<span>Transpiration is considered as necessary evil because during CO2 and other gaseous exchange, water also escapes and that could be harmful to the plant.</span>
The correct answer is gravity.
The planets and other heavenly bodies are attracted towards the earth by the earth's gravity. The pull which is formed by the gravity of earth holds the objects near the earth surfaces. The planets and atmosphere is also kept around the earth's surface by the gravity.
The general term of it all is Cytology, which means the study of the cells. This in turn is split into smaller sciences like Cytophysiology (Study of cell physiology and anatomy), Ctyopathology (Study of diseases effects on cells), and Cytochemistry (study of cell biochemistry).
