There are inner part of the stem and outer part of the stem; in a plant the inner part supports the movement of the water and nutrients. You stop the flow of the vital elements of plant growth when you snap the stem and this is the reason why the plant die due to starvation of the nutrients.
Without the green house effect, earth would be like a solid ball of ice and rock. Life as we know it wouldn't exist without greenhouse gases. The temperature would drop below 0° Fahrenheit!