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The reproductive systems in both plants and animals play a vital role in the continuation of a species, as well as for maximum success of the species. Reproduction allows for beneficial alleles to increase in frequency within a gene pool, and aids in removing deficient mutations. There are a number of clear physical differences in the reproductive systems and behaviours of plants, but upon closer inspection, it becomes apparent that there are in fact a handful of similarities between the two kingdoms.
Fig.1 : The flowers of the Ophrys kotschyi, or Cyprus Bee Orchid, which are used to attract bees in order to transfer pollen To the untrained eye, the most obvious difference between reproduction in plants and animals is the behaviours displayed. In the animal kingdom, one can often observe males fighting for females, in order to maximise their contribution to the gene pool. This is often referred to as inclusive fitness (Cowie, 2009). This in itself is a form of natural selection, known as sexual selection. In other animal species, such as in the Arctiidae family, or Tiger Moths, courtship pheromones and ultrasound are used to attract females (Weller, Jacobson, & Conner, 2008). Clearly this does not take place in plants, although there are methods used to attract insects and other vectors which are required to transfer pollen form plant to plant. Many plants have brightly coloured flowers, which attract butterflies and other such insects. Some plants use a more clever method, such as the Ophrys kotschyi, which mimics the shape and colour of a bee in order to attract the insects, which will be covered in pollen when they land on the imitation. This is an example of a mutualistic symbiotic relationship. From this, it can be seen that although the behaviours of the two kingdoms are very different, they are in fact simply different interpretations of the same thing.
A further difference between the two kingdoms is the method of fertilisation. In most animals, the two sex cells, the spermatozoa and the oocyte, meet during sexual intercourse. This requires the two animals to physically interact with each other within close proximity. However, in plants the sex cells must be transmitted by a vector, such as an insect or a bird. This happens when an insect, for example, lands on the pollen rich anther, which contains 4 microsporangia, or pollen sacs. These sacs contain microsporocytes, which are diploid cells. When forming male cells, or pollen grains, these undergo meiosis, to form 4 microspores for each microsporocyte. These eventually give rise to a gametophyte, which is a haploid cell. This gametophyte consists of a generative cell and a tube cell. During maturation, the generative cell passes in to the tube cell, completing the spore wall. This tube cell produces a pollen tube, which can quickly deliver sperm to the female gametophyte. (Campbell & Reece, 2008)
Another major similarity between plants and animals is also one of their major differences. Both plants and animals reproduce sexually, producing a male and female gamete, which fuse to form a zygote. However, plants are also able to reproduce asexually, unlike most animals. Furthermore, although both plants and animals are able to reproduce sexually, the mechanisms they use to do this starkly contrast one another.
In sexual reproduction in animals, the two gametes are the spermatozoa and the oocyte, which when fused become the ovum. Using humans, Homo sapiens, as an example, the spermatozoa are produced by spermatogenesis. During this process, human stem cells called spermatogonia divide by mitosis, forming spermatocytes. These cells then undergo meiosis, forming spermatids, which are haploid gametes. This means that as daughter cells, they have half the amount of genetic material compared to that of the parent cells. These spermatids are relatively unspecialized, so they have to undergo spermiogenesis in order to differentiate to spermatozoa. Once they have done so, the spermatozoa are ready for fertilization. The spermatozoa are produced in the testes, and travel within the epidydimis, to the vas deferens, onwards to the ejaculatory duct and finally through the urethra. Other organs involved in the successful transportation of these spermatozoa include seminal vesicles, the prostate gland, and the bulbourethral glands. Each of these secretes various fluids into the ejaculatory ducts, and the urethra (Martini, 2004). It is worth noting also that some animals are able to reproduce using asexual reproduction. These species are few and far between, though it does occur in some species of lizard and insect. Asexual reproduction in animals occurs either by vegetative means, such as budding as seen in Hydra, or by fragmentation as seen in Planaria, or by regeneration as seen in Echinoderms.
To contrast, plants use a very different method of sexual reproduction, in which the production of the gametes and the production of a zygote is very different to the methods used in animals. The female gametophytes, or embryo sacs, form through a variety of different methods, though the most common occurs entirely within the carpelââ‚¬â„¢s ovary. During this process, two integuments surround each megasporangium entirely, apart from one small section, called the micropyle. Female gametophyte production begins when the megasporocyte enlarges and undergoes meiosis to produce four haploid megaspores. Of these four, only one survives. This then undergoes mitosis three times, without cytokinesis, resulting in one cell with eight haploid nuclei. This cell then divides into a multicellular gametophyte, which is the embryo sac. One of these becomes the ovule, which eventually forms a seed. The production of the male gametophyte results ultimately in a pollen tube, the process by which is described above.
The process during which a male and female gametophyte meets is known as pollination. This is the transfer of pollen from an anther to a stigma, and is usually done by insects. If pollination is successful, a pollen grain produces a pollen tube, which then grows down into the ovary via the style. Pollination can be done by a number of organisms, including bees, flies, wasps, bats, birds and also by wind.
Despite such a large number of differences in the methods of gamete production, and of fertilization, the final stage of reproduction; growth of the offspring, is startlingly similar. Both a seed and an embryo act in the same way, in that they become multicellular. Within the womb in animals, an embryo has all the nutrients it requires to begin growth, and any other material it requires is delivered through the placenta. In plants, a seed also has all the nutrients it requires to begin growth within the angiosperm, and once it has germinated it can begin to get any further nutrients it requires from its root.
Asexual reproduction in plants is also very different to that in animals. The methods used by animals are mainly budding, fragmentation, regeneration and parthenogenesis. However, in plants, there are many different methods. In the strawberry plant, Fragaria chiloensis, the stems can themselves take root and form new plants (Hancock & Bringhurst, 1980). Leaves can also form new plants, when mitosis at meristems along leaf margins takes place. Roots can also be a source of asexual reproduction, as seen in the dandelion, Taraxacum officinale. Propagation is also widely used to preserve desirable traits, and is performed simply by planting cuttings of the desired species.
From this analysis of the reproductive systems of the two kingdoms, there is one more small difference which has also become apparent. In animals, the offspring is an embryo, or in a few cases an egg, whereas in plants the offspring is the seed. It is also apparent that both kingdoms use meiosis and mitosis to maintain correct chromosome number, although there is the potential for sympatric speciation to occur within plants and animals, a prime example being that of polyploidy in Hibiscus rosa-sinensis (Rao L. N., 1941) and of salamanders (Fankhauser G. , 1939)
In conclusion, there are clearly a number of differences in reproduction in plants and animals, with the most striking being the use of asexual reproduction in plants. The ability of plants to reproduce asexually is one which has proven to have true advantages to the successful survival and evolution of a species, and although it is not to be relied upon, I believe it is a fantastic method to ensure the continuation of a set of alleles within a gene pool.
However, even within sexual reproduction, both kingdoms use very different methods to achieve the same outcome. The behaviour in both kingdoms also distinguishes them from one another, particularly as plants lack the range of movement posessed by most animals. Clearly both kingdoms have evolved to use very distinct methods, but through natural selection and evolution, both have come to find a method which is successful.