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The following points highlight the fifteen important Thorne’s principles (1958) of plant classification.
Thorne’s Principle # 1. Existing species have descended with change from pre-existing species and are therefore, the products of evolutionary forces.
Thorne’s Principle # 2. Ancestral conditions and trends of specialisation are often recognisable in the organs, tissues and cells of living and fossil angiosperms.
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Thorne’s Principle # 3. The primitive, ancestral condition of any given characteristic can be no more specialised than its condition in derived, existing species most primitive for that characteristic.
Thorne’s Principle # 4. The presence of vestigial rudiments of organs, or sometimes the presence of vestigial vascular supply to greatly modified or missing organs, often furnishes evidence of evolutionary reduction, loss, fusion or other major modification of structures.
Thorne’s Principle # 5. The prevalence of parallel and convergent evolution in habit, function and structure is a predictable consequence of the relatively limited means, angiosperms have for effective reproduction and for adaptation to available environmental niches.
Thorne’s Principle # 6. All parts of plants at all stages of their development may produce evidence that is valuable in establishing relationships.
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Thorne’s Principle # 7. Evolution may tend towards elaboration and diversity or towards reduction and simplicity.
Thorne’s Principle # 8. The role and direction of evolution may vary in different organs and tissues of plants.
Thorne’s Principle # 9. Most existing angiosperms are highly specialised and greatly modified from their primitive, generalised ancestors.
Thorne’s Principle # 10. Evolutionary trends are sometimes reversible under the influence of change in environmental factors.
Thorne’s Principle # 11. Once lost, organs usually are not regained.
Thorne’s Principle # 12. New angiospermous structures have arisen as modifications of or as outgrowths from the pre-existing structures.
Thorne’s Principle # 13. The sporadic or restricted occurrence of unusual or uncommon characteristics lacking apparent evolutionary significance is often an indication of relationship when correlated with other characteristics.
Thorne’s Principles # 14. The occasional attainment of certain characteristics of certain levels of evolutionary development is frequently valuable in determining the affinities of families and orders.
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Thorne’s Principles # 15. Embryos and seedlings of related though dissimilar plants often resemble each other more than do the adult plants because of their apparent retention of primitive characteristics.
