Sunday, May 20, 2018

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."