(a) In plants, there are tiny pores called stomata on leaves and lenticels in stem which facilitate the exchange of gases. CO2 is taken in and O2 given out (during photosynthesis) and vice-versa during respiration.
(b) Mechanism of Transport of Water and Minerals in a Plant
• The vessels and tracheids of roots, stems and leaves in xylem tissue are interconnected to form a continuous system of water-conducting channels reaching all parts of the plant.
• The cells of the roots in contact with the soil actively take up ions which creates a difference in the ion concentration between the root and the soil.
• Thus, there is steady movement of water into root xylem from the soil, creating a column of water that is pushed upwards.
• Plant uses another strategy to move water in the xylem upwards to the highest points of the plant body.
• The water which is lost through the stomata is replaced by water from the xylem vessels in the leaf.
• Evaporation of water molecules from the cells of a leaf creates a suction which pulls water from the xylem cells of roots.
• This loss of water is transpiration which helps in the absorption and upward movement of water and minerals dissolved in it from roots to the leaves.
• Transpiration becomes the major driving force in movement of water in the xylem during the day when the stomata are open.
• This mechanism is also known as cohesion of water theory or transpiration pull.
