Step 1: Understanding the Concept
Quantitative growth is a measurable increase in size, number or weight.
Step 2: Qualitative growth
Qualitative growth is a change in structure or function, such as differentiation of cells into tissues.
Step 3: Compare the options
Dry weight, cell number and volume are all measurable quantities, so only differentiation is qualitative.
Final Answer:
Differentiation of cells represents qualitative growth.
\[ \boxed{\text{(C) }\text{Differentiation of cells}} \]