Plant Photosynthesis

It has been found useful to classify living organisms on the basis of their nutritional requirements for carbon containing compounds as either autotrophs or heterotrophs. Organisms able to utilize CO2 as their sole source of carbon for the formation of organic food by the process of photosynthesis are called autotrophs (self nourishing). In addition to CO2, autotrophs require H2O and several inorganic ions.

In contrast to autotrophs, many bacteria, fungi and certain higher plants are incapable of photosynthesizing organic food. They lack chlorophyll. Such plants are completely or partially dependent on some other source for their nutrition and are called heterotrophs. When some plants are incapable of photosynthesising and obtain water, mineral and certain organic compounds from other autotrophs, they are called heterotrophs and the type of nutrition is referred to as heterotrophic nutrition.

Evidently, degree of heterotrophism varies (complete or partial). Such individuals are either parasites or saprophytes. Still some act as insectivorous plants.

(a) Parasites. Parasites obtain their nutrition from living plants or animals, which are called hosts. These parasites maintain physical contacts with the host plant through haustoria or suckers. These haustoria penetrate into host tissue and make connections with the conducting elements of host.

Total parasites like Cuscuta and Orobanche are never green, and consequently they have no power to prepare their own food. They get their food supply from the host plant on which they are parasites.

Rafflesia another root parasite, vegetative parts of the plant are highly reduced and represented by cellular filaments resembling fungal mycelium. These mycelium like structures get embedded into host tissue and flowers emerge out. Each flower of Rafflesia weighs about 11 kg. in weight with diameter of one diameter are largest in plant kingdom.

Some examples of parasites are:

Total stem parasite: Cuscuta
Partial stem parasite: Lomnthus, Viscum
Total root parasite: Orobanche, Balanophora, Rafflesia
Partial root parasite: Santalum, Striga, Thesium

Parasites can be further classified into obligate parasites and facultative parasites. Obligate parasites can live only on living hosts and facultative parasites have the facility to live as saprophytes besides being parasites, as long as no living organism is available.

In Cuscuta, haustoria from stem of parasite send small outgrowths into the, body of the host. Xylem and phloem of naustoria come in contact with the xylem and phloem of the host. Haustoria are metabolically very active and provide a channel system for the transport of water and other metabolites of host to the parasite. The haustoria exhibit intense activity of several enzymes e.g., oxidases, esterases, phosphatases etc. So, the penetration of haustoria into host tissue is not only mechanical but enzymatic also.

Orobanche inhabits the roots of many plants. The young seedlings of Santalum (sandal-wood tree) grow independently up to one year of age. After this, few roots develop haustoria which make contact with roots of neighboring trees. Viscum, a partial stem parasite bears green leaves and thus is capable of manufacturing food, but for water supply, it is dependent upon the host plant.

(b) Saprophytes. Saprophytes get their nutrition from dead and rotting organic matter. Many bacteria and fungi e.g. Agaricus exhibit saprophytic mode of nutrition. Neottia and Monotropa are the examples of flowering plants, which act as saprophytes. In such cases roots of plant constitute a mycorrhizal association with fungal hyphae, which help in absorption. Neottia (Bird’s nest orchid) and Monotropa (Indian pipe) grow in forest soil which is rich in humus formed from the fallen leaves. In pteridophytes Botrychium and some species of Lycopodium are partial or complete saprophytes.

(c) Insectivorous plants. Another category of heterotrophic plants can be identified as insectivorous or carnivorous plants. They are said to be partly autotrophic and partly heterotrophic. These autotrophs supplement their nutritional requirements by trapping and digesting insects and other small animals. Researchers have shown that most of the insectivorous plants can live without feeding on insects; however, their growth is stimulated when they digest the insects. The leaves of these plants are modified in several ways for trapping insects. Some of the insectivorous plants are discussed below:

(i) Pitcher plants. Nepenthes and Sarracenia are the examples of pitcher plants which behave as insectivorous plants. In such cases, leaf lamina is modified into a pitcher-like structure. The lid of the pitcher is a modified leaf apex. The flattened basal portion is the leaf base and stalk of the pitcher is the petiole of the leaf. Petiole contributes to tendrillar modification. The lid of pitcher is lined with a ring of nectar glands. The colored lid and the nectar attract the insects. The upper region of pitcher bears many digestive glands. This region is followed by a slippery surface. At the bottom of the pitcher acidic fluid is present. Insects after entering into pitcher fall into the acidic fluid of the pitcher.

Hairs present near the rim of the pitcher prevent the insect from coming out as they are pointed downwards. Digestive juices are secreted by the digestive glands. Thus body of the insect is digested by enzymatic action and finally digested food is absorbed by the pitcher. Nepenthes khasiana is found in north eastern India and has been identified as an endangered plant.

(ii) Sundew. In the herbaceous plants of sundew (Drosera), each leaf is covered on upper surface by glandular hairs called as tentacles. Each tentacle bears a gland at its tip. Glands are reddish in color and secrete viscous fluid which glitters in sun like dew drops, hence called sundew. When any insect, mistaking the glistening substance as honey, comes in contact with leaf, gets entangled viscous sticky fluid. Tentacles bend upon the insects and the lamina of leaf assumes cup-like structure. Digestive juices are secreted and digested food is absorbed.

(iii) Venus fly trap. Dionaea muscipula or venus flytrap is a herbaceous plant and grows on damp mossy places. Each half of leaf blade bears three long pointed trigger hairs placed triangularly on the leaf surface. The midrib of the leaf functions as a hinge between two lobes of lamina. The upper surface of leaf bears reddish digestive glands. When the insect sits on the leaf sensitive pointed hairs bring about sudden closure of leaf. Glands secrete the enzyme pepsin and hydrochloric acid, thereby digesting the protein contents of trapped insect.

(iv) Bladderwort. The Utricularia or bladderwort is a water plant. The leaves are highly dissected and bear small narrow segments. Few segments of the dissected leaves are modified into bladders or sac-like structures to entrap small water fleas. Each bladder is having the diameter size of 3-5 mm and is provided with a trap door entrance. This trap door functions as a valve which can open inwardly.

Small water insects after entering into bladder along with water fail to come out as valve shuts itself. Digestive glands present on the inner surface of bladder secrete digestive enzymes. The small insects are digested within few days and finally absorbed.