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2. H. Tada, P. Paris and G. Irwin, ?The Stress Analysis of Cracks Handbook? (Del Research Corporation, Providence, Rhode Island, 1973).
3. S. A. Wainwright, W. D. Biggs, J. D. Currey and J. M. Gosline, ?Mechanical Design in Organisms? (Edward Arnold, London, 1975).
4. P. S. Evans, J. Agric. Sci. Camb. 67 (1969) 171.
5. Idem, ibid. 67 (1969) 175.