Monocot Diversity in the Tropics

Many of the most striking plants in the tropics belong to the botanical group known as the monocots.  This group includes bananas and taro, coconut and bamboo, ti and ginger, agave and aloe, pineapple and sugar.  There are also monocots that grow in temperate regions, but they often aren’t as individually dramatic — partly because they don’t grow as large.  Why is this?

There are several basic features that distinguish monocots from other flowering plants.  For one thing, their flowers are divisible by 3 (so that they will have 3 petals, or 6 petals, etc.).  Leaves characteristically connect to the stem with a sheath, and the veins in the leaves are generally parallel (striate) rather than branching (reticulate).  Finally, the vessels which transport water up and down the plant are distributed throughout the stem, rather than being in a ring at the edge of the stem.

TypDicotStemXSLab300.jpg (59140 bytes) CornStemXSLowMagLab200.jpg (56035 bytes)
Cross Section of a typical Dicot Stem Cross Section of a typical Monocot
[Illustrations from Dave Webb’s Botany 201 Lab Class at the University of Hawai‘i at Mānoa]

This last feature means that the stems of monocots are filled with moisture, and have no core of solid wood inside them.  So they are very vulnerable to freezing temperatures!  While a woody “dicot” could drain all its sap down into its root system for the winter, a monocot with a wet stem would be in danger of freezing and breaking.  What this means for the distribution of monocots (i.e. their biogeography) is that the only monocots found in temperate climates are those which have some kind of adaptation for getting through cold winters.

When the weather is freezing, the above-ground parts of monocots die back.  In some cases the whole plant dies, but only after producing seeds which will sprout the next year, and so these plants are called “annuals.”  These usually produce abundant crops of seeds —which is a plus for humans who have used various members of the Grass family as grain crops: wheat, corn, rice, wild rice, oats, barley, rye, millets and more.

Other temperate monocots have parts that stay alive underground, so they are sometimes called “geophytes” (from the Greek words geo for earth and phyte for plant).  These include some common crops like onions and garlic, and other edible plants like camas root and sego lily that people traditionally gather for food.  Also in this group are the bulbs that people grow as ornamental flowers: tulips, irises, gladioli, daffodils and others.  Most of these are in or related to the Lily family.

So: in temperate regions, monocots are usually either annuals or geophytes, in the grass or lily groups.  However, in the tropics, where monocots are not in danger of freezing, there are many more forms and families.  Plants can grow for many years — some of the agaves are even called “century plants”!  They can grow huge leaves like the ‘ape or elephant ear.  They can form forests, like the loulu palms which formerly covered most of the ‘Ewa Plain on O‘ahu, or tangles of long vines that climb up trees, like the “swiss cheese plant” (Monstera deliciosa) commonly used in landscaping.

Monocots also often reproduce through side shoots.  In the Pacific, this quality is important to farmers, who plant taro using the ‘ohā that sprout from the side of the makua corm, and grow new banana plants from the keiki that shoot up near the base of the older ones.

In Hawai‘i, the trunks of the banana are also important for cooking, because they are traditionally cut up and used to line the ‘imu pit oven, where they keep the food from burning on the hot rocks and provide steam to cook the food.  Banana leaves are used to cover the top of the oven and keep the steam in.  This cooking technique works precisely because the bananas are monocots and so their trunks are full of moisture.

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Bunchy Top at Ricky's