Phototropism in plants can be elicited by the process of water flow. The lateral light cause stem to grow differentially due to asymmetrical auxin gradient. But the rapid response may be evaporative inhition of elongation or the change of microtules structure in the cell wall and phototropic growth of the new branches will be increased longer at the sunny side than the shade side of main stem and so keep on growing secondly to the direction such as upper, horizontal and below. The difference of irradiation on both sides of the main stem would influence differential water utilization at one sunny side competitively and resulted better growing toward the lighted direction. Phototropism is stem bending of the vector both elongation and eclipse of tree annual rings. The stage of phototropic response can be defined as the early elongated growth, the lateral branching and the last annual ring growth. The overall stages must be related to water utilization. Hence, the analogy of tropic growth is the relation of road web and market industry. Auxin may be traffic signal and light can be the director of road building to regulate the road traffic flow. Such an analogy for water flow and utility by way of light can explain the other tropisms as well as phototropism.