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16
A flowering plant is planted in an earthen pot and irrigated. Urea is added to make the plant grow faster, but after sometime the plant dies. This may be due to ............. .
Explanation
The plant dies due to exosmosis. As the solution outside the plant is an hypertonic solution, and the plant celsl are hypotonic in nature, so there is a gradual movement of water from plant cell to outside urea solution leading to plasmolysis of root cells and plant dies gradually.
17
Absorption of water from soil by dry seeds increases the ............ thus helping seedlings to come out of soil.
Explanation
Due to imbibition of water by seed materials (starch and protein), pushes the seedlings out of the soil, thus the seed swells and imbibition pressure increases inside the seed, which contributing for germination of seeds.
18
Water moves up against gravity and even for a tree of 20 m height, the tip receives water within two hours. The most important physiological phenomenon which is responsible for the upward movement of water is ............ .
Explanation

Transpiration pull is the physiological phenomenon which is responsible for the upward movement of water in trees of height up to 20 m . As the water molecules transpire from stomata, it pulls water molecules up ward to the leaf from the continuous chain of water molecules carried by xylem.

This transpiration from leaf creates a pressure gradient between the atmosphere and sub-stomatal cavity and intercellular spaces of leaf. This gradient is transferred to photosynthetic cells and then on to xylem of leaf vein.

It results in a force called "transpirational pull" that can lift water upto 130 m high, through a xylem column.

19
The plant cell cytoplasm is surrounded by both cell wall and cell membrane. The specificity of transport of substances are mostly across the cell membrane, because ............. .
Explanation

The transport takes place by the cell membrane because cell membrane is semi-permeable and composed of lipids that are arranged in bilayer structure along with proteins and carbohydrate.

The lipid soluble molecules pass readily across lipid bilayer of cell membrane whereas movement of other molecules of small size is facilitated by porins, pores made up of proteins. While cell wall does not helps in transport of substances because it.

Cell wall is basically a non-living rigid structure forming outer covering over plasma membrane and it provides shape, mechanical strength and protection to the plant cell from damages and infection.

20
The $\mathrm{C}_4$ plants are twice as efficient as $\mathrm{C}_3$ plants in terms of fixing $\mathrm{CO}_2$ but lose only ............ as much water $\mathrm{C}_3$ plants for the same amount of $\mathrm{CO}_2$ fixed.
Explanation
Half, The evolution of the $\mathrm{C}_4$ or plants photosynthetic system is probably one of the strategies for maximising the ability of $\mathrm{CO}_2$ while minimising water loss. $\mathrm{C}_4$ plants are twice as efficient as $\mathrm{C}_3$ plants in terms of fixing carbon in the form of glucose, but lose only half as much water as $\mathrm{aC}_3$ plant for the same amount of $\mathrm{C}_2$ fixed.