During experiments with shoot tips' response to light, the Darwins concluded that phototropism is caused by , Boysen-Jensen added that phototropism is caused by , and Went further added that phototropism is caused by .

D. a signal; a chemical; auxin                      

Explanation:

Phototropism is described as a phenomenon that involves the growth of a specific organism as an outcome towards a light stimulus. It is generally observed or found in plants yet occurs in a few other organisms, for example, fungi. The different cells that are being found on the plants that are placed far from the light posses a specific chemical known as 'auxin' that generally reacts when the "phototropism" occurs.

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the old way of life was represented by superstition, an angry god, and absolute submission to authority. the thinkers of the age of reason ushered in a new way of thinking. this new way championed the accomplishments of humankind. individuals did not have to accept despair. science and reason could bring happiness and progress. kings did not rule by divine right. they had an obligation to their subjects. europeans pondered the implications for nearly a century. americans put them into practice first.