Plants don't move around, that we all know, yet they manage to get all that they want, grow big, really big! I wonder what makes them grow? Yes, the basic answer comes to my mind; sunlight, soil, and water, but closer introspection of this answer tells me what they need to grow, not how they grow. So this article is an attempt to answer all these questions.
Plants are living organisms, comprising a vast variety of species such as trees, shrubs and bushes, herbs, grasses, vines, ferns, moss, and green algae. They belong to the kingdom Plantae. With over 350,000 (identified) species and still counting, plants are the largest group of organisms found on this planet. Plants consist of six basic parts; roots, stem (branches), flowers, fruits, seeds and leaves.
Each part plays a role in the plant's growth. Plants extract their food from the atmosphere using the process of photosynthesis with the help of chlorophyll, a green-colored pigment. With this process plants use the sunlight's energy to convert atmospheric carbon dioxide into simple sugars, which is then used to build the plants structural component. Plants also rely on soil for nutrients (nitrogen, phosphorus, magnesium, etc) and water. This explains the basic needs that makes plants grow. But then there are plants that grow without soil, or in the dark without sunlight. Plants use the method of photosynthesis and respiration to produce food, and on this premise will grow anywhere they get it.
Growth of Plants
Plants grow from seeds, because the seeds contain within themselves an embryonic plant and stored food. The seed is a result of pollination, where the ripened ovule is protected by the seed coat, an outer covering of the ovule. Basically, the seed contains the whole immature plant (roots and leaves), the seed's leaves are called the cotyledons; a seed with one leaf is known as monocotyledonous or monocots, while a two-leaved seed is known as dicotyledonous or dicots. For a seed to germinate, right conditions such as quality of the endosperm (food within the seed), moisture, temperature, humidity, light, and quality of the nutrients in the soil, all come in play.
In the Dark
To most of us this may seem strange, for we have been schooled in the thought that plants need sunlight to grow. Which is quite right. Some may grow in shade, some indoors, but they all need light, full, partial, or diffused. Plants follow the principle of phototropism, which means that their growth is determined by the direction of the light source. Here, even the concept of gravitropism (gravity), also known as geotropism comes to play, the plant shoots exhibit positive phototropism (growth towards light) and negative gravitropism (defying gravity by shooting upwards), while the roots exhibit exactly the opposite; negative phototropism and positive gravitropism.
When seeds are sown, 'keep it in a dark place' is generally a common instruction, but by dark one never means absolute dark, it just refers to keeping the seeds away from direct sunlight. Plants need a source of light to grow, to produce their food through the process of photosynthesis. If plants do not receive light, they will not be able to produce chlorophyll, eventually losing their green color and they will die. This rule also applies to plants that have different colored leaves too. Some plants may grow in the dark, but they are equipped to source their light from the atmosphere. Try a simple experiment at home, take a small plant and keep it in dark, one may have to cover it, and see for yourself, in a few days your plant will wither and die.
In the Ocean
For plants to grow in the ocean they do need sunlight, in fact, all marine life is dependent upon the light and the process of photosynthesis. Sunlight penetrates the ocean up to a thousand meters. It is up to 100 meters in depth that receives a good amount of sunlight, this place is called the euphotic zone, while below that till 1000 meters receives diffused sunlight, and is known as disphotic zone. The layer where no light penetrates is called the aphotic zone.
Basically there are two types of plants found in the ocean; ones with roots that grow on the ocean bed, and rootless ones that float about with the water. The ones that float receive all the light they need directly for photosynthesis as they are on the surface of the water, whereas the ones that are rooted are found only in shallow waters in the euphotic zone. No plants are found below this zone as there is not enough sunlight to maintain photosynthesis in deeper waters. Besides sunlight and water (which they have in plenty), marine plants such as phytoplankton, algae, seaweed, kelp, etc., draw carbon dioxide from the atmosphere. Carbon from carbon dioxide is needed for photosynthesis.
Plants have been growing in water and without soil for years now. It is only in the past decade the concept has caught on. Most epiphytic, parasitic, and symbiotic plants do not need soil to grow, but find other mediums to ensure a steady flow of nutrients. The principles of hydroponic gardening are applied to grow plants in water. All nutrients required by a plant are provided through the water, by the way of fertilizer application; combined with sunlight they receive, plants have no trouble surviving. Another method is the aeroponic gardening; here the plants are suspended in air and their roots are kept moistened with nutrient-rich water. This method is being extensively studied for the purpose of commercial agriculture, especially for places where soil conditions are poor.
Exploration of space brings to mind whether plants will be able to grow in space or not. Plants haven't been found growing on Mars or Venus, but they are being grown in space shuttles. A specialized chamber called 'Advanced Astroculture' a.k.a. ADVASC or a container called Biomass production is used for this process. This chamber is experimentally controlled to provide the plants with water and a supply of carbon dioxide to help them in their own food production through photosynthesis.
I hope this article has helped you understand the growth mechanism of plants a little better, if you do know more, share it with me.