MECHANISM OF TRANSPORTATION IN PLANTS

The lower plants

All the plants from the single-celled Chlamydomonas, bacterium, Phacus to the simple multicellular Spirogyra, Funaria (Bryophyta) that lack conducting vessels use their cytoplasm for the movement and distribution of materials. Most of these lower plants live in water, moist places and the shade of larger higher plants. There is water to hydrate their cytoplasm if not completely surrounded by it. Digested food, dissolved gases and manufactured food are moved around the cell and from cell to cell by the streaming of cytoplasm.

Carbon (IV) oxide diffuses out of the cells as the cytoplasm streams around. Other metabolic wastes diffuse out or diffuse into cell wall where they are converted into insoluble forms, stored and kept away from contact with cytoplasm.

Reserved carbohydrate in the form of simple sugar gets moved in similar manner to the pyrenoids where it is stored in the form of starch.

 

Transportation in the vascular plants

Some vascular plants are the ferns (Pteridophyta), pinus (Gymnospermae),  sunflower and maize (Angiospermae). These plants develop channels through which water flows in their body. The materials for transportation (gases, soil water, solutes, manufactured food, hormones and metabolic wastes) are carried in water that flows in these channels. The structures that serve as transport channels are called vascular bundles (tissues) comparable to the blood vessels and lymphatic vessels of animals.

The Vascular bundles are the conducting elements of the plants. Each vascular bundle consists of the phloem on the outside and the xylem inside.

The phloem is a complex tissue made up of sieve tube cells, phloem parenchyma and phloem fibres. Of these four types of phloem cells only the sieve tube cells translocate manufactured food from the leaves down to all living cells and storage organs of the plant.

The xylem is also a complex tissue made up of vessels, xylem, tracheids, wood parenchyma and wood fibres. Of these four types of xylem cells only the xylem vessels and tracheids are responsible for the upward conduction of soil solution containing dissolved mineral substances from the roots to the leaves.

 

Mechanism of transportation of water in higher plants (root pressure)

The soil water which has entered the cell sap of the root hairs through osmosis and by capillarity passes to the cortical cells of the root. From the cortical cells, the soil water enters the endodermis then to the pericycle and finally to the xylem vessels of the root. From the xylem vessels of the root, the soil water is passed to the xylem of the stem under root pressure.

 

Experiment to demonstrate root pres-sure in a young potted plant

Method

The shoot of a young potted plant is cut off sharply a few centimetres above the level of soil thus leaving a stump. Within a short time water (xylem sap) exudes from the cut end of the stump. A glass tube is fixed tightly to the stump with a short rubber band. Water is introduced into the glass tube with a pipette to level marked “A”.

 

Result

After a short time water is seen rising from A to B in the glass tube due to root pressure. The force responsible for pushing the sap up the tube is called root pressure. The pushing effect of root pressure is sufficiently strong to account for ascent of water in low growing plants. The root pressure is not enough to raise water up to heights of trees over 34 metres tall.