Some exceptions in Biology

• Physical sciences are governed by a set of laws, such laws are rigid and show no deviations or exceptions.
• But, biology is a science of life. It deals with living organisms.
• The living organisms show enormous variations and are capable of change.
• Therefore, the laws of physical and chemical phenomena associated with life processes show deviations or exceptions.
• Examples of such exceptions are as follows:

1. Dicotyledons have two cotyledons in their seeds. But the seeds of parasitic plants like Cuscuta are devoid of any distinct cotyledons.
2. Dicot leaves have reticulate venation while monocot leaves have parallel venation. However, a number of monocots (e.g., Smilax, Colocasia, Dioscorea) have reticulate venation. However, some dicots (Calophylum, Corymbium) have parallel venation.
3. Stems are normally aerial but in number of plants like Banana, Colocasia, Ginger has underground stems.
4. Roots are normally positively geotropic. But in plants like mangrove (Sonneratia) some of the roots are negatively geotropic, come out of saline marsh and take part in aeration. They are called pneumatophores.
5. Roots are non-green and non-photosynthetic. They are green and photosynthetic in Tinospora.

                plants having green roots                       pneumatophores having plant

6. Blood of all vertebrates is red due to the presence of hemoglobin but a shark, Carcharhinus, has colorless blood.

7. Adventitious roots are commonly developed from the nodes but in Ivy they grow from internodes as well.

8. All living cells have a nucleus except mature red blood cells of mammals.

9. DNA is hereditary material in all organisms except in some virus, called riboviruses. Here RNA functions as the hereditary material.

10. DNA is double stranded but in virus x 174, it is single stranded.

11. RNA is single stranded but it is double stranded in Rheovirus.

12. The hearts of reptiles is three chambered except crocodile.

13. Lungs are absent in fishes but lungfishes have lung as well as gill.

                           lungfish                                                         double-stranded RNA

14. Birds can fly but Ostrich and Kiwi do not do so.

15. Mammals are mostly terrestrial. Bats are able to fly like birds while Whale and Dolphin are able to swim as well.

16. Mammals are usually viviparous (give birth to their young ones directly) except Duck-Billed Platypus (Ornithorhyncus) and spiny ant eater (Echidna).

17. RBCs or erythrocytes of mammals are circular and biconcave except Camel and Ilama as they have oval and flat RBCs.

 flightless ostrich

                                                                               nucleated RBC of camel 

18. Larval stage is usually juvenile stage as they cannot reproduce but Axolotl larva of American salamander can reproduce. The phenomenon is called padogenesis or neoteny.

19. Young ones are produced by fertilization of eggs with the help of sperms. However the drones of honey bee develop from unfertilized eggs.

20. Mitochondria are the respiratory organelles of most organisms, but in prokaryotes mesosomes perform the function of mitochondria.

21. Euglena shows both plant and animal characters. It lacks cell wall but bears chloroplast.

References:

i) https://www.studyadda.com/notes/11th-class/biology/the-living-world-%E0%A4%9C%E0%A4%B5-%E0%A4%9C%E0%A4%97%E0%A4%A4/biology-is-a-science-of-exceptions/9520#:~:text=Following%20are%20noteworthy%20exceptions%20in,devoid%20of%20nucleus%20except%20camel.

ii) https://www.quora.com/What-are-the-exceptions-of-biology

Some exceptions in Biology