Reproduction in Aquatic Plants

Most aquarium plants propagate vegetatively, putting out new plants from ro&t stocks or runners, and seldom flower and produce viable seeds under cultivation. In spite of this generalization some species can be reproduced only from seeds (as Apanoge-ton among others), seldom or never producing vegetative offshoots. Because of the beauty and value of the flowers of aquatic plants, sexual reproduction is discussed here in some detail even though it must be remembered that many common aquarium plants will never flower in the home.

Bisexual flower of Cabomba. Both male and female structures are present in such flowers.

Bisexual flower of Cabomba. Both male and female structures are present in such flowers.

Flowers and fruit of Echinodorus cordif alius

In plants the flowers are termed perfect or bisexual if they have both the male (stamens) and female (pistils) organs. The ovary is at the base of the pistil, while pollen is produced toward the tip of the stamens in the anthers. Bisexuality is probably the most common condition in aquatic plants.

If the flowers are not bisexual, two other conditions can exist. The plants can be monoecious (literally, one house) and have separate male (staminate) flowers and female (pistillate) flowers on the same plant, or they can be dioecious (literally, two houses), having the male and female flowers on different plants. Typical monoecious plants include the Sagittaria, while examples of dioecious plants are Vallisneria, Elodea, and Lagarosiphon among

Female flowers of Sagittaria graminea.

Bisexual and monoecious plants can be either self-sterile or self-fertile. Self-sterile plants must be pollinated by pollen from a different plant, while self-fertile plants can be successfully pollinated by pollen from the same or a different plant. Self-fertile plants such as Aponogeton, Ottelia, Sagittaria, some Echinodorm, Nuphar, and Barclaya produce seeds with relative ease in the aquarium.

It must be clearly understood that self-fertile species can be pollinated by pollen of either the same plant or a different individual of the same species; self-fertility is not an exclusive character. Almost all self-fertile plants can be cross-pollinated {fertilized with the pollen from a different plant), with the few exceptions of those species in which the flower never opens or the flower opens under water. If the flower never opens (a situation known as cleistogamy) self-fertilization is of course necessary because pollen from different flowers cannot enter. Underwater flowers have about the same problem as it is unlikely that pollen will be transmitted from one plant to another.

Echinodorus corditolius is seen here blooming and fruiting in the emersed condition.

With self-sterile plants, self-fertility is almost impossible under normal conditions. It must be remembered that if we take two plants grown vegetatively from the same plant, they will also be self-sterile. This is because the plants have the same genetic makeup and are in reality not two different individuals, but just separate parts of the same original individual. Thus if a cutting is made from a self-sterile plant and allowed to come to flower, it cannot be used to fertilize flowers on the 'mother' plant from which the cutting was obtained; the nature of pollen from the two plants is identical. The only exception occurs when the cutting is grown under very different conditions than the 'mother* plant.

Ball-shaped fruit of Echinodorus cordlfolius containing a large number of seeds called achenes.

Artificial pollination is usually necessary in order to ensure fertilization and viable seeds from a self-sterile plant. Pollen from a flower on one plant (never one produced as a cutting or other vegetative offspring of the other plant) is carried by a fine brush or other tool to a flower on another plant. This necessitates that ripe pollen be present on two different plants at the same time, making artificial pollination far from easy. In the cryptocorynes, for example, the sexual questions are so complicated that they are often the topic of serious scientific research.

Seeds are the product of sexual reproduction, a process termed generative as opposed to vegetative. It is rather slow and tedious and is seldom used in the cultivation of aquatic plants, with three classes of exceptions including only a few species.

1) Species not reproducing vegetatively (Aponogeton, among others) and annual species that die after flowering (Ottelia, etc.). With these species the generative reproduction from seeds is either necessary to continue the species under aquarium conditions (annuals) or necessary if we wish to increase the number of plants.

2) Species that reproduce slowly by vegetative means but flower and produce seeds easily. These are mainly large species of the genus Echinodonis. During one vegetative period they are able to render only 2 to 30 young plants, but in the same period may produce up to 10,000 seeds.

Irregular flower (left) and fruit (right) of Utricularia

3) Species that do not flower in aquariums or do so only rarely yet also do not reproduce vegetatively. Such plants are raised in emersed cultures (paludariums or terrariums) and usually flower there quite easily. Seeds are sown in humid soil and the young plants can be replanted underwater into aquariums. Under these conditions generative reproduction is very laborious but extremely productive. It has one disadvantage: plants grown from seeds do not always resemble their parents in quality and appearance, as do plants produced vegetatively.

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