All stem of the plants and trees are green in colour when they young. But why does the colour of the stem change to brown as they grow older

Because the plants 'mature' and get older, which causes them to have a harder shell like trees so they don't die as easy, and plants are typically brown they are already dead.

Explanation:

the rebels’ chances of victory at lexington & concord were fairly slim, given the fact that most of them very untrained and they were up against the world's greatest military force.  

abraham lincoln issued the preliminary emancipation proclamation on september 22nd, 1862. it stipulated that if the southern states did not cease their rebellion by january 1st, 1863, then proclamation would go into effect.