Evolutionary Biology Topics allow us to trace the history of life on Earth.
What are various modes of nutrition in Heterotrophic Plants?
Most of the plants have green pigment called chlorophyll and can make their own food. They are called autotrophic. Some plants, however, do not contain chlorophyll and hence cannot synthesise their food. They are called heterotrophic and depend for food on other organisms. Depending on their mode of obtaining food, all the heterotrophic plants can be divided into two main groups :
- Parasites, and
Thus, just like animals, some plants have also heterotrophic modes of nutrition. We will now discuss the two types of plants called parasites and saprophytes which have heterotrophic mode of nutrition.
A plant (or animal) which lives on or inside another organism (called host) and derives the food from it, is called a parasite. Those non-green plants which obtain their food from the living bodies of other plants (or animals) are called plant parasites (or just parasites). The living organism (from whose body food is obtained) is called ‘host’ of the parasite. The parasite plants climb on the host plants from which they get all the food. A parasite plant produces certain special type of roots (called sucking roots) which penetrate into the host plant. The parasite sucks the food materials from the host through these special roots.
An example of parasite plant is Cuscuta. It is called Amarbel in Hindi. Cuscuta plant does not have chlorophyll (so it is not green in colour). We can see Cuscuta plant as a yellow, tubular structure twining around the stem and branches of a tree (see Figure). The tree on which Cuscuta plant climbs is called its host. Cuscuta is a non-green plant having a yellow colour. Since Cuscuta plant does not have the green pigment called chlorophyll, it cannot synthesise its own food. Cuscuta plant takes readymade food from the tree (or plant) on which it climbs. Since Cuscuta plant deprives the host tree of valuable nutrients, it is called a parasite. We find that Cuscuta plant depends on other plants for obtaining food, therefore, Cuscuta plant has heterotrophic mode of nutrition. Please note that another name of Cuscuta is Dodder. In addition to Cuscuta, some other examples of plant parasites are: Mistletoe, Wheat rust and Corn smut.
In the above discussion, at many places we have used the term ‘plant parasite’ instead of just ‘parasite’. This is because many animals also live as parasites. For example, the animals such as lice, bed-bugs, leeches and mosquitoes (which suck our blood) are also parasites.
Those non-green plants which obtain their food (or nutrition) from dead and decaying organic matter are called saprophytes (Dead and decaying organic matter means dead and decaying plants and animal remains). The non-green plants called fungi (read as : funjaaee) derive their food from dead and decaying organic matter, so fungi are saprophytes. Some of the common fungi are mushrooms, bread mould and yeast, so we can also say that the fungi such as mushrooms, bread mould and yeast are saprophytes. In other words, the fungi such as mushrooms, bread mould and yeast have saprophytic mode of nutrition (which is a heterotrophic mode of nutrition). Certain bacteria are also saprophytes. They are called saprophytic bacteria.
Many times we see small, fluffy, umbrella-like plants growing on rotting wood during the rainy season (see Figure). These are a kind of fungus called mushrooms (Fungus is the singular of fungi). They take their nutrition from the rotting wood of a dead tree, so they are saprophytes. The saprophytic plants (like fungi) do not have chlorophyll and hence they cannot make their own food by photosynthesis. The non-green saprophytic plants (such as fungi) obtain their food as follows : The saprophytic plants (fungi) secrete digestive juices on the dead and decaying organic matter and convert it into a solution. They absorb the nutrients from this solution. This mode of nutrition in which plants take in nutrients from dead and decaying organic matter is called saprophytic nutrition. The plants which use saprophytic mode of nutrition are saprotrophs.
To Grow Fungus on Bread
We can grow fungus on bread ourselves as follows : Take a slice of bread and moisten it with water. Keep it in a moist and warm place for 2 or 3 days. We will find that some fluffy patches appear on the surface of the slice of bread (see Figure). These patches are of fungus plants. The patches of fungus may be white, green, brown, black or any other colour. If we look at this slice of bread through a magnifying glass, the fungus plants growing on its surface will appear to be cotton-like threads spread on the slice of bread.
We will now describe how the fungus grows on the slice of bread. The tiny spores of fungus plants are always present in air (but we cannot see them with naked eyes). When these tiny spores land on wet objects under warm conditions, they germinate and grow into new fungus plants. It was one such air-borne spore which settled and grew on the moist slice of bread kept aside by us at a warm place for a few days. Please note that the spores are a kind of seeds of fungus plants.
Fungi also grow on pickles, leather objects, clothes and other articles which are left uncleaned in hot and humid weather for a considerable time. The growth of fungi may spoil many things during the rainy season. It is due to the growth of fungus that our leather shoes kept in the house sometimes get spoiled during the rainy season.
Fungi can be useful as well as harmful. For example, fungi such as mushrooms and yeast are useful. Mushrooms are eaten as a vegetable whereas yeast is used for producing alcohol. Some fungi are also used for making medicines. For example, the Penicillium fungus is used in making an antibiotic called penicillin. Fungi also cause diseases in plants and human beings. Sometimes the whole crops standing in the fields are destroyed by fungus. The skin disease (called ringworm) in humans is caused by fungus.
In addition to parasitic and saprophytic modes of nutrition in some plants, there are also some other plants whose modes of nutrition are somewhat peculiar. These are called insectivorous plants and symbiotic plants. Let us discuss these peculiar modes of nutrition in plants in detail, one by one.
There are some green plants which obtain their food partly from the soil and atmosphere, and partly from small insects. Those green plants which obtain their food partly from insects are called insectivorous plants.
Insectivorous plants are also known as carnivorous plants. The insectivorous plants have specialised leaves to catch the insects. The insectivorous plants grow only in those soils which do not contain sufficient nitrogen mineral. These insectivorous plants trap insects by various methods, kill them and digest them to obtain nitrogen compounds (like amino acids) for their growth. Some common examples of the insectivorous plants (or carnivorous plants) are : Pitcher plant, Sundew, Venus fly-trap and Bladderwort.
The insectivorous plant called pitcher plant is shown in Figure. The pitcher plant uses a pitcher like organ to trap insects and digest them. This is explained below. In the pitcher plant, the lamina (or blade) of the leaf is modified into a hollow tube called pitcher (see Figure). The leaf apex (top part of leaf) forms a lid which can open or close the mouth of the pitcher. Inside the pitcher, there are hair which are directed downwards.
When an insect falls in the pitcher, the lid closes automatically. The trapped insect gets entangled in the hair of the pitcher and hence cannot come out. After some time, the insect dies in the pitcher. The walls of the pitcher secrete digestive juices which digest the proteins present in the body of insect to form simpler nitrogen compounds (like amino acids). These simpler nitrogen compounds are absorbed by the walls of the pitcher and used by the whole pitcher plant.
Please note that insectivorous plants are green and carry out photosynthesis to obtain a part of the food required by them. But they do not get the ‘nitrogen nutrient from the soil in which they grow. So, insectivorous plants feed on insects to obtain the nitrogen compounds needed for their growth. We can now say that the insectivorous plants are “partial heterotrophs”. They get some food by photosynthesis and some by eating insects.
The living together of two different species of plants as if they are parts of the same plant and help each other in obtaining food is called symbiosis (and such plants are known as symbiotic plants). This type of nutrition involving symbiosis occurs in the plants called ‘lichens’ (see Figure). In lichens, the green coloured plant called ‘alga’ (autotroph) and non-green plant ‘fungus’ (saprophyte) live together. The fungus holds the alga cells in its mat of web-like hyphae (thin filaments). The fungus plant absorbs the water and mineral salts from the surroundings and supplies them to alga. The alga plant being green, prepares the food by photosynthesis and shares it with fungus. Thus, both alga and fungus gain mutually from one another by living together. This is an example of symbiosis.
The plants which exhibit symbiosis are called symbiotic plants. The condition where two different organisms live together and help each other to survive is called symbiotic relationship. Symbiotic relationship is advantageous to both the organisms.
Another example of symbiotic relationship is provided by Rhizobium bacteria and leguminous plants. Rhizobium bacteria cannot make their own food. Rhizobium bacteria live in the root nodules of leguminous plants (such as gram, peas, beans, pulses, etc.). Rhizobium bacteria convert nitrogen gas of air into water soluble nitrogen compounds (called nitrates) and give them to the leguminous plants for their growth. In return, leguminous plants give food and shelter to Rhizobium bacteria. Thus, Rhizobium bacteria and leguminous plants have a symbiotic relationship.
Certain fungi live in the roots of trees. These fungi take up water and minerals from the soil and supply them to the roots of the tree. The tree, in return, provides food to these fungi. This symbiotic relationship is important for the survival of fungi as well as the tree.