Final answer:
The Durham tube in the fermentation tube is used to detect the presence of gas produced during fermentation.
Step-by-step explanation:
The Durham tube is used in the fermentation tube to detect the presence of gas produced during fermentation. When a gas is produced, it will collect in the Durham tube, causing it to rise. This allows for the identification and quantification of fermentation.
For example, in the fermentation of sugar by yeast to produce ethanol, carbon dioxide gas is released. The production of gas can be observed by the rise of the Durham tube.