Choose the correct answer from the alternatives given.
Which of the following statements about plant hormones is not true?
A. Each naturally occurring hormone has a specific chemical structure.
B. Hormones are among the principal internal regulators of plant growth.
C. Responses to plant hormones may be observed in nearly any part of a plant.
D. All known growth regulators of plant activities are chemicals produced naturally in plants.

Hint: There are 5 types of plant-growth-regulating compounds or phytohormones: Auxin, Cytokinin, Gibberellin (GA), Ethylene, and Abscisic acid (ABA). For each type there are both naturally occurring hormones as well as synthetic substances.

Complete Answer:
- Cytokinins have precise results on cytokinesis (cell division) and have been observed as kinetin (a changed shape of adenine, a purine) from the autoclaved herring sperm DNA.
- Thus, kinetin occurs both naturally as well as chemically.

So, the correct answer is option (D) 'All known growth regulators of plant activities are chemicals produced naturally in plants'.

Additional points:
Plant Growth Regulators may be of a various chemical composition which includes gases (ethylene), carotenoid derivatives (abscisic acid) or terpenes (gibberellic acid). They also are called phytohormones or plant hormones.
Depending on their action, they can be of following types:
• Plant Growth Promoters – They promote, cell enlargement, cell division, flowering, fruiting. E.g. Auxins, Cytokinins and Gibberellins.
• Plant Growth Inhibitors – These chemical substances inhibit boom and sell dormancy and abscission in plants.
E.g. abscisic acid.

Note: Ethylene can be a promoter or an inhibitor, but is largely a Plant Growth Inhibitor.