaves that elongate from the base, and short-lived upright reproductive shoots (see How the grass leaf got its stripes).  Leaves of monocots, which can  be relatively large, are heavily dependent on bundles of fibers for support against both gravity and wind, as well as sometimes for protection against herbivores.  As they spread to a wide variety of habitats, some monocots got larger and developed upright stems with increased density of supporting fibers.  Important commercial fibers come from a variety of monocots, including Manila hemp (from a type of banana), sisal (from a species of Agave), and New Zealand hemp (from Phormium).  Fiber can also be teased our of bamboo stems and the leaves, stems, and fruits of many palms. 

Tropical monocots tend to be evergreen, another way they differ from temperate herbs.  Banana plants, which are tree-like, but clearly herbaceous, remain above ground for several years.  Others, such as agaves, aloes, and birds-of-paradise have permanent tufts or rosettes of above-ground foliage, typically arising from underground rhizomes.  No one would confuse such plants with woody shrubs, and these must be considered  perennial herbs.   Other monocots, including many grasses (e.g. canes) have upright stems that are reinforced with fibers and may last for several years. Bamboos are giant grasses with sturdy upright stems that live for many years (see The grasses that would be trees).   Should they be called herbs or woody plants?  Neither, actually.

The whole point of this long diatribe is to once again to point out how different monocots are from other vascular plants.  Their growth forms cannot be classified in the same terms as dicots.  They have mimicked the forms of many other kinds of plants (e.g. palms vs cycads), but with very different patterns of growth and tissues. Some of the elaborate classifications of the past (try googling: "plant growth forms") included special categories for palms and bamboos, but many did not.  In my opinion, the term "woody" should not be used for any monocot.  We can substitute the word "fibrous," which will be much more accurate and informative.  Many tropical and xeric monocots can be referred to as evergreen perennial herbs.  That would cover agaves, aloes, yuccas, and birds-of-paradise, as well as smaller palms.  Tree-like monocots, such as coconut palms, bamboos, screw-pines, Joshua trees, and dragon trees, might be called "fibrous arborescent perennials."  

Flowers, Compound Flowers, and Superflowers

In the simple inflorescences of Indigo
(Indigofera spp., Fabaceae) new flowers
are produced at the tip for an extended
period of time, opening first at base, and
lasting only for a day or two.  Many
inflorescences with more complex
branching patterns still open just a few
flowers at a time.

An inflorescence can be defined as an aggregation of flowers on a specialized shoot that lacks ordinary leaves.  The lack of full-sized, photosynthetic leaves is the key to defining an inflorescence, as opposed to a series of single flowers along a leafy branch.  Within an inflorescence, leaves may in fact be present, but they are either smaller than normal, specialized in shape, conspicuously colored, or all of the above.

One common type of inflorescence, is one in which flowers open up one or a few at a time, for an extended period.  Examples include lupines, snapdragons, gladioli, and foxgloves.  Such inflorescences are adapted to induce "repeat visitors" - insects, birds, or other animals that remember the location of the plants and drop by each day to collect nectar or pollen from freshly opened flowers. This is a behavior known as trap lining.


In such flowers, a common means of avoiding self-pollination, is for the stamens and pistil within each flower to mature on different days. For example, pistils may be active and receptive to pollen on the day the flower opens, with the anthers opening to shed pollen 24 hours later. The common Amaryllis follows this pattern.

A common means of avoiding self-fertilization within individual flowers is to have pollen shed on one day (left) and stigmas receptive on another day (right).  This effectively makes the flowers male on one day, and female on another day.  

Compound, or false flowers, such as those of the sunflower family or the spectacular poinsettias, are actually condensed inflorescences adapted to look like a single large flower to pollinators, but still opening their flowers a few at a time to attract trap-lining animals.

In the compound flower heads of the Sunflower
Family, such as this Ice Daisy, the tiny flowers
(visible as the yellow rod-like structures)
around the outside of the central disk open first,
to be followed by flowers progressively closer
to the center.

In other inflorescences, the flowers mature all at once to create a single, massive pollination event.  I like to call these "superflowers." Super flowers don't necessarily look like a single flower, but they behave like one.  In the most specialized of these kinds of inflorescences, flowers are unisexual, and the opening of the male and female flowers is offset, such that the entire inflorescence behaves as a single, short-lived flower. Examples of these are found most spectacularly in the Aroid and Palm Families.

The Titan Arum, Amorphophallus titanum,
blooming at the U.S. Botanical Garden in
Washington, D.C., posted on Wikipedia.

The Titan Arum, which makes the news whenever it blooms in a botanical garden, produces a gigantic inflorescence in which all the tiny flower buds are mature when the large bract, or spathe, opens to reveal them.  They follow a similar strategy as the amaryllis flowers above, with separate female and male phases.  In this case, all the female flowers are active first, followed by the male flowers in a day or two.  This avoids self-pollination, as insects arrive during the female phase, bearing pollen from another inflorescence.  They then leave during the male phase, freshly dusted with new pollen.

Unlike the simple inflorescences
mentioned at the beginning of this post,
palm inflorescences complete most of
their development within their large,
protective bracts.  No new branches or
flowers will form after the bracts open. In
many, such as this Rhopaloblaste, the
flowers will continue to expand for a
while, and may open over a prolonged
period of time in order to attract trap-
lining insects. 

The inflorescences of palms, despite being rather large, appear to be fairly simple assemblages of small flowers, yet some of them behave in much the same way as the Titan Arum.  I made my discovery of these palm "superflowers" as a graduate student, first in Costa Rica, and later in New Guinea.

After waiting for a few hours at the edge of a swamp in Costa Rica, I found that inflorescences of a species of Bactris opened abruptly at dusk, displaying unopened male flowers and active female flowers nestled within them. The inflorescence emitted a musky odor, which attracted a variety of small flies, bees, and beetles. The male flowers opened to release their pollen 24 hours later. I observed the same thing in another species of Bactris later.

A year later, I was in Papua New Guinea and observed a nearly identical process in a species of Hydriastele.  I was able to get more detailed pictures of male and female flowers, along with their insect visitors, which I share below.

Though it consists of a number of branches, the inflorescences
of Bactris guineensis behave the same as that of the Titan Arum. Female
flowers, hidden among the larger male flowers, are
receptive to pollen soon after the large, fibrous bract opens.

When the bract of a Hydriastele microspadix inflorescence splits
open, the flowers are all mature, and arranged in triads of
two large male flowers with a tiny female flower between them.  

   

When the flowers are first exposed, the stigmas of the tiny female flowers, seen at the left between pairs of male flowers, are exposed, sticky, and receptive to pollen..  The male flowers (right) only open 24 hours later to release their pollen.  During the female phase, tiny flies and weevils are already present, attracted by the scent of the unopened male flowers.

One further example from the palm family comes from the mangrove palm, Nypa fruiticans, which lives in brackish water around river deltas and estuaries throughout the old world tropics. I spent a day observing them.  Here the female flowers, which are rather bare and uninviting, are borne in a tight globose head at the top of the inflorescence. Male flowers are borne on dense orange-colored spikes below the female flowers.  Brief observations suggested that pollination is accomplished by small flies that land first on the female flower heads, and then crawl down to the spikes of unopened male flowers where they lay their eggs.  Larvae develop within the spikes, feeding on the unopened male flowers, and mature in a few days.  When the new adult flies emerge, the male flowers have opened and are shedding pollen.  The flies are then covered with the sticky pollen and fly off to start a new cycle on another Nypa inflorescence.

Female flowers of Nypa form a dense globose head (left), and appear to provide no nutrition for insects.  The dense male spikes (right), however, provide a place for fly larvae to develop as they feed on the tissues of the unopened male flowers.