Give scientific reasons:
$(1)$ Cytokinin is an evergreen hormone.
$(2)$ Growth is regulated by multiple factors.

(N/A) $(1)$ Cytokinin stimulates cell division and cell expansion in plants. Furthermore,its effect reduces apical dominance and delays senescence (aging). It helps in the retention of chlorophyll in leaves,preventing them from turning yellow and falling off. Thus,it keeps the plant fresh and green,which is why it is called an evergreen hormone.
$(2)$ The phenomenon of growth depends on several factors. For instance,oxygen is essential for respiration. Water is necessary to maintain cell turgor pressure. An optimal temperature range of $28^{\circ}C$ to $30^{\circ}C$ is required. Light is essential for photosynthesis. Nutrients are required for the synthesis of protoplasm,and minerals are also indispensable. Thus,growth is regulated by multiple internal and external factors.