3rd Hour Biology
Tuesday, January 22, 2002
(last updated)

Monocot

Dicot

For many life-bearing beings, birth is not the way into the world. There are some, that take to the challenge first as a seed. It is these beings, these organisms, that make up a section of the plant kingdom.

• A seed increases chances of successful reproduction
• It remains dormant during extreme conditions until conditions are favorable, at that time, germination takes place.
• Metabolism rate slows considerably.
• Scarification - the need for a nick of the seed coat (perhaps chewing) to thin the coat. Some, such as raspberries and strawberries, do this by passing through the digestive tract of an animal.
• Factors of germination: some plants require pounding river currents, animal attacks, animal ingestion, or light penetration.
• Some seeds contain inhibitors that stunts germination, thus the seed must be rid of the inhibator for germination to take place. This is usually so that the plant will grow in the proper season.

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�����For many life-bearing beings, birth is not the way into the world. There are some that take to the challenge first as a seed. It is these beings, these organisms, which make up a section of the plant kingdom.

�����Within the seed plants, there are five divisions among the species. They are Cycadophyta, Ginkgophyta, Coniferophyta, Gnetophyta, and Anthophya.

�����Cyadophyta lived successfully mostly during the Mesozoic Era, and about 100 species of these plants live now.

�����Ginkgophyta plants also thrived during the Mesozoic Era. The most common is probably ginkgo biloba (a supplement taken for memory).

�����Coniferophyta are more commonly known as conifers. There are many widely known variations including, but not limited to: pines, cedars, yews, firs, junipers, and spruces.

�����Gnetophyta are a less common seed plant. There are about 70 species, with angiospermlike vascular systems.

�����Anthophyta, or angiosperms as they are most often referred to as, are flowering plants with a protective fruit encasing the seed for protection. These plants have resided among the soil of Earth for an estimate of 135 million years. They have survived successfully for so long because of the protection provided by the encompassing fruit to the seed. The rate of successful reproduction is thus increased, making it a positive adaptation.

�����Anthophyta can be separated into two major groups--monocotyledoneae (monocots) and dicotyledoneae (dicots). These two have some major differences, beginning with the number of cotyledons present in each of the two seeds. Monocots, as the name suggests, have one cotyledon, whilst dicots have two. The veins of a monocot run parallel, and those of a dicot are netted. Dicots are noted for their occurring in fours or fives and monocots normally appear in symmetries of three.

�����Plants can reproduce sexually and asexually. Vegetative reproduction can take place as fragmentation, adventitious rooting, or apomixis. Fragmentation involves the separation of a part of a plant, and the regeneration of a clone from that part. This can be observed in chlorophytum comosum (or more commonly referred to as spider plants). The reason that plants can do this, but animals can't is because of the presence of meristems. They have the ability to produce whole new plants or plant parts. Adventitious root systems act like fragmentation in which a plant sends out adventitious roots that themselves grow shoots in the reformation of a plant. Apomixis is reproduction without the formation of gametes via meiosis. An apomictic seed is one that develops without the fertilization of a male gamete, therefore it will hold all the genetic material from the mother, and will be a female plant. These seeds can be distributed as all other seeds are. Sexually reproducing organisms may carry this action out through cross-pollination, self-pollination, or both. In the example of Gregor Mendel's pea plants, he self-pollinated the peas to obtain a pure strain of plants, but they also cross-pollinate. Self-pollination is considered sexual reproduction because it includes the joining of both gametes, even if it is from the same plant.

�����Seeds are normally a product of sexual reproduction. An embryo sac surrounded by the nucellus develops into a seed after fertilization, then, embryogenesis takes place resulting in a mature seed. A seed consists of an embryonic axis (root and axis), food reserves, and a seed coat. The dicot is believed to be more primitive, evolving first. Within their embrionic axis, a plumule, hypocotyl, and radicle exists. The plumule would be the undeveloped bud of a plant. The hypocotyl is the part of the embryo plant beneath the cotyledons, and the radicle later develops into a root. The two cotyledons serve as the food reserves. They are composed of protiens, starches, and lipids. Before the plant can photosynthesize its own food, it relies on the food sources of the cotyledons. If this hoard became too far depleted before the plant was able to do this, it would die. The exterior of a seed is crucial. It is the barrier between safety and the cruel, cold world. The safety is that of which allows the seed to live through harsh conditions before becoming a beautiful, self-sufficient organism. By these means are those that almost ensure plant reproduction. The hilium is a structure of the seed coat that shows where it was once connected to the mother plant (a botanical belly button). The micropyle is also a part of the seed coat; the part, which was where the tube the pollen travels down to the egg, was once connected.

�����A typical monocot may contain some of the same structures of a dicot, with the exception of a protective cap over the plumule, and monocots have a scutellum. The scutellum acts to transfer food from the endosperm to the seedling.

�����A seed increases the likelihood success rate that a plant will reproduce. They are alive, but you would barely be able to tell. It will remain dormant; it will not grow, until conditions are favorable. Life processes in a seed take place, although they are very slow, and scarcely noticeable. In order to grow up later, one must be alive now. Seeds can remain dormant for different periods of time, mostly depending on the breed of seed. Some can last up to thousands of years.

�����Also varying with the seed is the conditions under which germination will take place. The seed can sense the right time by inside or outside mechanisms that appear to the seed, such as the presence of water or suitable temperature, or also scarification of the seed coat (for those with a thick coating). This may be caused by the pounding current of a river, an animal that has chewed the seed coat partially, or even passing through the digestive tract of an animal. Some seeds with thinner seed coats may germinate with the absorption of water, or even the penetration of light.

�����When germination takes place, the seed will take up water, start to rely on its' food reserves, break out of the seed coat, and begin the growth of the stem and root structures. The radicle stretches down, while the hypocotyl expands upwards. The seedlings metabolism rate increases as it grows and utilizes its' hoard for these actions.

�����The shoot emerges from the soil, and photosynthesis begins to produce a food source in contrast to the usage of the storage, which, by now shouldn't have expended itself, for if it has, the plant would have died.

�����Life prevails. Through natural selection, the advancement of seed coverage evolved to better the organisms' likelihood to reproduce successfully.