observations of gamma-irradiated and control wheat plants showed that the oldest leaves and coleoptiles of irradiated wheat were shorter than the oldest leaves and the coleoptiles of the control plants. Microscopy was used to establish if the differences in coleoptile length in 7-day plants were due to changes in the gamma-irradiated wheat at a cellular level. Wheat coleoptiles consist of two basic cell types: cortical cells and epidermal cells. A large number of cells of both types were measured in gamma-irradiated coleoptiles and control coleoptiles. The mean cell lengths were calculated and used to calculate the cell index for each type of cell in gamma-irradiated wheat and control wheat.

(i) Why was it necessary to measure the lengths of "a large number of cells"?

(ii) In Stage 6 and Stage 12 of activity D you prepared tissue before viewing it under the microscope. What general preparations are necessary for viewing biological material under the microscope and why are they required?

(iii) Explain what is meant by the term "index of cell number" What would be the index of cell number ( to the nearest whole number) for the epidermal gamma cells of a 7-day gamma irradiated wheat plant which was known to have a coleoptile length of 14 mm and a mean cell length of 318 µm.

(iv) Cortical cells in control wheat have a higher index of cell number than cortical cells in gamma-irradiated wheat; explain this finding in terms of cellular processes that may be taking place in the coleoptile.