The segments labelled 1,2,3 are as follows:

1: Lag phase: During this period, the cells are active metabolically only increasing their cell size and synthesizing enzymes required for cell division under environmental conditions that are given.

2: Log phase: In this phase, the cell increases in number in a logarithmic pattern resulting in balanced increase of cell constituents. This phase shall continue unless nutrients are depleted and toxic materials accumulation reaches a threshold level.

3: Stationary phase: This phase occurs when the nutrients get depleted and toxic by-products accumulate thus not leading to anymore increase in cell number.