Aquarium Frontiers Aquatic Horticulture

AQUATIC HORTICULTURE

BY KAREN RANDALL

KAREN RANDALL

Anubias barteri is one of the many flowering plants or angiosperms we enjoy in the home aquarium. Anubias are monocotyledons.

Aquatic Angiosperms

Continuing with our journey through the evolution of plant life on the planet earth, the next group of plants to make their appearance were the gynmosperms. Represented most prominently among modern plants by the conifers, the gymnosperms include no aquatic species.

The origin of flowering plants

The first signs of true flowering plants, the angiosperms, finally began to appear in fossils from the Cretaceous period. Looking at the fossil record alone, it would seem that the angiosperms sprang into great abundance and variety in almost the blink of a prehistoric eye. The truth is, however, that this is very unlikely.

Scientists postulate that what actually happened was that the first angiosperms developed in areas that were not conducive to the formation of fossils — areas that were dry and unlikely to develop layers of sediment in which specimens could become trapped and preserved. It is thought that angiosperms may have existed in the late Paleozoic era, a good 150 million years before their first appearance in the fossil record.

So, what is an angiosperm, and what modifications do they add to the evolution of plant life? Actually, the majority of living plant species are angiosperms, or flowering plants. They vary in size from trees over 300 feet in height to the tiniest flowering plants, the aquatic Wolffia sp., barely 1 millimeter long. They include annuals, perennials, trees and shrubs. They have adapted to live under conditions from the arctic tundra to the equatorial rainforests, and from the driest deserts where they can exist in seed state for years between rainfalls to fully aquatic conditions.

KAREN RANDALL

Flowers

Although the gametophyte stage is even more highly reduced than in more primitive vascular plants, the most noticeable and probably most important difference between angiosperms and their predecessors is the development of flowers. Flowers made it possible to depend on the reliable transport of sexual reproductive products from one plant to another by a variety of animal vectors, as opposed to relying on the vagaries of wind or water currents. They also made it possible to attract the specific types of creatures that were best suited to pollinate particular species of plants. From the dawn of the angiosperm onward, the evolution of certain insect groups and their flower-bearing plant partners have been intricately intertwined.

Flowers consist of several parts that have developed to serve specific purposes. Some of these structures can be highly modified, depending on the particular species and the ecological niche it has evolved to fill. The parts of a flower are:

anther — pollen-bearing portion of the stamen

carpel — a leaf-like structure with a basal swollen ovary and a stalk-like style and stigma for receiving pollen

petal — part of the corolla of the flower, often conspicuously colored. Most are sterile stamens used to attract insects, but some are derived from sepals instead

sepal — specialized leaf modified to protect the flower bud

stamen — the pollen-producing organ of the flower, usually composed of the anther and filament.

Fruits

The result of the successful combination of the reproductive products of an angiosperm is the formation of fruit — the ripened ovary and the seeds it contains. Like the flowers themselves, these have become highly individualized based on the ecological pressures that have come to bear on a particular species.

In some species, where wind is the primary method of dispersal, fruits are lightweight and often have wings or tufts of “fuzz” to help them remain airborne over long distances. In other cases, plants have evolved to shoot seeds explosively in order to disperse them over as large an area as possible.

Seeds and fruit that are primarily dispersed by water are protected with resistant coatings and often float. Some seeds are spread through the intestinal tracts of animals. These have developed coatings of tasty flesh, while others have evolved to stick to the fur and feathers of their animal transporters as they hitchhike to a new home.

All’s fair in love and war

Another important factor that has contributed to the tremendous success of the angiosperms is the development of a number of secondary plant substances. These substances are often chemically unrelated compounds that have evolved independently in a number of plant groups. They include alkaloids, quinones, terpenoids, glycosides and flavonoids, among others.

These substances have developed to restrict the palatability of various plants. These plants are apt to be eaten only by very specific groups of insects that have evolved to benefit from a food source avoided by most other animals. The same chemicals that repel most insects are powerful attractants for the specific feeders and actually act as feeding stimuli (for instance, Monarch butterflies feed only on milkweed, a plant toxic to many other species). These insects are often brightly colored to warn their potential predators that they now carry those same noxious chemicals within their tissues.

KAREN RANDALL

Two types of angiosperms

The huge class of plants known as angiosperms is further broken down into two subclasses, the monocotyledons and the dicotyledons. Monocots account for some 60,000 species, including a number of aquatic plants and all marine angiosperms. There are about 190,000 species of dicots, which include most non-coniferous trees and shrubs, and most annual herbs, except for the grasses.

TABLE I Major differences between dicotyledons and monocotyledons
	Dicots	Monocots
Flower parts	usually in fours or fives	usually in threes
Leaf venation	usually net-like, pinnate or palmate	usually parallel
True secondary growth with vascular cambium	commonly present	absent
Cotyledons	two	one
Vascular bundles in stem	in a ring	scattered
Pollen	tricolpate (with three furrows or grooves)	usually monocolpate (having one furrow or groove)

Aquatic angiosperms

The aquatic members of this diverse group of plants have clearly derived from their terrestrial ancestors. They have returned to the water after having adapted to life on dry land. In returning to the water, they have undergone further adaptations. There has been a reduction in the vascular strands, and the plants have often developed pithy stems and/or floating plant parts. Some have evolved to be able to become pollinated even in the submerged form, although many semi-aquatics must still flower above water for successful pollination to take place. Leaves in many of these plants have become thin and much dissected. Next month we will begin to examine the individual families of angiosperms that contain aquatic representatives.

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AQUATIC HORTICULTURE