25.

(a) Describe the cohesion and transpiration pull theory of ascent of sap.

(b) Explain the sequence of events between pollination and fertilisation in plants.

(c) State and explain Blackman’s Law of Limiting Factors.

(a) Cohesive and adhesive properties of water molecules form an unbroken continuous water column in the xylem. When water transpires from the aerial part of the plant it creates a transpiration pull or tension is exerted on this water column. Xylem vessels are tubular structures extending from roots to the top of the plants. Cells are placed one above the other, with their end walls perforated forming a continuous tube. These are supported by xylem tracheids which are characterised by having pores in their walls .one end of xylem tube is connected with the root hairs via pericycle, endodermis and cortex and another end is connected with the sub stomatal cavity in the leaves via mesophyll cells. This tube is filled with water. The water is filled inside the xylem capillaries and due to cohesion and adhesion properties of water, it forms a continuous water column. The water column cannot be broken or pulled away from the xylem walls because of cohesion and adhesion of water.

(b) Pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower.

When the pollen is shed from anther it has usually two cells a generation cell and a tube cell. The generation cell forms two mole gametes. Once the pollen has landed on the receptive stigma, germination starts. It absorbs water and swells up to produce a pollen tube. Entire contents of pollen travel into pollen tube. The pollen tube grows and enters the ovule. The entry of pollen tube into the embryo sac.

After entering into the embryo sac, the tip of the pollen tube bursts and two male gametes are discharged. The tube nucleus disorganizes before bursting of the pollen tube. The male gametes is released and it shows amoeboid movements. One moves towards the egg and other to polar nuclei. One male gametes fuses with the egg and results in the formation of a diploid zygote. This fusion of male and female gametes is known and fertilization. This is the sequence of steps from pollination to complete fertilization.

c) According to this Blakman's Law of Limiting Factors - When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the ‘slowest’ fac­tor.

For example, the rate of photosynthesis depends on a number of factors like light intensity, CO₂ concentration etc.  If the light intensity is minimum then the rate of photosynthesis will depend on the amount of light intensity rather than the other factors.