BIOLOGY----- A SCIENCE OF EXCEPTIONS...

The laws that govern the physical and chemical phenomena of non-living bodies are rigid and no deviations or exceptions are noticed. Although, physical and chemical phenomena are associated with life processes, the laws of physical and chemical phenomena are not always followed in their true sense, because, life-forms exhibit enormous variations and adaptations in their functions. Life-forms are constantly evolving and changing in forms and functions to get advantage. As a result of this, variations in the laws that govern the phenomena of life functioning are observed.Therefore, biologist must be prepared to accept exceptions of laws and principles. 

      

        Some examples of such exceptions are given below:

1. Euglena has both plant characters as

well as animal characters. They lack

cell wall, but bear chloroplasts. 2. Viruses do not observe definite set rules

of living being. 3. Mangrove produces respiratory roots

(pneumatophores) which grow against

the force of gravity and towards light. 4. Roots are non-green and non-photo synthetic. They

green

and photosynthetic in Tinospora, Trapa etc. 5. Organised nucleus which is essential for cell functions is absent in

many organisms (e.g., Bacteria). 6. All living cells are nucleated except

mature R.B.Cs of mammals and sieve

tubes in plants. 7. DNA is the hereditary material of most

organisms, but in plant viruses RNA is

the hereditary material. 8. DNA is double stranded, but it may be

single stranded also (e.g., virus 0 x 174). 9. RNA is single stranded but it may be

double stranded also (e.g. Ribovirus). 10. Mature red blood cells of mammals

lack nuclei with the exception in

camel. 11. In cactus the stem becomes green in

colour and takes the functions of leaves. 12. Mitochondria

the respiratory organelles of most organisms, but in primitive forms like bacteria mesosomes perform the function of

mitochondria. 13. Reproduction can take place even at the

larval stage (e.g., American

Salamander). 14. Birds fly but Ostrich and Kiwi are

flightless birds. 15. Birds never excrete urine. 16. Mammals are viviparous (give birth to

young ones) but Duck Billed Platypus. and Spiny Ant Eater (Echidna) are

oviparous (egg laying mammals). Dolphins are Viviparous. 8. Heart in reptiles is three chambered but

it may be four chambered in crocodiles. 19. Blood of all vertebrates is red due to

the presence of haemoglobin but a shark, Carcharhinus, has colourless

blood. 20. Cuscuta is a dicot but there is no

cotyledon. 21. Leaves of dicots have reticulate

venation but in Eryngium and Calophyllum there is parallel venation. Smilax, a monocot has reticulate

venation. 22. Lungs are absent in fishes but lung

fishes have lungs as well as gills.