wwnature – Page 5 – Worldwide Nature

The flowering plants are divided into two main groups, monocots, and dicots. In this article, I explain the basic differences in flower, leaf, stem, and root systems so that you can distinguish one from another when you are next out. Monocots differ from dicots through their flowers, stems, leaves, and roots. The term monocot derives from the fact that the seed of a monocot contains only a single cotyledon or seed leaf. The cotyledon serves to supply food for the young developing plant. Monocots have one cotyledon (vein), while dicots have two. Almost all botanists agree that flowering plants (angiosperms) may be divided into two groups. There are the monocotyledonous species, known as monocots, which include grasses, lilies, bamboo, and palm trees, while the other group is the dicotyledonous species (dicots), an extremely varied group encompassing such plants as daisies and oak trees. The division of flowering plants into these two groups has long been recognized and is generally accepted as reflecting a natural partition. Monocots The monocots represent only a quarter of all flowering plant families. They are herbaceous, while some, such as palm trees and bamboo, give the impression of woodiness. Some members of the group, like the grasses, are very evolutionary advanced and also provide most of the world's staple foods, such as wheat and rice. The term monocot derives from the fact that the seed of a monocot contains only a single cotyledon or seed leaf. The cotyledon serves to supply food for the young developing plant. It may remain in the seed husk, in which case it actively absorbs nutrients from other parts of the seed, or it emerges from the seed husk, turns green, and manufactures food by photosynthesis. In most monocots, the cotyledon remains inside the seed husk. One of the most obvious characteristics of monocots is in the leaves. These are typically linear or oblong and have several veins parallel to the leaf's long axis. The bases of these leaves encircle the stem and lack a stalk or petiole. Rarely are the leaf margins toothed. The underground parts of many monocots take the form of food storage organs such as rhizomes, corms, or bulbs bearing numerous roots, and a simple, long, deeply penetrating tap root is absent. Since all monocots are herbaceous, their leaves and flowers typically die back during the winter, so to ensure rapid growth the following spring, food is stored underground in these bulbs, corms, and rhizomes. Monocot Flowers The floral parts of monocots are usually arranged in threes or multiples of three, with three or six sepals, three or six petals, and a corresponding number of stamens, styles, and stigmas. This is easy to see in some flowers, such as the iris, but in the more leaf-like structures, like orchids, the patterns can become smaller, enlarged, or fused to obscure the pattern. With grasses, although the male and female parts are usually in threes, their relationship to the other monocot flowers is not immediately obvious as they do not have petals. Because grasses are wind pollinated, they do not require petals to attract and guide insect pollinators. The pollen of most monocots is long and has a single furrow. Differences The dicots differ from the monocots in almost every feature and display more variation than seen in the monocots. Their cotyledons are usually produced in the seed, and many emerge to function as if they were true leaves. These leaves are readily identified in most cases because they tend to be fleshy, rounded structures, unlike normal leaves. The leaves of dicots typically have a net-like vein pattern. There is usually a central midvein, but the secondary veins (laterals) can radiate from around one point (palmate) or depart at intervals from the midvein (pinnate.) Compound leaves, such as the horse chestnut, are common. Dicot leaves are borne on stalks or petioles, while small leaf-like structures (stipules) may be found between the leaf and the stem. Unlike monocots, the leaves may have a toothed appearance, lobed, or have spines typical of holly. In contrast to the monocots, the underground organs of dicots often take the form of a single tap root with secondary side roots. Dicot flowers The floral parts of dicots are arranged in fours or fives, unlike the monocot multiples of three. Like monocots, dicot flowers can be modified to enhance pollination by a particular method. The petals may be absent in wind-pollinated species such as willow, or the petals may be fused, and the flowers massed into compound heads as in the daisy family. The simple oval-shaped pollen with a single furrow, typical of the monocots, is only seen in primitive dicots. The usual dicot pollen grain has three furrows or pores and tends to be spherical. Despite all these differences, it can be difficult to tell the difference between monocots and dicots. The feature that gives the groups their respective names, the number of cotyledons, can be different in many monocots. Some may have two cotyledons, while some dicots can have only one or sometimes three or more. The absence of the tap root in monocots represents a modification in how the root develops, and many dicots also lack a primary root system. Examples of this are seen in the water-lily and umbellifer families. Differences In Stems If you compare part of a dicot stem and part of a monocot stem under a microscope, you see that the bundles of xylem and phloem cells are arranged differently. In dicots, they are organized in a ring near the outside of the stem, but in monocots, there is no regular pattern, though they are less dense in the center. Evolutionary History The similarities between the monocots and dicots suggest that they evolved from the same non-flowering plant ancestors. However, there are obvious differences between the two groups indicating that they diverged relatively early on in the evolutionary history of flowering plants. This is supported by the fact that fossilized pollen and leaves of monocot and dicot types have been found in rocks 120 million years old. Which group is the oldest, then? The fossil record has been of little help to answer whether monocots or dicots are older, as we find evidence of plants in both groups. Botanists are therefore forced to try and answer the question by examining living plants to see which have the most primitive features. Here the evidence is not that clear, but the dicots are usually regarded as more primitive and, therefore, older than the monocots.