The amount of water that condensed at the top of the terrarium change as plants sprouted and grew because water, oxygen and nutrients are very essential elements for growth.
The plant cells grow in size by cell enlargement which in turn requires water. Turgidity of cells helps in extension growth. Thus, plant growth and further development is intimately linked to the water status of the plant. Water also provides the medium for enzymatic activities needed for growth. Oxygen helps in releasing metabolic energy essential for growth activities. Nutrients (macro and micro essential elements) are required by plants for the synthesis of protoplasm and act as source of energy.
Thus as plant grows it needs more water to provide to it's larger plant body and which causes decrease in the amount of water that condensed at the top of the terrarium.
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